GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: rsterne on July 06, 2015, 11:14:17 PM
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If you have been following the thread on boattail development.... http://www.gatewaytoairguns.org/GTA/index.php?topic=93524.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=93524.0) .... you will have realized that when I figured out it was possible to duplicate the ballistics of a .300 AAC Blackout with a powerful enough PCP that I changed horses in mid stream.... First a little history.... Rizen 1 asked me if I could develop a bullet for a .308 that could start at about 1500 fps and stay supersonic for 200 yards.... I told him that it might be possible, but when I started looking into it, I quickly realized that was the worst possible velocity range to work in for wind drift.... In addition, that was 750 FPE, and I thought that was a stretch, even for an Extreme .308 on Helium.... I started working on optimizing a 200 gr. boattail for subsonic flight, instead of the 150 gr. for supersonic, and the result is shown below....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/308%20BBT%20Whiteout_zps2gwmcn6l.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/308%20BBT%20Whiteout_zps2gwmcn6l.jpg.html)
All of life is a compromise, and bullet design is no different.... The numbers told me that an 8R Ogive was the best choice, but when I drew it out, it just didn't look right, and I had to shorten that a bit so that the nose ended up under half the overall length.... A 2" radius gave me a 6.67R Ogive, and to keep the 200 gr. weight the bullet is 1.285" long.... The optimum twist is 8", perfect for 1050 fps, and it should still be stable at Mach 1, with a Stability Factor of 1.3.... The Ballistics Coefficient peaks at 1000 fps, so the optimum muzzle velocity over 200 yards is 1030 fps, and it should still be going 960 fps at 200 yards, and have less than 2 MOA of wind drift in a 10 mph wind at that range.... The JBM Ballistics Calculator says that over that velocity range the G1 BC is a staggering 0.80-0.855.... I think a fairer measure would be to use the G7 (boattail) standard, where the BC over that velocity range is 0.351-0.367....
I have sent the design to Veral at LBT to make sure that he can make a mold for it, and I've got my fingers crossed that his production methods don't dictate further changes.... We shall see.... BTW, the name "Whiteout"?.... obviously a play on the word Blackout, and suitable for a bullet from the Great White North, don't you think?.... ;D
Bob
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This makes me want a .308! Dangnabbit Bob!
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I love it! cant't wait to see how this all comes together.....
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If you have been following the thread on boattail development.... http://www.gatewaytoairguns.org/GTA/index.php?topic=93524.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=93524.0) .... you will have realized that when I figured out it was possible to duplicate the ballistics of a .300 AAC Blackout with a powerful enough PCP that I changed horses in mid stream.... First a little history.... Rizen 1 asked me if I could develop a bullet for a .308 that could start at about 1500 fps and stay supersonic for 200 yards.... I told him that it might be possible, but when I started looking into it, I quickly realized that was the worst possible velocity range to work in for wind drift.... In addition, that was 750 FPE, and I thought that was a stretch, even for an Extreme .308 on Helium.... I started working on optimizing a 200 gr. boattail for subsonic flight, instead of the 150 gr. for supersonic, and the result is shown below....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/308%20BBT%20Whiteout_zps2gwmcn6l.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/308%20BBT%20Whiteout_zps2gwmcn6l.jpg.html)
All of life is a compromise, and bullet design is no different.... The numbers told me that an 8R Ogive was the best choice, but when I drew it out, it just didn't look right, and I had to shorten that a bit so that the nose ended up under half the overall length.... A 2" radius gave me a 6.67R Ogive, and to keep the 200 gr. weight the bullet is 1.285" long.... The optimum twist is 8", perfect for 1050 fps, and it should still be stable at Mach 1, with a Stability Factor of 1.3.... The Ballistics Coefficient peaks at 1000 fps, so the optimum muzzle velocity over 200 yards is 1030 fps, and it should still be going 960 fps at 200 yards, and have less than 2 MOA of wind drift in a 10 mph wind at that range.... The JBM Ballistics Calculator says that over that velocity range the G1 BC is a staggering 0.80-0.855.... I think a fairer measure would be to use the G7 (boattail) standard, where the BC over that velocity range is 0.351-0.367....
