GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on July 04, 2015, 07:20:47 PM
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I have had a few questions (thanks for all the work, Daryl) about how Boattails compare to Flat Base bullets at airgun velocities.... Using Geoff Kolbe's Drag and Trajectory Calculator, I compared the latest .308 cal Bore-Riding Boattail I designed to a Flat Base bullet of the same weight and nose profile.... Unfortuantely, while I was inputting the data for the Flat Base bullet I made an error, which I have since corrected.... The data below, and the comments about it, now reflect those corrections.... My profuse apologies, it was a case of GIGO (garbage in, garbage out)....
Here is the Boattail.... The Flat Base has the same nose, but parallel at 0.308" diameter from there back to the base, and the overall length is 0.85" to keep the weight the same....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/308%20cal%20BBT%20152%20gr_zps0blk15f4.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/308%20cal%20BBT%20152%20gr_zps0blk15f4.jpg.html)
Here are the calculated Drag Curves for the two bullets.... First the Flat Base....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20FB_zpst20cgyea.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20FB_zpst20cgyea.png.html)
Next the Boattail.... Note the subsonic drag is less than half that of the Flat Base, but there is a little spike in the drag at just over Mach 1.... Above Mach 1.2, the Boattail has slightly less drag....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png.html)
When the trajectory and wind drift are calculated for various muzzle velocities, for a rifle zeroed at 100 yards, here are the results.... Solid lines are the Boattail, Dotted lines are the Flat Base....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20BBT%20Drift%20and%20Drop_zpscbual1wt.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20BBT%20Drift%20and%20Drop_zpscbual1wt.jpg.html)
Note that the Boattail has a flatter trajectory and less wind drift at all velocities, although the advantage in drop is small at over 1300 fps.... Subsonic, the Boattail shows an advantage in trajectory, and a huge advantage in reduced wind drift....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20BBT%20Drift%20and%20Drop%20Subsonic_zps1mqscrz6.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20BBT%20Drift%20and%20Drop%20Subsonic_zps1mqscrz6.jpg.html)
There are a couple of other things about the wind drift that are interesting.... The least wind drift occurs for the Boattail when starting at 900 fps.... while for the Flat Base bullet the least drift needs a higher muzzle velocity of 950 fps.... However the Boattail always has less drift, and in fact at 900 fps it is predicted to have less drift at 400 yards than the Flat Base at 200 yards.... Refering to the top graph, you will note that in all cases, there is a muzzle velocity where the wind drift is the worst, increasing with range.... For both bullets, that occurs at a muzzle velocity of about 1300 fps at 200 yards, 1500 fps at 400 yards, and 1700 fps at 600 yards.... This is important for anyone considering shooting supersonic in hopes of having more success in hitting distant targets.... If you contemplate doing that, these results tell you that most achieveable velocities, even with Helium, will put you right where the drift is the worst.... To stay supersonic all the way to the target, means you need about 1600 fps to get to 200 yards, and over 2000 fps to reach 400 yards.... Even at those extreme velocities, the wind drift will still be greater than if you stayed subsonic....
These calculations are only valid for this one comparison, but of course it is possible to compare any two bullet by this method, just very time consuming.... I feel that they are typical of what you will find, however, when looking at what are the possible advantages of a Boattail for use in airguns.... One other thing I did not address is that for a given length, Boattails are longer, and the shape also requires additional stability.... They therefore need a faster twist rate to be stable.... For the two bullets in this example, the optimum twist rate, to give a Stability Factor of 1.5 at Mach 1 (worst case), is a 10" twist for the Boattail, while the Flat Base only needs a 16" twist.... If kept below 1050 fps, the Boattail can use a 12" twist, and I have found that in most cases the twist correction for my Boattails is exaggerated, they will work in a slower twist than the calculated one....
I hope that this information gives you some insight into the advantages and disadvantages of using a Boattail design in airguns.... I feel that there are significant gains to be made right at the muzzle velocities we are striving for, in the 900-1050 fps range.... It is worth noting what the retained velocities are for the two bullets, both starting out at 1000 fps.... For the Flat Base, they are 865 fps at 200 yards, 761 fps at 400 yards, and 671 fps at 600 yards.... For the Boattail, the retained velocity is 921 fps at 200 yards, 864 fps at 400 yard, and 813 fps at 600 yards... The Boattail is travelling faster at 600 yards than the Flat Base at 300, and over 140 fps faster by the time they are both at 600 yards....
I apologize for the earlier error....
Bob
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Thank you for such an informative post... I wish my brain worked like yours! LOL....
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I read every post Bob makes knowing I am going to have a head ache afterwards, I just wish I understood some of them better, I get the drift of them but hands on is a different storey for me. If you really want a head ache read some of Lloyd's and Bobs post , Its like a foreign language. David
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I have had a few questions (thanks for all the work, Daryl) about how Boattails compare to Flat Base bullets at airgun velocities.... Using Geoff Kolbe's Drag and Trajectory Calculator, I compared the latest .308 cal Bore-Riding Boattail I designed to a Flat Base bullet of the same weight and nose profile.... Unfortuantely, while I was inputting the data for the Flat Base bullet I made an error, which I have since corrected.... The data below, and the comments about it, now reflect those corrections.... My profuse apologies, it was a case of GIGO (garbage in, garbage out)....
