GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: Tofazfou on August 17, 2015, 02:49:23 AM
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Bob,
I recently bought a new Caldweld Chrony and i love it to say the least. I get no errors any longer and the numbers seems to be spot on compared to when my old chrony was NEW.......lol! Plus, with the new numbers, my Mildots are lining up when i use a my ballistics program.
(http://i39.photobucket.com/albums/e184/tofazfou/Caldwell%20Ballistics%20Chrony%203.jpg) (http://s39.photobucket.com/user/tofazfou/media/Caldwell%20Ballistics%20Chrony%203.jpg.html)
(http://i39.photobucket.com/albums/e184/tofazfou/Caldwell%20Ballistics%20Chrony%204.jpg) (http://s39.photobucket.com/user/tofazfou/media/Caldwell%20Ballistics%20Chrony%204.jpg.html)
Anyway, down to business. So today, i go shooting and I bring all the gear with me with the intent to get some downrange numbers of both your BBT 7mm bullet and the LBT 130 gr 7mm Flat Point Bullet (HPs actually). I set up the chrony at 3' from the muzzle and then 65 yards downrange. A 62 yard difference. I also shot my gun regulated and weight sorted the bullets from 98.0-98.9 grs with most of them being in the mid to slightly upper range of that weight. With the bullets, i shot 3 of each to get the average at both ranges of near and far. I then repeated the process to make sure the overall averages where, well, AVERAGE...... ;D
The number are as follows:
7mm BBT @ 98.5 grs
NEAR (3 yards)
983 fps
976
977
Avg is 978 fps
7mm BBT @ 98.5 grs
FAR (65 yards)
938 fps
935
938
Avg is 937 fps
7mm LBT @ 130 grs
NEAR (3 yards)
869 fps
873
875
Avg is 872 fps
7mm LBT @ 130 grs
FAR (65 yards)
843 fps
846
853
Avg is 847 fps
So entering the near and far numbers into JBM's ballistics program gave me these BC numbers:
7mm BBT = .203
7mm LBT = .249
I was at 4000 ft in elevation and the humidity was roughly 35%.
These numbers seem MORE LIKE IT. How does the BC of your BBT compared to the slugs sectional density? What do you think?
Regards,
Cedric
PS - I recasted some newer BBTs for this outting which contain a little bit of Antimony instead of being pure lead. Im always trying new things to get different or better results than the last time i might shoot or try a slug. I can say that while i did not do any EXTREME RANGE shooting, the groups were much better at 65 yards and 100 yards. On todays outing, the BBTs outperformed the LBT's and that's sayin something.
(http://i39.photobucket.com/albums/e184/tofazfou/GROUPS-08-16-15%207.jpg) (http://s39.photobucket.com/user/tofazfou/media/GROUPS-08-16-15%207.jpg.html)
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Great data, Cedric.... Here is how the bullets compare.... The Form Factor is a measure of how "slippery" the bullet is, and the lower the better.... If it is over 1.00 it means the bullet has more drag than the G1 Standard Projectile....
98.5 gr. BBT:
Your BC = 0.203 @ average velocity of 958 fps
SD = (98.5/7000/0.284/0.284) = 0.175
FF = 0.175/0.203 = 0.859
130 gr. LBT:
Your BC = 0.249 @ average velocity of 860 fps
SD = (130/7000/0.284/0.284) = 0.230
FF = 0.230/0.249 = 0.923
As you realize, the BC you calculated is only valid at that velocity, how the BCs compare at some different velocity can't really be determined without multiple data points to discover the shape of the Drag Curve for each bullet.... However, we CAN calculate the Drag Coefficient for each data point by multiplying the Cd of the G1 Projectile by the FF determined, and then we can plot those to see how they compare to the standard G1 and G7 Drag Models.... Here is that chart....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/Cd%20284%20and%20Models_zps0bye6j8a.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/Cd%20284%20and%20Models_zps0bye6j8a.jpg.html)
The distance the data point is below the G1 curve is an indication of how good the bullet is, taking Sectional Density (bullet weight) out of the equation.... The further it is below the G1 line the better.... Since the LBT bullet is pretty close in shape to the G1 Model, we might expect it to have a drag curve that is quite similar to the model.... while the BBT, having a boattail, might have a drag curve somewhere in between the G1 and G7 models, because it does have a boattail, but not an extreme one.... What that means is, that if you could push the LBT 100 fps faster, it should have a lower BC than the BBT.... and the same thing if you slowed down the BBT 100 fps.... However, that isn't going to happen with the gun you have, you would normally take advantage of the velocity it can produce with either bullet.... and the retained velocity and energy of the two bullets are really all that matters....