I have sent the design to Veral at LBT to make sure that he can make a mold for it, and I've got my fingers crossed that his production methods don't dictate further changes.... We shall see.... BTW, the name "Whiteout"?.... obviously a play on the word Blackout, and suitable for a bullet from the Great White North, don't you think?.... ;D
Bob
Hello Bob
I just have to ask where did you come up with 1500 fps with a 200 gr bullet(will duplicate a 300 black out) you are over 400 fps faster than a sammy speck 300 black out.Not trying to be a wise guy but you are way off base on this one.
David
208 GR. HDY A-MAX
Starting Loads Maximum Loads
Manufacturer Powder Bullet Diam.
C.O.L.
Grs.
Vel. (ft/s)
Pressure
Grs.
Vel. (ft/s)
Pressure
IMR IMR 4198 .308" 2.260" 11.0 1,052 20,200 CUP
Hodgdon H4198 .308" 2.260" 11.2 1,042 18,800 CUP
IMR IMR 4227 .308" 2.260" 10.3 1,073 24,500 CUP
Winchester 296 .308" 2.260" 9.4 1,066 20,100 CUP
Hodgdon H110 .308" 2.260" 9.4 1,066 20,100 CUP
Hodgdon Lil'Gun .308" 2.260" 8.3 1,047 19,400 CUP
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gracious!!!!!
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jr460 your reading comprehension skills are about as lacking as your ability to complete a sentence without a grammatical error. Please read rsterne's post once more and realize he designed this bullet for subsonic use and at no point recommended/suggested it be used at 1500fps. I personally am tired of your post as I feel they add nothing to the thread. This is not because of your lack of understanding of ballistics because it looks as if you have plenty of knowledge in that area but your inability to read rsterne's post and not somehow see something that's not there.
I hope you somehow are able to read back over the post and understand what's actually being posted so that maybe we can learn from what you and rsterne obviously know more about than the rest of us. As of now all I do is read your post and wonder how you managed to put two totally separate facts posted by rsterne together to create something you can disagree with.
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You really need to read what I wrote.... I thought it was quite clear that I was originally working (at the request of Rizen 1) on a 150 gr. bullet at 1500 fps.... The drawing for that is in the other thread.... When I studied it in detail, I realized those velocities were NOT the place to be for wind drift and also realized that we could duplicate the ballistics of a .300 AAC Blackout (200 gr. @ 1050 fps) with a modern PCP.... That is when I suggested the change to a 200 gr. bullet, subsonic, to take advantage of the vastly decreased wind drift.... As stated above....
I started working on optimizing a 200 gr. boattail for subsonic flight, instead of the 150 gr. for supersonic, and the result is shown below....
The rest of that post, below the drawing, talks about the ballistics when subsonic, so I would have thought that clear.... You accused me of changing from a 150 gr. bullet to a 200 gr. because the original wouldn't do what I wanted.... In fact, my view was that the task itself (staying supersonic to 200 yard, starting at about 1500) was flawed, as I showed with this diagram, based the 150 gr. boattail and a flat based version of the same....
(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)
The results are quite clear, I bring your attention to the blue lines on the upper graph, which are the 200 yard case that currently interests us.... The solid line is for a boattail and the dotted line is for a flat based bullet.... Whether you agree with the superior ballistics of the boattail or not, you cannot ignore what happens to the wind drift when you stay subsonic instead of going supersonic.... With the 150 gr. flat base bullet, starting at 1500 fps, the wind drift over 200 yards is 6.2 MOA, but starting at 1050 fps it is only 4.4 MOA with a retained velocity of 893 fps.... With the 150 gr. boattail, starting at 1500 fps it is 5.7 MOA, but starting at 1050 fps it is only 3.1 MOA with a retained velocity of 943 fps.... By changing to a 200 gr. at 1030 fps, that is reduced to 1.9 MOA and the retained velocity is 961 fps.... Simply stated, starting at 1030 fps, the 200 gr. ends up faster at 200 yards, and drifts less, than either the 150 gr. boattail or flat base starting at 1050 fps....