Here is the Boattail.... The Flat Base has the same nose, but parallel at 0.308" diameter from there back to the base, and the overall length is 0.85" to keep the weight the same....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/308%20cal%20BBT%20152%20gr_zps0blk15f4.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/308%20cal%20BBT%20152%20gr_zps0blk15f4.jpg.html)
Here are the calculated Drag Curves for the two bullets.... First the Flat Base....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20FB_zpst20cgyea.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20FB_zpst20cgyea.png.html)
Next the Boattail.... Note the subsonic drag is less than half that of the Flat Base, but there is a little spike in the drag at just over Mach 1.... Above Mach 1.2, the Boattail has slightly less drag....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png.html)
When the trajectory and wind drift are calculated for various muzzle velocities, for a rifle zeroed at 100 yards, here are the results.... Solid lines are the Boattail, Dotted lines are the Flat Base....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20BBT%20Drift%20and%20Drop_zpscbual1wt.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20BBT%20Drift%20and%20Drop_zpscbual1wt.jpg.html)
Note that the Boattail has a flatter trajectory and less wind drift at all velocities, although the advantage in drop is small at over 1300 fps.... Subsonic, the Boattail shows an advantage in trajectory, and a huge advantage in reduced wind drift....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20BBT%20Drift%20and%20Drop%20Subsonic_zps1mqscrz6.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20BBT%20Drift%20and%20Drop%20Subsonic_zps1mqscrz6.jpg.html)
There are a couple of other things about the wind drift that are interesting.... The least wind drift occurs for the Boattail when starting at 900 fps.... while for the Flat Base bullet the least drift needs a higher muzzle velocity of 950 fps.... However the Boattail always has less drift, and in fact at 900 fps it is predicted to have less drift at 400 yards than the Flat Base at 200 yards.... Refering to the top graph, you will note that in all cases, there is a muzzle velocity where the wind drift is the worst, increasing with range.... For both bullets, that occurs at a muzzle velocity of about 1300 fps at 200 yards, 1500 fps at 400 yards, and 1700 fps at 600 yards.... This is important for anyone considering shooting supersonic in hopes of having more success in hitting distant targets.... If you contemplate doing that, these results tell you that most achieveable velocities, even with Helium, will put you right where the drift is the worst.... To stay supersonic all the way to the target, means you need about 1600 fps to get to 200 yards, and over 2000 fps to reach 400 yards.... Even at those extreme velocities, the wind drift will still be greater than if you stayed subsonic....
These calculations are only valid for this one comparison, but of course it is possible to compare any two bullet by this method, just very time consuming.... I feel that they are typical of what you will find, however, when looking at what are the possible advantages of a Boattail for use in airguns.... One other thing I did not address is that for a given length, Boattails are longer, and the shape also requires additional stability.... They therefore need a faster twist rate to be stable.... For the two bullets in this example, the optimum twist rate, to give a Stability Factor of 1.5 at Mach 1 (worst case), is a 10" twist for the Boattail, while the Flat Base only needs a 16" twist.... If kept below 1050 fps, the Boattail can use a 12" twist, and I have found that in most cases the twist correction for my Boattails is exaggerated, they will work in a slower twist than the calculated one....
I hope that this information gives you some insight into the advantages and disadvantages of using a Boattail design in airguns.... I feel that there are significant gains to be made right at the muzzle velocities we are striving for, in the 900-1050 fps range.... It is worth noting what the retained velocities are for the two bullets, both starting out at 1000 fps.... For the Flat Base, they are 865 fps at 200 yards, 761 fps at 400 yards, and 671 fps at 600 yards.... For the Boattail, the retained velocity is 921 fps at 200 yards, 864 fps at 400 yard, and 813 fps at 600 yards... The Boattail is travelling faster at 600 yards than the Flat Base at 300, and over 140 fps faster by the time they are both at 600 yards….
I apologize for the earlier error....
Bob
Bob all is ok on paper, but will it shoot as well as this plain base 30 cal has done in many air guns see attachment
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We don't know yet... The bullet is a prototype and has not been tested.....
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Bob all is ok on paper, but will it shoot as well as this plain base 30 cal has done in many air guns see attachment
For curiosities sake which .30 caliber airgun did you shoot that group with?
Sorry to side track your thread a little bit Bob, as always your data is outstanding and I look forward to seeing how this design performs!
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I admit, that is an amazing 5 shot group with an air rifle. The bullet/ barrel/ pressure combination must be right on... It would be interesting to see what that bullet does @200,300,&400yds with a slight crosswind.......
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jr460.... you're right, of course, I should just give up and crawl back under my desk.... ::)
Bob
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jr460.... you're right, of course, I should just give up and crawl back under my desk.... ::)
Bob
No No Bob
Your idea is very sound,but as you have pointed out 1300 fps is the magic number for it to out perform a plain base bullet.
The 1300 is just not attainable with air guns made at this time even the custom ones can not reach 1300 fps (unless you add helium).
As for all asking what did this group it is a custom Jack Haley Jack built to my specks 34 inch barrel 1/8 twist at 3200 psi it will do
1030 fps in this gun with this 132 gr bullet.
Keep up the good work Bob the boat tail design is just ahead of the guns being made in this day and time.
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You either didn't read, or didn't understand (or believe) what I wrote.... BELOW Mach 1 the gains are far superior to over 1300 fps.... something not fully understood or appreciated by airgunners.... Boattails can have double the BC of flat base bullets in the subsonic region.... That is why I have been working on them for about two years now....
Bob
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Ok Bob
BOTAIL just do not work that well below the speed of sound and going from the speed of sound and drooping below it in flight
does a lot of strange things to projectiles( which you have left out of your data),Why has the Olympics shooters not cum up with this idea in the 22's they compete with and the air gun pellets they use.In the Olympics a 20 percent advantage would not be over looked no matter the cost.As I have said keep up your good work but please complete the math.
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BOTAIL just do not work that well below the speed of sound
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20FB_zpst20cgyea.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20FB_zpst20cgyea.png.html)
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png.html)
I've done the math.... perhaps you should.... the boattail has less than half the drag below Mach 0.85 (950 fps).... Of course I suppose it is possible that all the research done by the military at places like the Aberdeen Proving Grounds may be wrong, and you may be right.... Look at the Drag Coefficient in the subsonic region.... particularly at 900-1000 fps.... These drag models were downloaded from the JBM Ballistics website, and are the official versions....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20Coefficients%20Below%202000%20fps_zpsq9mxrmkw.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20Coefficients%20Below%202000%20fps_zpsq9mxrmkw.jpg.html)
The G1 Model used above.... flat base spritzer....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/g1_zps09zm9zd2.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/g1_zps09zm9zd2.jpg.html)
The G7 Model used above.... spitzer boattail....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/g7_zpsscbkua7h.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/g7_zpsscbkua7h.jpg.html)
Everybody, including me, thought that boattails were intended for extreme range and high supersonic flight.... because that is their typical use.... One look at those drag curves convinced me that maybe.... just maybe.... we had overlooked something.... I am well aware that boattails, fired from barrels without a fast enough twist to take into account their inherent stability problems just as they drop through the speed of sound have been known to tumble.... The obvious solution, to me, and particularly since we don't have the power to launch them at 3000 fps, is to stay subsonic where they don't have a stability problem.... and have about half the drag of a flat based bullet....