All in all, I am quite pleased that the boattail is doing what it is supposed to do.... reducing the drag, and therefore the wind drift, at any given velocity.... and it looks like you are getting great accuracy as well....
Bob
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Great data, Cedric.... Here is how the bullets compare.... The Form Factor is a measure of how "slippery" the bullet is, and the lower the better.... If it is over 1.00 it means the bullet has more drag than the G1 Standard Projectile....
98.5 gr. BBT:
Your BC = 0.203 @ average velocity of 958 fps
SD = (98.5/7000/0.284/0.284) = 0.175
FF = 0.175/0.203 = 0.859
130 gr. LBT:
Your BC = 0.249 @ average velocity of 860 fps
SD = (130/7000/0.284/0.284) = 0.230
FF = 0.230/0.249 = 0.923
As you realize, the BC you calculated is only valid at that velocity, how the BCs compare at some different velocity can't really be determined without multiple data points to discover the shape of the Drag Curve for each bullet.... However, we CAN calculate the Drag Coefficient for each data point by multiplying the Cd of the G1 Projectile by the FF determined, and then we can plot those to see how they compare to the standard G1 and G7 Drag Models.... Here is that chart....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/Cd%20284%20and%20Models_zps0bye6j8a.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/Cd%20284%20and%20Models_zps0bye6j8a.jpg.html)
The distance the data point is below the G1 curve is an indication of how good the bullet is, taking Sectional Density (bullet weight) out of the equation.... The further it is below the G1 line the better.... Since the LBT bullet is pretty close in shape to the G1 Model, we might expect it to have a drag curve that is quite similar to the model.... while the BBT, having a boattail, might have a drag curve somewhere in between the G1 and G7 models, because it does have a boattail, but not an extreme one.... What that means is, that if you could push the LBT 100 fps faster, it should have a lower BC than the BBT.... and the same thing if you slowed down the BBT 100 fps.... However, that isn't going to happen with the gun you have, you would normally take advantage of the velocity it can produce with either bullet.... and the retained velocity and energy of the two bullets are really all that matters....
All in all, I am quite pleased that the boattail is doing what it is supposed to do.... reducing the drag, and therefore the wind drift, at any given velocity.... and it looks like you are getting great accuracy as well....
Bob
Great info Bob. I follow 100% and agree with you. Hmmmmm, now i really wonder how the other 7mm bullet mold (110 gr) you designed will do? LOL! I need to start a Christmas list ;D
I like the speed and BC of your 98.5 gr BBT and its doing well out to 430 yards. Shoots much flatter than the LBT at that range also. I have another older LBT mold in 7mm thats was HP'd by Erik but the bullets are too small for my current barrel. Eric is going to open the bearing grooves up .002" and the bullets weight about 120 gr in HP config. So its gonna be interesting to see what they can also do.
Stay tuned for future info on your 7mm BBTs and thank you for what you do.........................
Tofazfou
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Cedric, all you guys shooting at long range are playing a balancing act between trajectory and wind drift.... If you look at that graph, and think of wind drift on the vertical axis and trajectory on the horizontal axis, you will have a pretty good grasp of the problem.... Low Cd means low wind drift, but high velocity means flatter trajectory.... Since the velocity is predominately a factor of bullet weight, and the faster you go, the closer you are to Mach 1 where the drag takes a huge increase, you are trading off one for the other....
A more pronounced boattail, in a slightly longer and heavier bullet might the best compromise.... Look at the 5 designs in this thread for a .257 cal....
http://www.gatewaytoairguns.org/GTA/index.php?topic=94920.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=94920.0)
In 7mm, those designs would be 88, 104, 120, 135, & 150 gr.... Looking at the 104 gr. and 120 gr., they would have an SD of 0.184 and 0.213.... You are shooting the 98 gr., with an SD of 0.175 at 980 fps (210 FPE) and the 130 gr., with an SD of 0.230 at 870 fps (220 FPE), so we might reasonably expect the 104 gr. to shoot about 212 FPE (~960 fps) and the 120 gr. about 217 FPE (~900 fps).... The theoretical twist required is about 12" for the 104 gr. and 11" for the 120 gr.... I have a gut feeling that the 104 gr. would be OK in your 14" twist, but the 120 gr. would be marginal or unstable, so let's look at the 104 gr. more closely....