I have never made any accuracy claims for this bullet, that would be ludicrous since it hasn't been built yet.... The entire development of the design is based on mathematics, using the formulas developed by Robert McCoy during his work at the Aberdeen Proving Grounds, and tested against real time data there using Chronographs and Doppler Radar to prove the validity of the calculations.... The stated accuracy of the calculations is 3% supersonic (over Mach 1.2), 6% subsonic (below Mach 0.8 ) and 11% transonic (between the two).... The Drag Calculator on the JBM Ballistics website at http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi (http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi) uses these calculations to convert dimensions of a bullet into the drag coefficient for velocities from Mach 0.5 to Mach 4.5, with extreme detail between Mach 0.8-1.0 where things change so rapidly.... That is great for airgunners (if we choose to take advantage of it), because that is precisely the velocity range we spend so much time in.... I also use the Drag Calculator by Geoffry Kolbe at http://www.geoffrey-kolbe.com/drag.htm (http://www.geoffrey-kolbe.com/drag.htm) which uses the same mathematical model, and presents it in graphic form, along with the ability to input a muzzle velocity and see the trajectory and wind drift at various ranges....
You may not put much stock in mathematical models, preferring to use your own vast experience to develop your own opinons about what works and what doesn't.... I don't have that experience to draw from, but that gives me the ability to question the status quo, and hopefully further the sport or airgunning by developing exciting new concepts.... I would ask that you be tolerant in your criticism of my approach, and simply let me do my thing.... You may be correct and I may be totally wasting my time.... but maybe.... just maybe.... the projectiles I am suggesting to be suitable for airgunning might end up offering superior ballistics to flat base bullets.... and in my experience, once something is proven to be more efficient.... people will work on the concept until any deficiencies are eliminated....
I'm not trying to reinvent the wheel.... just trying to make it roll easier.... ;D
Bob
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I have just received word from Veral at LBT that he can, indeed, make a mold for the Whiteout.... However it will require two new patterns, one for the 8* boattail (I knew that) and one for the main body.... The pattern charge will be $200, after which his normal mold prices will apply.... He says he can deliver the mold 2 weeks after payment is received....
Bob
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And I will step in ONE more time to gently remind everyone......let's be respectful with posts on open forum. If you have a disagreement that you feel requires you to attack a post, do it in private message, email, phone call or face to face. But NOT on open forum. If I have to step into another of these type of exchanges, it will get ugly. So, let's keep this informative and enjoyable.
ONLY warning!!!!
We are running a special on free vacations. Will entail a 14 day retreat to begin with.
As a refresher, see Rules #2 and #13.
http://www.gatewaytoairguns.org/GTA/index.php?topic=919.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=919.0)
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I have just received word from Veral at LBT that he can, indeed, make a mold for the Whiteout.... However it will require two new patterns, one for the 8* boattail (I knew that) and one for the main body.... The pattern charge will be $200, after which his normal mold prices will apply.... He says he can deliver the mold 2 weeks after payment is received....
Bob
Being a bore rider I'm assuming he'll need the exact bore dimensions? Should we send him a slug from the barrel after pushing it through the bore?
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A slug would be a good idea.... My concept is for the bore-rider to be 0.001" over the land diameter so that it touches them firmly (0.0005" per side) to guide it perfectly straight.... The driving band could likewise be about 0.001" over, maximum, I don't think it will matter if it is that much over since there is only one to provide friction (and a gas seal).... Veral could easily have a different opinion, and if he is working from a slug, and does his own thing, who am I to argue?.... I would not want to see the bore-riding portion smaller than the land diameter....
The chamber would be shorter than normal, with the leade closer to the breech, because the driving band is further back, of course.... 50% engagement on the lands should be about right, IMO....