If you want to have a positive input on this discussion, then find some numbers that support your statement that I quoted above.... or some hard data that does the same.... don't just be a naysayer because you have a different opinion without facts to back it up.... You might like to watch this video, they don't mention boattails, but with only 100 fps loss over 300 yards, that leads me to believe they must be boattails....
https://www.youtube.com/watch?v=Ruvoc1-uZRY (https://www.youtube.com/watch?v=Ruvoc1-uZRY)
Or read these articles on subsonic sniper rounds that use boattail bullets out to 600 yards....
http://www.sadefensejournal.com/wp/?p=751 (http://www.sadefensejournal.com/wp/?p=751) .... http://www.sadefensejournal.com/wp/?p=1318 (http://www.sadefensejournal.com/wp/?p=1318) ....
and lastly, a couple of types of commercially available subsonic centerfire ammunition using boattails for the .300 AAC Blackout, which has a reputation for superb accuracy....
http://www.midwayusa.com/product/381996/lapua-subsonic-bullets-30-caliber-308-diameter-200-grain-full-metal-jacket-boat-tail-box-of-100 (http://www.midwayusa.com/product/381996/lapua-subsonic-bullets-30-caliber-308-diameter-200-grain-full-metal-jacket-boat-tail-box-of-100)
http://www.midwayusa.com/product/486942/doubletap-ammunition-300-aac-blackout-240-grain-sierra-matchking-hollow-point-boat-tail-box-of-20 (http://www.midwayusa.com/product/486942/doubletap-ammunition-300-aac-blackout-240-grain-sierra-matchking-hollow-point-boat-tail-box-of-20)
Conventional thinking?.... no.... but to me the potential is obvious.... By using Helium, high pressure and long barrels, we can duplicate the performance of the .300 AAC Blackout with an airgun.... food for thought....
Bob
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BOTAIL just do not work that well below the speed of sound
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20FB_zpst20cgyea.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20FB_zpst20cgyea.png.html)
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20BBT%203%20cal%20Long%20BR_zpsobsigepz.png.html)
I've done the math.... perhaps you should.... the boattail has less than half the drag below Mach 0.85 (950 fps).... Of course I suppose it is possible that all the research done by the military at places like the Aberdeen Proving Grounds may be wrong, and you may be right.... Look at the Drag Coefficient in the subsonic region.... particularly at 900-1000 fps.... These drag models were downloaded from the JBM Ballistics website, and are the official versions....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20Coefficients%20Below%202000%20fps_zpsq9mxrmkw.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20Coefficients%20Below%202000%20fps_zpsq9mxrmkw.jpg.html)
The G1 Model used above.... flat base spritzer....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/g1_zps09zm9zd2.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/g1_zps09zm9zd2.jpg.html)
The G7 Model used above.... spitzer boattail....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/g7_zpsscbkua7h.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/g7_zpsscbkua7h.jpg.html)
Everybody, including me, thought that boattails were intended for extreme range and high supersonic flight.... because that is their typical use.... One look at those drag curves convinced me that maybe.... just maybe.... we had overlooked something.... I am well aware that boattails, fired from barrels without a fast enough twist to take into account their inherent stability problems just as they drop through the speed of sound have been known to tumble.... The obvious solution, to me, and particularly since we don't have the power to launch them at 3000 fps, is to stay subsonic where they don't have a stability problem.... and have about half the drag of a flat based bullet....
If you want to have a positive input on this discussion, then find some numbers that support your statement that I quoted above.... or some hard data that does the same.... don't just be a naysayer because you have a different opinion without facts to back it up.... You might like to watch this video, they don't mention boattails, but with only 100 fps loss over 300 yards, that leads me to believe they must be boattails....
https://www.youtube.com/watch?v=Ruvoc1-uZRY (https://www.youtube.com/watch?v=Ruvoc1-uZRY)
Or read these articles on subsonic sniper rounds that use boattail bullets out to 600 yards....
http://www.sadefensejournal.com/wp/?p=751 (http://www.sadefensejournal.com/wp/?p=751) .... http://www.sadefensejournal.com/wp/?p=1318 (http://www.sadefensejournal.com/wp/?p=1318) ....
and lastly, a couple of types of commercially available subsonic centerfire ammunition using boattails for the .300 AAC Blackout, which has a reputation for superb accuracy....
http://www.midwayusa.com/product/381996/lapua-subsonic-bullets-30-caliber-308-diameter-200-grain-full-metal-jacket-boat-tail-box-of-100 (http://www.midwayusa.com/product/381996/lapua-subsonic-bullets-30-caliber-308-diameter-200-grain-full-metal-jacket-boat-tail-box-of-100)
http://www.midwayusa.com/product/486942/doubletap-ammunition-300-aac-blackout-240-grain-sierra-matchking-hollow-point-boat-tail-box-of-20 (http://www.midwayusa.com/product/486942/doubletap-ammunition-300-aac-blackout-240-grain-sierra-matchking-hollow-point-boat-tail-box-of-20)
Conventional thinking?.... no.... but to me the potential is obvious.... By using Helium, high pressure and long barrels, we can duplicate the performance of the .300 AAC Blackout with an airgun.... food for thought....
Bob
YOUR DRAWING REFERENCE IS NOT THE BOAT TAIL YOU ARE NOW SHOWING IN THIS QUOTE!!!COME ON DO THE MATH RIGHT.
AS FOR TRYING TO DUPLICATE A 300 BLACK OUT ROUND WITH AIR IT HAS ALREADY BEEN DON WITH AIR! NO HELAEM REQUIRED ALL THAT IS NEEDED 1/8 TWIST BARREL 38 inches long AND 4200 PSI AIR GUN AND MY 300 BLACK OUT BULLET WHICH IS A FLAT BASE AND SOLD BY MIDWAY USA.
Your boat tail in your {drawing} was abandoned back in the 60's because it was to hard on barrels and gave 1/2 the barrel life of todays boat tail designs .You are using out of date data,and quoting obsolete military data enough said on this subject .Here is a new problem for you to put in your computer If you sight in at 500 yds @ 900 fps facing dew west NO WIND NEEDED and you turn around and shoot the exact same going due east what is your correction?