In .284 cal, at 2.5 calibers long, it would have a LOA of 0.71".... The nose radius, if scaled up, would be 0.88", but Veral doesn't have one of those, but he does have a 0.8" and a 1.0" Ogive pattern.... Either one of those will provide a much sleeker, lower drag nose than the 98 gr. BBT you have now.... The big change, however, is the longer, shallower 8 degree boattail.... The combination of those should reduce the drag to a significant degree.... Using the same FF as the 98 gr. of 0.859, the BC should increase to 0.214, but at the slightly lower muzzle velocity that should increase a bit.... With the changes to the nose and tail, if should go even higher....
If you're interested, I think maybe we should start with the 104 gr. 2.5 caliber long boattail, and tweak it a bit to meet your needs.... Veral now has the new 8 degree boattail pattern done, it is used on the Whiteout.... He has the 0.8 and 1.0" Ogive patterns, either of which could be made to work.... If you don't like the bore-riding nose, there are two alternatives, one with the back of the Ogive at groove diameter and the area between it and the rear driving band 0.002" smaller than the lands.... The other would have conventional drive bands and grooves.... Veral can make any of them without making new patterns.... If you let me know what you want, including the weight, I'll draw up my suggestion.... I would suggest staying between 100-115 gr. at the outside....
EDIT: I had some time tonight, so came up with a suggestion for you to look at....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/284%20cal%20LBT%20105%20gr_zpszqwxgsoe.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/284%20cal%20LBT%20105%20gr_zpszqwxgsoe.jpg.html)
The dotted line is where the Ogive would start to engage the rifling, so the total length engaged by the rifling would be well over 1 caliber.... The center section is about 0.276", just clear of the lands.... The stability is 1.3 at 1000 fps in a 13" twist, but I am quite confident it would work fine in your 14" twist.... I'm not sure how much longer you could go, however, this might be pushing it.... Here is the existing bullet for comparison....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/284%20cal%20Airgun%20Boattail%2094%20gr_zpsxscq5rcp.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/284%20cal%20Airgun%20Boattail%2094%20gr_zpsxscq5rcp.jpg.html)
As you can see, the new design is quite a bit sleeker, and should have a significantly better BC, even though it is not a lot heavier....
Bob
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Cedric, all you guys shooting at long range are playing a balancing act between trajectory and wind drift.... If you look at that graph, and think of wind drift on the vertical axis and trajectory on the horizontal axis, you will have a pretty good grasp of the problem.... Low Cd means low wind drift, but high velocity means flatter trajectory.... Since the velocity is predominately a factor of bullet weight, and the faster you go, the closer you are to Mach 1 where the drag takes a huge increase, you are trading off one for the other....
A more pronounced boattail, in a slightly longer and heavier bullet might the best compromise.... Look at the 5 designs in this thread for a .257 cal....
http://www.gatewaytoairguns.org/GTA/index.php?topic=94920.0 (http://www.gatewaytoairguns.org/GTA/index.php?topic=94920.0)
In 7mm, those designs would be 88, 104, 120, 135, & 150 gr.... Looking at the 104 gr. and 120 gr., they would have an SD of 0.184 and 0.213.... You are shooting the 98 gr., with an SD of 0.175 at 980 fps (210 FPE) and the 130 gr., with an SD of 0.230 at 870 fps (220 FPE), so we might reasonably expect the 104 gr. to shoot about 212 FPE (~960 fps) and the 120 gr. about 217 FPE (~900 fps).... The theoretical twist required is about 12" for the 104 gr. and 11" for the 120 gr.... I have a gut feeling that the 104 gr. would be OK in your 14" twist, but the 120 gr. would be marginal or unstable, so let's look at the 104 gr. more closely....
In .284 cal, at 2.5 calibers long, it would have a LOA of 0.71".... The nose radius, if scaled up, would be 0.88", but Veral doesn't have one of those, but he does have a 0.8" and a 1.0" Ogive pattern.... Either one of those will provide a much sleeker, lower drag nose than the 98 gr. BBT you have now.... The big change, however, is the longer, shallower 8 degree boattail.... The combination of those should reduce the drag to a significant degree.... Using the same FF as the 98 gr. of 0.859, the BC should increase to 0.214, but at the slightly lower muzzle velocity that should increase a bit.... With the changes to the nose and tail, if should go even higher....