Bob
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Hi Bob,
It is always fascinating to read your theory on new bullet designs...and the ensuing controversy it creates! ;) I have a question for you to think about...What do you think of the idea of designing airgun bullets to be swaged in a Corbin press, and then instead of having drive bands on the bullets to minimize bore drag, simply run them through a Corbin HCT-2 Bullet Knurling Tool to create a lower bore drag. Do you think a knurled bullet flying sub-sonic would work as well as a bullet with smooth sides? Or do you think it would be more affected by the wind?
I ask because it seems to me that although swaged bullets are far superior to cast bullets due to their greater uniformity in weight and consistency, they cannot have driving bands on them or they would not release from the die. This makes swaged bullets less than ideal for most airguns because there is not enough velocity to overcome the energy robbing drag of a swaged bullet. Which leaves the only obvious alternative which is cast bullets with drive bands...and the inherent drawbacks of trying to control all the variables involved in the casting process.
I just keep wondering if a swaged bullet with a rebated boattail, that is made to be .001 over bore size to seal adequately, and that has been knurled and propelled at sub-sonic speeds would shoot accurately, or if it would be adversely affected by the wind because of the knurling. I also wonder if knurling the bullets would reduce the bore drag to a level acceptable to airgun velocities, or if the drag would still be prohibitive. I wish I had the money to buy a Corbin press, and dies, and a knurling machine, and you to design some bullets for it so I could test the knurled airgun bullet idea.
Just interested in what your thoughts on the subject might be...
Best regards, Chuckster
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Chuckster, one of our own GTA members, Bill G has exactly the setup you are talking about.... He can produce swaged bullets to just over the land diameter and then raise knurled driving bands.... or a single band.... to a precise, and adjustable, diameter.... I have seen some of his prototypes, and feel that he should be able to make this bullet with no problem.... He makes his own dies, and therefore once he has a nose and a boattail die, he can vary the length and therefore the weight.... I sent him an email about this project, but haven't heard back yet....
Bob
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Chuckster, one of our own GTA members, Bill G has exactly the setup you are talking about.... He can produce swaged bullets to just over the land diameter and then raise knurled driving bands.... or a single band.... to a precise, and adjustable, diameter.... I have seen some of his prototypes, and feel that he should be able to make this bullet with no problem.... He makes his own dies, and therefore once he has a nose and a boattail die, he can vary the length and therefore the weight.... I sent him an email about this project, but haven't heard back yet....
Bob
Why not use a technique that's available for all of us, cast bullets, sized to land diameter or slightly over and then knurled? Or is there something I'm missing in this thought?
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jr460 your reading comprehension skills are about as lacking as your ability to complete a sentence without a grammatical error. Please read rsterne's post once more and realize he designed this bullet for subsonic use and at no point recommended/suggested it be used at 1500fps. I personally am tired of your post as I feel they add nothing to the thread. This is not because of your lack of understanding of ballistics because it looks as if you have plenty of knowledge in that area but your inability to read rsterne's post and not somehow see something that's not there.
I hope you somehow are able to read back over the post and understand what's actually being posted so that maybe we can learn from what you and rsterne obviously know more about than the rest of us. As of now all I do is read your post and wonder how you managed to put two totally separate facts posted by rsterne together to create something you can disagree with.
Lets connect the dots .
1 There is no drawing or mention of the same bullet in a flat base version being used to prove the rebated boat tail will do what is being calmed.
2 what rate of twist will be used
3 specifications of barrel; chamber speck, lead speck,lands and groves speck, how many,right hand twist,left hand twist,bore diameter,rifling twist rate, rifling cut,micro grove ,hammer forged or button, air gauged{ MATCH} or standard, LENGTH OF BARREL.
4 at what rpm was this data generated with.
5 How can you have a mould made with out having this data?
6 Most important what lead is this mould being cut for,lead tin,lead tin antimony,lead antimony or pure lead ?
With out these answered the mould and drawing is just a wast of time and money and just a theory no facts will ever be proven.
Yes I did read THE POST it is just a rambling with no facts to show how the answer came to be.