I really fiend it funny that all of a sudden you are saying you are trying to duplacte a powder burner with helium thought this was for air guns???
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Some of us do run helium... I personally find Bobs information useful and beneficial for all of us big bore shooters. I find it hard to follow someone with such an argumentative personality. All I have to go on is the 20 post vs the 11,535 post that Bob has..... Not to mention all the PROVEN research that Bob has presented and represented... Instead of arguing or trying to belittle someones efforts, why not show us? We all come here to share or learn.. I personally come here to learn. Just show us some detailed research or results that you have achieved.
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I am no longer going to argue with those who prefer to stick their head in the sand and offer opinions only without science to back it up.... for I only have limited time to devote to do what I want, and that is to develop boattails for airguns.... I spent the afternoon pursuing the idea of duplicating the ballistics of the .300 AAC Blackout with an airgun.... The goal is to drive a 200 gr. bullet at 1050 fps, which is 490 FPE.... I seriously doubt that can be done with air, although a 38" barrel would certainly help, but fortunately Rizen 1, who IS excited about the project, plans to use Helium, so I am confident the power will be there.... The goal is to make a bullet similar to the Lapua 200 gr. Subsonic (link above), as I am assuming that the jacketed version may not work at 4500 psi.... If it does, then I'm spinning my wheels, and so be it, Rizen 1 can just use the Lapua....
When I started to look at optimizing the design, I found out that the length of the nose is critical to the drag between Mach 0.85-0.95 (950-1060 fps).... By using the JBM Ballistics Drag Calculator, I tried various Ogives from 3R to 10R, from Mach 0.5 to 1.5, with the following results.... The second graph focuses on the velocities we are interested in, which makes what is happening easier to see....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20200%20gr%20Ogives_zpszdijcgpr.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20200%20gr%20Ogives_zpszdijcgpr.jpg.html)
Note that as the length of the nose increases, the Ballistics Coefficient increases drastically between 950 fps and 1060 fps.... Now we can't keep on lengthening the nose forever, because that would shrink the bearing area of the bullet to the point it would tend to misalign in the bore.... I chose an 8R Ogive as the most suitable for a 200 gr. boattail, and input that data in the Kolbe Drag Calculator, to produce the following Drag Curve....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20200%20gr%20BBT_zpszqno3dzk.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20200%20gr%20BBT_zpszqno3dzk.png.html)
Note that the Cd below Mach 0.85 is similar to the curve above for the 152 gr.... but from Mach 0.95 and up the Cd is about 30% less.... On top of that, the SD is 32% higher, so the gains in the BC are huge.... about 30% subsonic and 70% transonic.... For both bullets starting at 1050 fps, the wind drift in a 10 mph crosswind at 200 yards for the 200 gr. drops by a third compared to the 152 gr., to about 2 MOA, and that advantage carries over to the longer ranges as well.... This bullet, on paper, equals the performance mentioned in the video linked above, losing only 100 fps over 300 yards....
Going this route does have some hurdles to overcome.... The .300 AAC Blackout uses a 7" twist, although for velocities below 1050, we "only" need an 8".... I'm sure there will be some other glitches along the way, and some discoveries that find more improvements as well.... Let the fun begin !!!
Bob
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I've locked this tread for administrative review.
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;D I for one can't wait to buy a .357 big bore and play with long range accuracy and possibly hunt larger pests with no fear of my kill being humanel
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Hey Bob,
I know we don't see eye to eye too much but I think I may be able to help "maybe" (if you want). I have been reading your posts for the last 2 years you have worked on this project and see merit.
I also read that you get alot of poop for it as well (from PB'ers and airgunners). I also know it is extremely time consuming to run the numbers for the perfect shape that can change the future of airgunning.
I use a program called minitab "statistical program". You can input variables "one or more" and through statistics predict the outcome. There's a valuable piece of software in minitab called the optimizer that allows you to adjust with the swipe of the mouse to predict.
I can "or you can download for 1 month" and run the numbers on the bullet design and optimize statistically "much faster and easier". I have it on my computer and have used it for marauder tuning with just 4 data point fairly accurately. If you choose to buy it, it is over $800.
I have already ran the program already and noticed on the drag calculator that Meplate diameter,base diameter, and boat tail length is the most important. Heck, I am only guessing.
If you want me to run numbers, I would do it. Just tell me what to run. Min/max and up to 30 variables.
like I said, we don't see eye to eye but, I have a big bore now (9mm/.357) and BC's aren't great and even if the gun was shooting over 120 fpe, it would still be a 50 yard gun.
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Shorty, if I understand correctly, to use statistical analysis you need real practical data to enter to predict trends. The way I interpret Bob's work in this thread is to use known ballistic data and modeling methods to determine parameters for for a new design then apply practical testing. Once practical testing is underway the parameters and methods can be evaluated for their accuracy, if they match we all go woohoo! if not we back up and modify our methods (models and input) and try again. I guess I'm asking where the data should come from for statistical analysis? Published BCs from bullet manufacture are notoriously incorrect for airgun velocities so some preliminary practical data would need to be worked up first?
I see the next step in bullet evolution is to determine how the CG, drive bands, boatails and other features affect accuracy and/or stability independent of the cD. Maybe another possible use for statistical analysis.
tom
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QVtom,
Your right, you really need practical data. Although, Bob has been using this for his hypothesis (http://www.geoffrey-kolbe.com/drag.htm (http://www.geoffrey-kolbe.com/drag.htm)). As for design work (on paper), I use this DOE method all the time at work when "practical" data is not there to dial something in without the "shoot from the hip" and time restraints for designing new production methods for "impossible tasks".
The DOE using the optimization program allows you to "get close" and then with practical data "dial in" to the best possible.
Think of it like an OQ and PQ. OQ finds the limits and PQ makes it perfect.
It's just a thought if Bob wants to give it a shot. I tried the "shoot from the hip" ( extremely not disrespectful ) and ran over 20 models of his .308 model using the 3 variables mentioned above in less than 60 minutes. Since I don't know any better about dimensional limits and what to look for except drag COD, my best design looked like a football. less than .05 below mach 1 and max peak of COD .7 without a turbulence. Which makes me giggle because of your comment about putting a bullet in backwards.