If you're interested, I think maybe we should start with the 104 gr. 2.5 caliber long boattail, and tweak it a bit to meet your needs.... Veral now has the new 8 degree boattail pattern done, it is used on the Whiteout.... He has the 0.8 and 1.0" Ogive patterns, either of which could be made to work.... If you don't like the bore-riding nose, there are two alternatives, one with the back of the Ogive at groove diameter and the area between it and the rear driving band 0.002" smaller than the lands.... The other would have conventional drive bands and grooves.... Veral can make any of them without making new patterns.... If you let me know what you want, including the weight, I'll draw up my suggestion.... I would suggest staying between 100-115 gr. at the outside....
EDIT: I had some time tonight, so came up with a suggestion for you to look at....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/284%20cal%20LBT%20105%20gr_zpszqwxgsoe.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/284%20cal%20LBT%20105%20gr_zpszqwxgsoe.jpg.html)
The dotted line is where the Ogive would start to engage the rifling, so the total length engaged by the rifling would be well over 1 caliber.... The center section is about 0.276", just clear of the lands.... The stability is 1.3 at 1000 fps in a 13" twist, but I am quite confident it would work fine in your 14" twist.... I'm not sure how much longer you could go, however, this might be pushing it.... Here is the existing bullet for comparison....
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/284%20cal%20Airgun%20Boattail%2094%20gr_zpsxscq5rcp.jpg) (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/284%20cal%20Airgun%20Boattail%2094%20gr_zpsxscq5rcp.jpg.html)
As you can see, the new design is quite a bit sleeker, and should have a significantly better BC, even though it is not a lot heavier....
Bob
Good info Bob. For me, the sleeker the better. In this caliber, i want the highest BC obtainable while keeping the speeds optimal. I want BC and SPEED for my own satisfaction. And i don't mean super speeds, i just want whats optimal for the flattest trajectory possible while maintaining efficiency.
Also Bob, with your twist rate calculator and the info that you give, is it SPECIFICALLY for BT bullets or is it the same info (pertaining to stability with a given twist rate) with Gas Check and Flat Base bullets. I ask because i shoot much longer and heavier bullets than your BBT's and i have no stability issues from 0 range out to 450 yards (so far)
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Boattails make the bullet act longer, both in terms of reduced drag and increased twist required, so at least in theory they need faster twists than the equivalent length flat base bullet.... The twist calculators have a correction for boattails, but it seems from the evidence we are slowly accumulating that the rebated boattails I use don't need the full correction factor that is built into the twist calculator.... A flat base bullet with the same nose and overall length, would in theory only need an 18" twist for the same stability as the boattail in a 13" twist.... My guess is that 15" is probably OK for the boattail, maybe even 16"....
The theoretical drag curve for the 105 gr. boattail above is virtually flat, out to Mach 0.85 (949 fps), and only increases about 20% at Mach 0.90 (1005 fps).... so it should be running right in the sweet spot in your gun.... The calculated Cd is 5% better than the original 94 gr. BBT at Mach 0.85 and below, and 28% better at Mach 0.90.... The blunter nose and tail of the original caused the rapid increase in drag above 950 fps (~50% increase by the time it reached 1000 fps), and you were already starting to get into that speed range.... With the 12% higher SD, that should, in theory, mean an increase in BC of about 18% up to 950 fps.... I hate making predictions, but that should mean a BC of about 0.24 at 950 fps....
Bob
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Boattails make the bullet act longer, both in terms of reduced drag and increased twist required, so at least in theory they need faster twists than the equivalent length flat base bullet.... The twist calculators have a correction for boattails, but it seems from the evidence we are slowly accumulating that the rebated boattails I use don't need the full correction factor that is built into the twist calculator.... A flat base bullet with the same nose and overall length, would in theory only need an 18" twist for the same stability as the boattail in a 13" twist.... My guess is that 15" is probably OK for the boattail, maybe even 16"....
The theoretical drag curve for the 105 gr. boattail above is virtually flat, out to Mach 0.85 (949 fps), and only increases about 20% at Mach 0.90 (1005 fps).... so it should be running right in the sweet spot in your gun.... The calculated Cd is 5% better than the original 94 gr. BBT at Mach 0.85 and below, and 28% better at Mach 0.90.... The blunter nose and tail of the original caused the rapid increase in drag above 950 fps (~50% increase by the time it reached 1000 fps), and you were already starting to get into that speed range.... With the 12% higher SD, that should, in theory, mean an increase in BC of about 18% up to 950 fps.... I hate making predictions, but that should mean a BC of about 0.24 at 950 fps....