David
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I have just received word from Veral at LBT that he can, indeed, make a mold for the Whiteout.... However it will require two new patterns, one for the 8* boattail (I knew that) and one for the main body.... The pattern charge will be $200, after which his normal mold prices will apply.... He says he can deliver the mold 2 weeks after payment is received....
Bob
This is great news! Expensive, but great!! I'll have to email Veral for specifics...
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Why not use a technique that's available for all of us, cast bullets, sized to land diameter or slightly over and then knurled? Or is there something I'm missing in this thought?
Veral at LBT is able to make molds for cast bullets... Bob is talking about a member that does swagged bullets- just another option over casting is all.
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Why not use a technique that's available for all of us, cast bullets, sized to land diameter or slightly over and then knurled? Or is there something I'm missing in this thought?
Interesting thought, I do not know enough about the effects of knurling on a bullet to comment on how well it would work.
It does seem to me that a mold would be much easier to make if there were no driving bands.
One could very easily make a jig to go between center on a lathe to knurl a few bullets for testing.
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rkr, I think I may have misinterpreted your question... :(
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rkr, you are completely correct, the bullet could be cast, or swaged, to just over land diameter and then the knurling process applied to form the drive band(s).... I can't see many guys taking the time or money to set up to knurl, I was merely pointing out that one of our members, Bill G, already has that setup for his swaged bullets.... Perhaps he is on holidays, I can't reach him at the moment....
Most home casters use anywhere from pure lead to 2.5% tin for airgun bullets, to keep the lead as soft as possible.... I personally use 1%, but I have never used a single drive band in a 7" twist, so it is possible that a harder alloy may be needed to stand the higher shearing force from the rifling.... These are the kinds of things that come during the development stage, once the original concept has been proven....
I am reminded of a story about an aerodymanicist who found, in an obscure Russian research journal, the mathematics whereby an object could be rendered nearly invisible to Radar.... He went to his boss, and told him about it, but finished with the statement "the only problem is, the shape is so weird, it can't be made to fly".... His boss asked him how large the radar signature was.... was it the size of an eagle?.... He replied that no, it was about the size of an eagle's eyeball.... His boss simply stated "if it's as good as you say it is, I'll MAKE it fly".... The boss was Kelly Johnson of Lockheed's Skunk Works, and the plane that resulted as the F-117 Stealth Fighter.... That is an example of what can be done through research and refusing to give in to those who say "it cant be done"....
Bob
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Why not use a technique that's available for all of us, cast bullets, sized to land diameter or slightly over and then knurled? Or is there something I'm missing in this thought?
Interesting thought, I do not know enough about the effects of knurling on a bullet to comment on how well it would work.
It does seem to me that a mold would be much easier to make if there were no driving bands.
One could very easily make a jig to go between center on a lathe to knurl a few bullets for testing.
Or we could just take a standard mold, size the bullet to land diameter of an individual rifle and then knurl a rear band to seal it to the bore. Since we have lots of variance in land diameters this would allow easy tailoring or bore riding (or is it land riding?) bullets for each barrel.
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Exactly the technique I am suggesting.... and that Bill G is set up to use for the swaged bullets he sells.... He would, however, have to make the dies for the nose and boattail, but would have the ability to change the length and weight, of course.... The advantage to knurling a band, or bands, is that you can tailor the number and width of them to prevent stripping in the fast rifling when being kicked by 4500 psi of Helium....
"Land riding" is I guess the correct term for bore-riding, but that is commonly used, and I suggest that it should JUST have the slightest rifling marks on the body (about 0.0005" or so) when pushed through.... The "correct" body size you should be able to push through by hand without difficulty, and yet leave rifling marks on the body....
Bob
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jr460 your reading comprehension skills are about as lacking as your ability to complete a sentence without a grammatical error. Please read rsterne's post once more and realize he designed this bullet for subsonic use and at no point recommended/suggested it be used at 1500fps. I personally am tired of your post as I feel they add nothing to the thread. This is not because of your lack of understanding of ballistics because it looks as if you have plenty of knowledge in that area but your inability to read rsterne's post and not somehow see something that's not there.