Your right though, good in good out. Bad in bad out. I am here to chug data.
I have some foot in this as well as I will be contacting this company in the next couple days (http://www.polycaseammo.com/ (http://www.polycaseammo.com/)). Approximately 30 % lighter with a BC of .033. Not only that, lead free. My .357/9mm needs to shoot faster and longer.
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I have been working on researching the effect of Meplat size on this bullet today, and if you hold the Ogive constant at 8R, and adjust the body length to keep the weight at 200 gr. I found some interesting trends as shown in the graph below....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20cal%20200%20gr%20Meplats_zpsy7wyxhd4.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20cal%20200%20gr%20Meplats_zpsy7wyxhd4.jpg.html)
The only curves that are complete up to a 70% Meplat are the ones I could get from the Kolbe Drag Calculator.... I double checked those using the JBM Drag Calculator (amazing agreement), but it only allows a Meplat up to 35% of Caliber, no larger.... First the obvious trends....
1. At Mach 1.5, the optimum meplat is 10%, although 0% is only 0.001 less.... The larger you go, larger than 10%, the worst the drag.... So for supersonic, large meplats are, literally, a drag.... A 50% Meplat had the same drag at Mach 1.5 as at Mach 1, larger was poorer,
2. At Mach 1.0, the optimum meplat is still 10%, but the curve is very flat from 0-20%....
3. At Mach 0.95, the curve is starting to shift towards larger meplats, peaking at 20%.... By the time the velocity is down to Mach 0.9 (1005 fps) the curve peaks at 30%, but it is very flat from 10-35%, and I couldn't assess larger than that....
4. Once the velocity drops to Mach 0.85 and down to 0.70 the graphs appear flat.... You will have to take my word for it that the best value for the BC is at a 30% meplat, but obviously it is a pretty flat curve.... The difference between a pointed and 30% is only 0.005, and for those curves I couldn't asses below 35%....
5. Down at Mach 0.5, the best values for the BC are from 0.35 and below, but again it is essentially flat....
For the area I am interested in primarily, from Mach 0.7-0.9 (781 to 1032 fps) the 30% Meplat is the best choice.... but in reality for any high subsonic use 20-30% will be essentially the same.... For hunting within 100 yards, where a larger Meplat is more important than the last tiny bit of drag, and with velocities below 950 fps, any Meplat up to 70% will be just fine.... For supersonic use, a 10% Meplat is optimum for a bullet of this shape....
shorty, thanks for the offer, I'll consider it....
Bob
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Hello BOB if I offended you and the GTA staff accept my apologies.
You and a few others here insets a boat tail will out preform a flat base bullet.
I give you real world groups. ALL 30 CALBER air gun.
Number 1 132 gr flat base Have never found a bullet that will out shoot it to 500 yds
Number 2 152 gr spire point gas check {closest you can get to your rebated boat tail in the drawing real life example} shoots very well and has been used in air gun competition with a lot of wines attributed to it bucks the wind ok good to 600 yards because of it weight and nose shape
Number 3 156 gr flat point with gas check mainly used for hunting and does the job very well have used to 300 yds
Now all three are average best groups for the bullets in the photos.
The rebated boat tail {gas check} just can not do what the 132 flat base will do in three real air guns Qb,haley,and new big bore from air hog .
I know some of you do not believe a flat base will out preform a boat tail but ask why is there no boat tail hand gun bullets especially
match ammo it is all flat base a boat tail just will not give a 20 percent increase{ as indicated in this posting} in the real world of air guns it is just the reverse . Photos show this
Please look at the proof and if you have better grouping with a boat tail of any type lets see it ,no more number crunching and graphs just real world proof.
David
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(https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRJpCNmlQ_JTnC76__n77s9QN7kV6UcYIQYLInU1IiZwl2KeXiD)
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(https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRJpCNmlQ_JTnC76__n77s9QN7kV6UcYIQYLInU1IiZwl2KeXiD)
::) ::) x2
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Hello BOB if I offended you and the GTA staff accept my apologies.
You and a few others here insets a boat tail will out preform a flat base bullet.
I give you real world groups. ALL 30 CALBER air gun.
Number 1 132 gr flat base Have never found a bullet that will out shoot it to 500 yds
Number 2 152 gr spire point gas check {closest you can get to your rebated boat tail in the drawing real life example} shoots very well and has been used in air gun competition with a lot of wines attributed to it bucks the wind ok good to 600 yards because of it weight and nose shape
Number 3 156 gr flat point with gas check mainly used for hunting and does the job very well have used to 300 yds
Now all three are average best groups for the bullets in the photos.
The rebated boat tail {gas check} just can not do what the 132 flat base will do in three real air guns Qb,haley,and new big bore from air hog .
I know some of you do not believe a flat base will out preform a boat tail but ask why is there no boat tail hand gun bullets especially
match ammo it is all flat base a boat tail just will not give a 20 percent increase{ as indicated in this posting} in the real world of air guns it is just the reverse . Photos show this
Please look at the proof and if you have better grouping with a boat tail of any type lets see it ,no more number crunching and graphs just real world proof.
David
Weeeeeellllll, to me, you shot 2-150+ gr GC bullets vs a 132 gr FP bullet. You will have to shoot the 132gr (which shows accurate in your gun) in both FB and GC configuration to prove that theory a little more solidly. This simply tells me your particular barrel doesn't really like bullets that heavy more than it does your gun NOT liking GC slugs or GC NOT being more accurate.................
My Corsair 308 with the 1-20 twist didn't like 150+ gr slugs but it loved the 130s and lighter too. As a matter of fact, i had the same FB 130 mold that you show up above and i had the RCBS 150 gr converted to HP and it was Gas Check. GUESS WHICH ONE IT LIKED MORE...........................NOT THE 130 gr FB bullet............................
Now for me, i've shot accurate BOTH. But none of them where the same bullets and that's the problem.
NOE HAS MOLDS for the SAME BULLETS IN BOTH CONFIGURATIONS (GC and FB). And in the same mold to................
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Not playing that game.... congrats on the great groups, BTW.... I can't shoot like that !!!