Bob
Oh, OK, i get it now. thanks for that info.
Also, 1 of the other test i have to do with the Corsair 7mm and your BBTs is to start off at slower speeds to see whats best. I can control speeds by turning down the regulator. Shooting at 3400 or 3500 should put that bullet into the sweet speed spot. Actually, just for giggles, i will start at 3200 then go up 100 psi per group of 5 shots to see what the bullet/twist rate/fill pressure likes.
More to come.
Maaaan, i love this stuff!
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Sounds simple, but barrel harmonics could easily lead you to make the wrong conclusion about the velocity the bullet likes (in terms of accuracy).... when it could just be the barrel and bullet getting into Zen with each other.... Finding the BC at various velocities would be VERY useful information, however....
Bob
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I really enjoy these posts and I have been following the Ballistic Modeling thread as well.
Bob, I was curious about the change in the design or the addition of the bore riding ogive in front of the 0.002" narrower space before the driving band. Is it like gas check and stabilizer with a benefit of lower friction from rifling? What does it gain over the 2.5 caliber long .257 design in the Ballistic Modeling design.
That is unless those are just adaptations to the .284 caliber and 14 inch twist of Cedric's rifle...
Do you think that that would be a useful addition to the other design that you posted on August 8th?
(http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/257%20cal%20BR%2065%20gr_zps5kuwihvg.jpg)
Assuming that the bore riding ogive could be incorporated into the 2 caliber long design without increasing the Cd.
Thanks again for all the work you guys do, I feel like I have learned a lot (So much more to learn) and it's just fascinating to read.
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The bore-riding nose is an attempt to reduce the friction in the bore.... There may be additional aerodynamic drag from the lube grooves in conventional bullets, I don't think that has ever been studied.... at the moment this is pretty much all theory.... The drawing you posted above was part of a study of Meplat diameter relative to bullet/nose length and ogive radius, the other designs came earlier.... I have no proof which is better, when you are comparing bullets of the same length....
If you compare two bullets, one that is 2.0 calibers long and ~65 gr. to one that is 2.5 calibers long and ~78 gr.... the longer, heavier bullet will have a higher SD, and therefore an advantage in terms of a higher BC just based on that.... Everything else is just a refinement to improve the Form Factor to get a slight increase in BC for the same SD.... The required twist rate will tend to be faster, the smaller the base diameter.... for a given length and general shape....
Bob
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Thank you for clarifying. It's starting to make sense.
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It's a lot of data to take in and make sense of -and much of what you are finding is counter intuitive to what many of us have assumed as fact or associated with factors of shooting powder burners at supersonic velocities. So I have to bookmark your posts to study and re-study. If it takes me that long to ingest, it must take a lot of time to compile and document so clearly. Thank you for all of that effort.
So after re-reading a bunch last night I started thinking along the lines of three things that stuck out in a couple of the threads and started me thinking (spooky, as a little knowledge is a dangerous thing)
1.Chuckster posted in the Boattail vs. flat tail thread this quote- "There is no technical reason why a double-ended bullet should not be used to overcome drag..." Dave Corbin
2. The shape of the classic pellet like those currently in the group buy threads. I recall some discussion about the flight stability of that pellet shape coming from the drag of the skirt/ low pressure behind the skirt. That the skirt was very necessary but the drag it imparts on the pellet could be a limiting factor in the pellets flight characteristics.
3. The shape of the .284 cal 2.5 caliber long slug design in this thread is nearly a hybrid of pellet and Boattail designs.
From those points, I was wondering if you were to use the design of that 2.5 caliber long slugs front end and use that again on the back end instead of the boattail, essentially like the Corbin quote but adapted for subsonic velocities. Your data has shown that the taper of the back end has a great deal of impact on the coefficient of drag of the bullet and that a meplat of 60 to 70 percent of the caliber of the bullet is most efficient at Mach 0.9. Having the back end mirror the front end would also give a longer taper than the boattail design as well. Arguably the equilateral design could impart an improvement in the coefficient of drag.
Of course the design as is with the boattail appears to have its center of gravity behind its midline, my idea would place the center of its mass at it's center and I'm not sure how that might impact the stability of the round in flight.
Bob, Would it be possible to run the equilateral or double headed slug idea through your programs? As if you had nothing but time on your hands.
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Nope, the McDrag program assumes a straight taper if the bullet has a boattail, no provision for a curved one.... Sorry....
Bob
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Well that's a McDrag :P