I hope you somehow are able to read back over the post and understand what's actually being posted so that maybe we can learn from what you and rsterne obviously know more about than the rest of us. As of now all I do is read your post and wonder how you managed to put two totally separate facts posted by rsterne together to create something you can disagree with.
Lets connect the dots .
1 There is no drawing or mention of the same bullet in a flat base version being used to prove the rebated boat tail will do what is being calmed.
2 what rate of twist will be used
3 specifications of barrel; chamber speck, lead speck,lands and groves speck, how many,right hand twist,left hand twist,bore diameter,rifling twist rate, rifling cut,micro grove ,hammer forged or button, air gauged{ MATCH} or standard, LENGTH OF BARREL.
4 at what rpm was this data generated with.
5 How can you have a mould made with out having this data?
6 Most important what lead is this mould being cut for,lead tin,lead tin antimony,lead antimony or pure lead ?
With out these answered the mould and drawing is just a wast of time and money and just a theory no facts will ever be proven.
Yes I did read THE POST it is just a rambling with no facts to show how the answer came to be.
David
once again I can not help but wonder about your tone...
Why not try to explain with your knowledge what would have the best chance of success and the problem still faced at that point and then try to come up with solutions...
It would be much more productive , in my mind at least... ;)
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I agree! Useful data even if it doesn't work is better than no data. I have no idea if the boat tails work or not but the data can be used to either make a working prototype or move in a new direction. Bob keep up the hard work your ideas and math to back them up is fascinating and I for one would love to test it out in a real world environment.
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The comment has been made that the models I have been using are for conventional boattails, and not rebated boattails, and that is completely correct.... However, Lapua and Corbin have been using rebated boattails very successfully for decades.... Considering that the math models show that the drag for conventional boattails have less than half the drag of a flat base bullet when subsonic, even if the rebated boattail is 25% more drag than a conventional one (and the BC of the Lapua bullets don't show that), the drag of a rebated boattail would still be a third less than a flat base.... I know from testing (mine and others) that the BC of my existing Bob' Boattails are significantly better than the same weight bullet when subsonic, so I know I'm on the right track.... So far, because of the relatively short bullets I have converted to boattails, I had not had the opportunity to try a "normally proportioned" boattail, which should work much like the Lapua and Corbin bullets when subsonic.... Until somebody bites the bullet (pun intended) and tries it, we won't have the hard data....Until someone shows me hard data that compares a boattail to a flat base bullet of the same nose shape, weight, and caliber (ie the only difference is the boattail) that show the flat base has less drag when subsonic.... I will proceed under the assumption that all the math models, and those that build boattails for subsonic use (eg. Lapua) know what they are doing....
Perhaps it would ease the mind of the person objecting to my efforts if he realized that I don't have any commercial plans, and never have.... All my Bob's Boattail designs are in the public domain, and free for any individual to use, make molds of, etc.etc.... If it is taken up commercially, my only expectation is that the manufacturer ask my permission, and once given, credit me with the design.... I have never asked, or received, any compensation for my design efforts, they are done solely for the purpose of furthering the sport of airgunning....
Bob
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In about 45 days, we are all going to know how well these bullets work, what (if any) changes need to be made, and what steps to take next to fulfill Bobs goal of making better airgun bullets.........Because C.Alls, Brent Hoag, and myself are going to be building and testing a gun that will shoot these... We are also coming out with a t shirt that says.... :o
Words don't put holes
in the 10 ring....
Bullets do!
SHUT UP AND SHOOT!
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Really enjoy reading this thread,wondering why nobody has mentioned johninthecamper? He has two Corbin presses and does very fine work ;)
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It is a scientifically proven fact that most of the drag is on the nose of a supersonic bullet, while most of the drag on a sub-sonic bullet is on the base. So if we can reduce the drag on the base of our sub-sonic airgun projectiles, it stands to reason that they will fly farther, more accurately, and with less wind drift.