Bob
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Reloading PB has shed the light on BT.. After using 175gr Sierra Match 175gr BT, I've never looked back.. Now after reading Your post I understand better.. Thanks
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Hello BOB if I offended you and the GTA staff accept my apologies.
You and a few others here insets a boat tail will out preform a flat base bullet.
I give you real world groups. ALL 30 CALBER air gun.
Number 1 132 gr flat base Have never found a bullet that will out shoot it to 500 yds
Number 2 152 gr spire point gas check {closest you can get to your rebated boat tail in the drawing real life example} shoots very well and has been used in air gun competition with a lot of wines attributed to it bucks the wind ok good to 600 yards because of it weight and nose shape
Number 3 156 gr flat point with gas check mainly used for hunting and does the job very well have used to 300 yds
Now all three are average best groups for the bullets in the photos.
The rebated boat tail {gas check} just can not do what the 132 flat base will do in three real air guns Qb,haley,and new big bore from air hog .
I know some of you do not believe a flat base will out preform a boat tail but ask why is there no boat tail hand gun bullets especially
match ammo it is all flat base a boat tail just will not give a 20 percent increase{ as indicated in this posting} in the real world of air guns it is just the reverse . Photos show this
Please look at the proof and if you have better grouping with a boat tail of any type lets see it ,no more number crunching and graphs just real world proof.
David
Weeeeeellllll, to me, you shot 2-150+ gr GC bullets vs a 132 gr FP bullet. You will have to shoot the 132gr (which shows accurate in your gun) in both FB and GC configuration to prove that theory a little more solidly. This simply tells me your particular barrel doesn't really like bullets that heavy more than it does your gun NOT liking GC slugs or GC NOT being more accurate.................
My Corsair 308 with the 1-20 twist didn't like 150+ gr slugs but it loved the 130s and lighter too. As a matter of fact, i had the same FB 130 mold that you show up above and i had the RCBS 150 gr converted to HP and it was Gas Check. GUESS WHICH ONE IT LIKED MORE...........................NOT THE 130 gr FB bullet............................
Now for me, i've shot accurate BOTH. But none of them where the same bullets and that's the problem.
NOE HAS MOLDS for the SAME BULLETS IN BOTH CONFIGURATIONS (GC and FB). And in the same mold to................
Please reread my post with photos These are average grouping out of 3 different air guns made by 3 different makers.
David
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Hello BOB if I offended you and the GTA staff accept my apologies.
You and a few others here insets a boat tail will out preform a flat base bullet.
I give you real world groups. ALL 30 CALBER air gun.
Number 1 132 gr flat base Have never found a bullet that will out shoot it to 500 yds
Number 2 152 gr spire point gas check {closest you can get to your rebated boat tail in the drawing real life example} shoots very well and has been used in air gun competition with a lot of wines attributed to it bucks the wind ok good to 600 yards because of it weight and nose shape
Number 3 156 gr flat point with gas check mainly used for hunting and does the job very well have used to 300 yds
Now all three are average best groups for the bullets in the photos.
The rebated boat tail {gas check} just can not do what the 132 flat base will do in three real air guns Qb,haley,and new big bore from air hog .
I know some of you do not believe a flat base will out preform a boat tail but ask why is there no boat tail hand gun bullets especially
match ammo it is all flat base a boat tail just will not give a 20 percent increase{ as indicated in this posting} in the real world of air guns it is just the reverse . Photos show this
Please look at the proof and if you have better grouping with a boat tail of any type lets see it ,no more number crunching and graphs just real world proof.
David
Weeeeeellllll, to me, you shot 2-150+ gr GC bullets vs a 132 gr FP bullet. You will have to shoot the 132gr (which shows accurate in your gun) in both FB and GC configuration to prove that theory a little more solidly. This simply tells me your particular barrel doesn't really like bullets that heavy more than it does your gun NOT liking GC slugs or GC NOT being more accurate.................
My Corsair 308 with the 1-20 twist didn't like 150+ gr slugs but it loved the 130s and lighter too. As a matter of fact, i had the same FB 130 mold that you show up above and i had the RCBS 150 gr converted to HP and it was Gas Check. GUESS WHICH ONE IT LIKED MORE...........................NOT THE 130 gr FB bullet............................
Now for me, i've shot accurate BOTH. But none of them where the same bullets and that's the problem.
NOE HAS MOLDS for the SAME BULLETS IN BOTH CONFIGURATIONS (GC and FB). And in the same mold to................
Please reread my post with photos These are average grouping out of 3 different air guns made by 3 different makers.
David
OK, and your point is by that?..................
Back to the beginning.
1st BOB NEVER said one was MORE ACCURATE than the other. He talks about efficiency during FLIGHT! Along with proper twist rates. He never said, a FB is INACCURATE or that a BT is MORE ACCURATE than a FB bullet.
2nd, his theory has long been proven in the powder burner world and should have validity in the airgun world also. Drag at the REAR of a bullet is the reason for the BT invention AND DRAG HAPPENS AT ALL SPEEDS. If there is AIR going around ANY object, there will be drag. Airplane wings and helicopter blades are designed to have less drag also just like the BT. This is why they don't make 4 sided SQUARE wings. While at 100 yards, you won't really see a difference IMO, you will out further and thats the point of a BT bullet.
3rd, I still stick to my haunches that you will have to shoot the same bullet in the same gun at the same fill pressures if YOU wanna prove which is more accurate. That whole 3 different guns by 3 different builders by 3 different shooters (possibly, not enough info) by 3 different bullets is not a CONTROLLED experiment. Again, NOE makes molds with the SAME bullets in both configs in the SAME mold.
4th, you talked about 500 yards good groups with that FB bullet vs other types but you showed a 100 yard group. How did the FB bullet shoot against the other GC bullets at 500 yards? A small group at 100 yards doesn't tell the tale of what will happen at 500 yards. I've never seen that post containing the pics........ Just asking
BTW, i'm not being argumentative. I'm just trying to figure out what your trying to show. I personnaly know FB bullets are accurate but unfortunately, there is NO REAL airgun info on them. No one shoots them. So if you have some stories or pics of extreme long range with airguns shooting FB bullets, I wanna see the post cause I love long range shooting with airgun is all.........
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I know some of you do not believe a flat base will out preform a boat tail but ask why is there no boat tail hand gun bullets especially
match ammo it is all flat base
David
I get where your going but also remember, a pistol bullet is for very short range with a few magnums being the SLIGHT exception. But in all, they are really short range guns used in combat and target shooting and hunting and this is accepted by the majority who own and shoot pistols like myself. While i can reach out to 200 yards with my pistols, THEY ARE NOT ACCURATE (or at least, i'm not....lol) at that range BY DESIGN. YOU DON'T NEED A BOATTAIL for 7-10-25-50 yard PISTOL shooting.
Can you lob a 357 or 44 mag bullet to 200 yards and hit a 24" still plate with a FB bullet? of course, but i can guarantee ya, it won't be accurate by the majority of pistol shooters. It would take Mr. Jerry Miculek to be somewhat accurate at these ranges and even then, would a boattailed pistol bullet be of any aid at these range? Well, we would certainly have to have someone do an actaul side by side comparison for this feat......................
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LOL, this threads Gimme an Headhake :) :) :)
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LOL, this threads Gimme an Headhake :) :) :)
Me to. David
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It has always been my understanding that in the pb world that flat base bullets shot better at short range and boattails at long range and the reason was in the difficulty of manufacturing the jacketed bullet as a boatail.
I can from being here a while understand that Bob is trying to help move the sport forward a bit at a time... Your tone has been to me the one of one who has a vested interest... I apologize if I am way off base...
Me I personally think it is a matter of way more than just bullet shape; rifling type, twist, fit in the barrel etc.etc. etc... a bunch of small factors that in the beginning lead to a large change in the gesault(Chaos theory), all this is barely charted waters...
I can see a bunch of benefits to a boatail one is that they can possibly use less rifling twist since they are more stable because of not being fat arsed...
If the bugs are found and figured out and this does move things forward well then all Bob gets is personal satisfaction (priceless to some)and our thanks...
so me I can see discussing the whys of how come it is currently working this way and finding solutions not I make this and your thoughts are flawed and here is my proof... WHY what are the factors...
This article is pretty much what I have thought of as how folks current thinking of flat vs boatial is...
Maybe it will give hints at accomplishing it with air and cast not powder and jackets... ::)
http://www.rifleshootermag.com/ammo/ammunition_rs_boattails_093009wo/ (http://www.rifleshootermag.com/ammo/ammunition_rs_boattails_093009wo/)
::)
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Boattails require a faster twist than the same weight flat base for two reasons.... First, they are longer.... and secondly, the boattail makes the bullet act even longer than it is, in the way it slips through the air, not only for lowering the drag, but increasing the effective length for stability....
That is a great article, and certainly looks at the pros and cons of boattails.... You will note that it stresses that at subsonic velocities the base drag is the dominant factor, and that is precisely what a boattail is designed to reduce, the turbulent wake area behind the bullet.... The manufacturing problems with a conventional boattails (concentricity where the boattail blends into the main body) that can cause inaccuracy, is one of the reasons for the rebated boattail design.... That, and the possibility of base distortion in a flat base bullet from the sprue plate, are the reasons I chose a rebated boattail for all my BBTs....
You are quite correct, I have no interests in developing boattails commercially, all my designs are in the public domain for private use.... If taken up commercially, all I have ever asked for is a request to use it, and recognition as the designer.... I have never asked for, or received, any compensation....
Bob
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Wait,,, :-[ I have been wrong about how I think for a long time then... I really did think that you could get away with less twist especially with subsonic just because the weight/cg is more forwards which to me meant it would be more stable... ? ???
Same nose and body/ length and let it lose the few grain compared to flat base...
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Even at the same length, a boattail, at least in theory, requires a faster twist.... and the smaller the base, the faster the twist.... The results I have seen with my shorter BBTs seem to show that the effect on the twist is less with a rebated boattail than what the calculators say.... and they can use a twist rate closer to a flat base.... Not enough testing to nail down all the variables, the idea is still too new....
Bob
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;D I have been saying this for over 2 years now Bob your threads are very informative but you Pac Protector and a very few others are dragging me up to a higher level where ballistics are concerned . In the past I just found a rifle I liked and ammo that worked well in it and was quite happy in my ignorance . So as I have said in the past thanks for the free education. Your posts give me a headache and require me to do research along with several re reads to completely understand all of what you are saying . Thanks for being the Ballistics Geek Master ;D ;D ;D
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Just and interesting anecdote...
"If the base of the bullet were made more streamlined, then the air would be put back together more smoothly, with less turbulence, and would fill the space left by the passing bullet more quickly. This would eliminate much of the base drag. Putting a point on both ends of the bullet accomplishes this, but it generally makes the bullet too long...There is no technical reason why a double-ended bullet should not be used to overcome drag..." Dave Corbin
And by the way, I hate the way a certain person is baiting Mr. Stern. Isn't it called "trolling" for controversy to continually criticize someone while offering nothing positive in return?
Chuckster
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Ok Bob
BOTAIL just do not work that well below the speed of sound and going from the speed of sound and drooping below it in flight
does a lot of strange things to projectiles( which you have left out of your data),Why has the Olympics shooters not cum up with this idea in the 22's they compete with and the air gun pellets they use.In the Olympics a 20 percent advantage would not be over looked no matter the cost.As I have said keep up your good work but please complete the math.
It may be a while but I would like to try a BBT .224 in the long run. While I can not afford a match rifle I do think it is woth trying...
Me I just ordered a Marauder and a Marlin .22 WMR barrel to mod for it...
"BOTAIL just do not work that well below the speed of sound"
In the PB world the world record is with the Lapua 200gr subsonic which looks very much like Bobs 200g design minus the adaptations for air...
http://www.midwayusa.com/product/381996/lapua-subsonic-bullets-30-caliber-308-diameter-200-grain-full-metal-jacket-boat-tail-box-of-100 (http://www.midwayusa.com/product/381996/lapua-subsonic-bullets-30-caliber-308-diameter-200-grain-full-metal-jacket-boat-tail-box-of-100)
It states sub 2" 10 shot groups at 300 yards and I think a well designed air rifle can do better because of consistency of velocity can be achieved more readily. I am not an expert on big bore air rifles but that is what I believe...
even if I am wrong about that why would Lapua make their subsonic a boattail if it did not work better given it is harder to make than a flat base... and I think they know more than I or even you...