A good read by Dave Corbin is titled: "A comparison of the Rebated Boattail Bullet Design with Flat base and conventional Boattail Bases..." Here are a couple excerpts below:
'The boattail reduces base drag at all velocities, but has the greatest percentage of effect when the bullet is flying at sub-sonic velocity.'
'A rebated Boattail base is an advanced form of the conventional boattail base, which adds a 90-degree step or shoulder between the angled base and the shank of the bullet, to act as a "spoiler" and break up muzzle gas flow, provide a better seal in the bore, and cause a more rapid and concentric transition from the muzzle to free flight than a boattail angle emerging from the barrel.'
'A conventional boattail bullet tends to focus the ...muzzle gas into a ball in front of the emerging bullet, just like a water hose nozzle focused the water into a stream. Then the conventional boattail bullet must pass through the turbulent ball of gas, and is buffeted about more than the rebated boattail, which passes through clear space as a ring of gas expands outward from the rebate step on the back of the bullet.'
'Hyper-velocity gas escaping around the bullet base at the muzzle will impinge upon the rebate shoulder, which acts as a "spoiler" and deflects the gas from a laminar pattern. It causes a ring of gas to be blown off in a turbulent forward rolling expansion, but leaves a clear space directly in front to the bullet. This eliminates the buffeting of the bullet by its own muzzle gas, a source of up to 15% of the total dispersion factor.'
I understand this to mean the bullet flies through the doughnut hole of quiet air instead of flying through a ball of violently turbulent gas...and if a rebated boattail has the potential to give me 15% better accuracy than a flat base bullet, then that's the one I want to be shooting. The other big advantage of the rebated boattail bullet base design, especially when it comes to cast bullets, is that the bullets consistently leave the barrel the same way each time. (consistency is accuracy). Whereas the Achilles heel of all cast bullets is the variations of the flat bases from bullet to bullet. No matter how perfect a bullet mold, if the base of the bullets it produces are all slightly different due to the casting process, the bullets will never be as accurate as swaged bullets produced in a press. This is why it is so important for cast bullets to have a rebated boattail, regardless of whether they are being produced to fly super or sub sonic.
It is just entirely amazing to me that Bob Sterne hit upon this idea for his cast bullet designs. I think Bob is laying the foundation for some astounding accuracy out of airguns with his bullets, versus just shooting drag stabilized diabolo shaped skirted pellets or cast flat base bullets. Keep up the great work you are doing Mr. Sterne...can't wait to see what you are producing a year or two down the road...
(For a technical study of the RBT versus the regular BT base, log onto www.Swage.com (http://www.Swage.com) and download the technical paper on rebated boattail bullets by Dr. Fitzgerald).
Fond regards, Chuckster
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Chuckster.... thank you for posting the info from the Corbin website.... It was one of the main influences in my decision to use the rebated boattail design, as I stated a couple of years ago when I started down this path.... Your independent summary of the reasons is concise, accurate, and exactly on point.... I simply saw the application of a known and proven technology to airguns and have given myself a goal to make it happen....
Bob
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I have had a couple of guys question the BC I quoted from the JBM calculator, and rightly so, as it was higher than the Lapua .308 Subsonic, and that makes no sense.... I guess I should have been clearer that I thought the number was exaggerated, just as it would be for a cast flat base bullet.... This is because the calculators make no allowance for grooves or ridges, drive bands or rebates.... However, the basics still apply.... boattails have a superior BC to a flat base bullet of the same caliber, weight, and nose shape (ie where the boattail is the only difference), particularly in the subsonic velocity range.... Corbin state that the BC of a rebated boattail is 10-15% less supersonic and 40% less subsonic, and I think that is an extremely useful figure to compare to an otherwise similar flat base bullet...
Bob
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Hi Bob often I am not clear as I thought I was...
In fact I think at times because of some probs I am not clear at all...
Heck in my above post about twist I meant a rebated Boat tail and did not make that clear... (in my mind I had )
About the B.C. well it could be very well the method of measurement, because well I just noticed that Midway states it very close to your figures... @
200g subsonic Bt
SD = .301
BC = .382
so of course it will vary some also as you have repeated for the novices many times... and until measured it is obviously just as good of an estimate as can be came up with...