Probably should have just dropped it but I did disagree with the statement...
"
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Just and interesting anecdote...
"If the base of the bullet were made more streamlined, then the air would be put back together more smoothly, with less turbulence, and would fill the space left by the passing bullet more quickly. This would eliminate much of the base drag. Putting a point on both ends of the bullet accomplishes this, but it generally makes the bullet too long...There is no technical reason why a double-ended bullet should not be used to overcome drag..." Dave Corbin
And by the way, I hate the way a certain person is baiting Mr. Stern. Isn't it called "trolling" for controversy to continually criticize someone while offering nothing positive in return?
Chuckster
except that it can have the same prob as a plain boattail ; waging its tail exiting the barrel and then recovering...
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Wow Kirby...thanks for your enlightening remarks. You pretty much closed the book on this subject.
Fond regards, Chuckster
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I finally got back to doing some more research on boattail design today.... I studied the relationship between drag and base diameter for four boattail lengths, from 25-100% of the caliber up to Mach 2.... The bullet used was my .308 BBT Whiteout, varying only the boattail, but keeping the weight at 200 gr.... All the calculations were done with the JBM Drag Calculator, but I didn't use the BC, I summed the Drag Coefficient for the Boattail and Base, so a smaller Cd is better.... Here are the results.... 100% diameter (on the right on each graph) is a flat base bullet, there is no boattail....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20vs%20BT%20Diameter%2025%20Length_zpsumfw2tur.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20vs%20BT%20Diameter%2025%20Length_zpsumfw2tur.jpg.html)
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20vs%20BT%20Diameter%2050%20Length.jpg_zpsixnk421e.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20vs%20BT%20Diameter%2050%20Length.jpg_zpsixnk421e.png.html)
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20vs%20BT%20Diameter%2075%20Length_zpsawpzdnir.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20vs%20BT%20Diameter%2075%20Length_zpsawpzdnir.jpg.html)
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Drag%20vs%20BT%20Diameter%20100%20Length.jpg_zps4ueutee6.png) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Drag%20vs%20BT%20Diameter%20100%20Length.jpg_zps4ueutee6.png.html)
I know, that is a lot of data, and not the easiest to read, so here is the summary....
1. Below Mach 0.85 (949 fps), the optimum BT base diameter is 50% of the caliber.... Yeah, that is VERY small, hard to cast, even harder to get a mold made....
2. Below Mach 0.85, the drag is the same regardless of the length of the boattail.... so even a short boattail will help significantly....
3. Above Mach 0.9, the longer the boattail is, the less the drag, at any given velocity.... Makes sense I guess, but again, that makes the mold and bullet hard to make....
4. Right at Mach 1, boattails don't have much of an advantage over a flat base, particularly if the boattail is short....
5. Above Mach 1, the optimum base diameter is between 75-95%, depending on the length of the boattail....
6. At supersonic velocities, a short boattail with a small base is a disadvantage over a flat base bullet.... the best shape is a shallow taper....
7. At speeds between Mach 0.85 and Mach 1 (949-1116 fps), there is an optimum base diameter for each length, and the best way to show that is below....
First, the big picture.... Each line is for a different length boattail, measured in calibers.... You can see that below Mach 0.85 you want a small base (50% is optimum).... However, above Mach 1, the optimum base diameter is much larger, as shown by the lines on the right.... The key is a shallow taper, which means longer boattails can (and should) have a smaller base....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Optimum%20BT%20Base%20Diameter_zpsqft6yf3d.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Optimum%20BT%20Base%20Diameter_zpsqft6yf3d.jpg.html)
Between Mach 0.85 and Mach 1, we need more detail, as shown in the graph below....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Optimum%20BT%20Base%20Detail_zps97vhwtrj.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Optimum%20BT%20Base%20Detail_zps97vhwtrj.jpg.html)
You will notice that for short boattails, as is typical of many of my designs that are made to fit magazines or work in existing twists, at 1000 fps the optimum base is 70% of the caliber.... Even that small a base is hard to obtain, Accurate Molds, for example, can't even approach making something that small.... As the boattail gets longer, for a given velocity, the optimum boattail diameter decreases.... However, when we look at what would be optimum for the .308 BBT Whiteout, we are pretty close.... We will be at the optimum at about 1070 fps.... but operating near the minimum drag for velocities well below that....
Bob
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Wow...
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After reading for years that ---
1. Boat Tail bullets only work at high velocities, I now see it de-bunked. ABOUT TIME !!! Bravo Bob!
2. That there is NO hydro static shock below 1500-1700 fps. I've blown that myth out of the water with my own testing. So-----
I'm testing a new bullet (for me) from NOE that I milled the length down by decking the mold. It has cut exactly thru the center of the radius of the half round lube grove, effectively making it a rebated short base boattail.
The bullet also has a very long ogive and a tiny meplat. It is ultra good at bucking the wind. This week, I will be doing side by side chrony test, and wind shooting with like weight bullet, from a mold from the same maker, with a flat base, and a wide meplat. So far, it (the Long ogive/rebated BT) is vastly superior.
As far as flat base bullets shooting better than GC bullets in our Air Guns, I, Tofazfou, Unrepentedsinner and a number of others, have found "Exactlyt the opposite!"
So far, it has been speculated by many here and on other forums, that it may very well be the case that the riffling marks are not being presented to the muzzle blast, making the bullet more stable, with the Gas check design. I can tell you this, I have tested several bullets, from otherwise identical molds, (same design, maker, weight) and the GC has so far, proven more accurate. By quite a bit!
I have posted many targets over on Talon AG over the years. A Lot of them are much tighter that the naysayer here has been brag'n about. LOL!!!
Mr.Bob, once again, Thank You Sir, for what you do! It is greatly appreciated by a LOT of us!!! ;)
Knife
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Interesting.
I thought this would lead into.
Reversable slugs, ie : where the cone becomes the tail.
If any one then made the nose conical.
My limited understanding as the boatail gets longer, (as in the Reversable slug) it evens out the BC curve along the mac numbers till obviously hits the mach 1 and beyond.
+ Makes the tail drag and skin drag not on the charts till after the mach 1 and beyond.
Rebated tale makes it act in a similar way.
(even the curve)