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oops the twist comment was the other thread (I think... ???)
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I just found this Lapua used Doppler for it and so their BC figure probably is not a single speed B.C. ...
http://www.lapua.com/en/customer-center/lapua-ballistics/lapua-ballistics-software.html (http://www.lapua.com/en/customer-center/lapua-ballistics/lapua-ballistics-software.html)
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Lapua is the only company, to my knowledge, that uses Doppler Radar data to develop the exact Drag Coefficient Curve for each bullet.... Since no bullet precisely matches the G1 or G7 model, this is the only method to precisely calculate the trajectory and wind drift as the velocity decays as the bullet travels downrange.... Instead of using the antiquated idea of BC, and quoting a BC that is really only correct at the velocity it was measured, they do it right.... and use the direct measurements of Cd in their calculations.... My hat's off to them for bringing ballistics into the 21st century....
Bob
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Bob and I finally made contact. Sorry gents, I've been off the grid for a bit for some family time. This project is extremely interesting but the time line is too short for me to be effective. IMO, your approach with the cast bullet is the fastest path to your goals. Now what I could do is knurl some bullets that have been cast and or sized to the lan diameter. The single driving band at the read of the bearing surface, this will likely make sizing difficult. I would have to make a knurl for a single band but have one that would do multiple bands ready for use. If the effort of making such a projectile was taken, I would gladly apply the knurl simply to see it done and tested. No charge, simply shipping.
That drawing is impressive and I think that a better name for it would be "Javelin". ;D Over an inch and a quarter, that thing looks like a miniature scale of the Paris gun shell!! :o.
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I checked the design with my new McDrag program, and I'm satisfied that the basic design of the bullet is optimized for the job.... Since it matches the G7 Drag Model very closely, I ran it through the JBM Trajectory and Drift Calculator, using several muzzle velocities from Mach 0.8-0.975 to see what happens to the Drop and Drift, and what the Retained Velocity is at 200 yards and 400 yards.... Here are the results....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20BBT%20Drift%20and%20Drop%20200_zpsaq4kzmpm.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20BBT%20Drift%20and%20Drop%20200_zpsaq4kzmpm.jpg.html)
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/308%20BBT%20Drift%20and%20Drop%20400_zpsktqdqit3.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/308%20BBT%20Drift%20and%20Drop%20400_zpsktqdqit3.jpg.html)
You can see in both cases that after you go above 1000 fps muzzle velocity, the Wind Drift starts to increase, and the Retained Velocity starts to decrease more rapidly.... The Drop (rifle sighted at 100 yards), however, decreases more rapidly below 1000 fps than above.... Therefore, it makes little sense to shoot below 1000 fps, you don't get less Drift, but you do get more Drop.... Going above that velocity your Drop doesn't change as much, but the Drift starts to increase.... You could undoubtedly push it a little bit, but I wouldn't go above 1050 fps.... Of course shooting is the only way to be sure.... these calculations only let you know what to expect.... To put some hard numbers on the calculated Wind Drift at 200 yards (10 mph crosswind) at various muzzle velocities....
900 fps - 2.2"
1005 fps - 2.4"
1033 fps - 2.8"
1061 fps - 3.2"
1089 fps - 3.9"
1116 fps - 5.5"
Bob
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Wow Mr. Sterne, it will be interesting to see how close your calculations on paper compare once you actually make some of these bullets and shoot them at 200 and 400 yards...You seem to be as enamored with the mathematical theory as you are with actually helping air gunners find the "magic bullet" for each caliber. It would be fun to have you in the same room with Mr. Dave Corbin for a brain storming session...only God and Werner Von Braun would be able to interpret the results...but I bet some good shooting bullets would be created. I think you are amazing Bob. 8)
Fond regards, Chuckster
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To tell you the truth, I would be satisfied even if the drag was twice that calculated.... It would still be a huge improvement over a flat base bullet at all velocities up to Mach 0.95....
Bob