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rsterne
06-15-2014, 01:12 PM
For a while now I have been developing a series of rebated boattail bullets for airgun use.... I do the design work and then pass the drawings on to Accurate or LBT molds, free of charge.... I received nothing, other than the satisfaction of trying to improve bullet design to keep up to what modern PCP airguns are capable of.... So far, the following molds are available....

# 25-52-BST from LBT Molds (designed for .250 cal airgun barrels of 16" and faster twist)
# 30-070A from Accurate Molds (designed for the new .300 cal airgun barrels, including the FX and Daystate)
# 31-079A from Accurate Molds (designed for a standard .308 cal barrel)
# 36-124A from Accurate Molds (designed for .357/9mm barrels)

The first three will all fit into .25 cal Marauder magazines (Lloyd at AirGunLab does a .30 cal conversion), that was one of the design criteria.... We also designed two new airgun barrels in .300 cal and .357 cal to fit the new JSB pellets, both of 26" twist, and the Bob's Boattail bullets are designed to work with those as well.... The barrels are available from Mike at TJ's, in either 1/2" OD (to fit a .25 cal MRod breech) or in 0.590" OD which can be machined to fit on a QB7X or others.... I'm currently working on a 95 gr. design in 7mm with Veral at LBT.... Here is a drawing so you can get an idea of what they look like....

http://i378.photobucket.com/albums/oo221/rsterne/Bullet%20Casting/284calAirgunBoattail96gr_zps7d756679.jpg (http://s378.photobucket.com/user/rsterne/media/Bullet%20Casting/284calAirgunBoattail96gr_zps7d756679.jpg.html)

The calculated subsonic Ballistics Coefficient for this latest design is 0.32, and it is designed to work in Mike's 14" twist .284 cal barrel.... The Bob's Boattail bullet that has received the most extensive testing so far is the 70 gr. in .300 cal, and it is regularly returning MOA groups out to 100 yards in the 850-1000 fps range....

Bob

HARRYMPOPE
06-15-2014, 07:28 PM
what does the boat tail gain you at subsonic speeds at those short ranges?

rsterne
06-15-2014, 07:41 PM
Surprisingly, MORE than it does at supersonic velocities.... It cuts the drag in half below Mach 0.85 (960 fps).... doubles the BC, and cuts the wind drift (which is the bane of airguns) in half as well.... Here is a great bullet drag calculator.... one of the few that deals with subsonic flight....

http://www.geoffrey-kolbe.com/drag.htm

One of the BIG advantages, however, is the perfect release of the rebated shoulder from the muzzle crown as it never gets distorted by the sprue cutter, sizing, or have fins added to the base of the bullet by the rifling lands....

Bob

HARRYMPOPE
06-15-2014, 09:00 PM
I never had a calculator for subsonic stuff,thanks.The base release and fins is an old schuetzen deal that i dont think matters all that much(but fun to worry about).Well the guys i shoot against in Spokane use flatbase bullets and have no trouble shooting sub 1/2" MOA in good conditions anyhow.Still have to use flags and read wind<G>

I am not totally doubting data but i played with plainbase fixed ammo for quite awhile and the twist rates and many of the things that made sense on paper didn't always show up on the target.Popes 308403 bullet for the 30-06 with its rear tapered base was supposed to take care of the finning but that bullets wasn't generally the most accurate and actually quite finicky.Not having powder burning the bullets base is a neat thing and i think airguns may have the advantage some day with plainbase once they can push a high BC bullet fast enough to shoot to 200 yards accurately.

If you take the boattail off of the above bullet whats the BC difference @ 800?

rsterne
06-15-2014, 09:38 PM
The BC is half, it drops from 0.32 for the boattail to 0.16 for the flatbase on the subsonic drag plateau from Mach 0.25 to Mach 0.85.... You might be interested in Kolbe's Twist Calculator as well.... It also has subsonic data, which again I've ever seen before....

http://www.geoffrey-kolbe.com/barrel_twist.htm

Both Calculators are based on the work done by Robert McCoy at the Aberdeen Proving Grounds in the 1980's and published as the "McDrag" and "McGyro" programs.... If you play with the programs, you will find out that a boattail bullet requires a faster twist.... Apparently not only does the bullet act "longer" from a drag point of view, but also from a stability point of view as well....

Bob

HARRYMPOPE
06-16-2014, 04:02 PM
hard to believe it will be a BC of .32 for such a short blunt bullet conidering the 311299 is long sleek and about .37 @ 1800.Something seems off?

rsterne
06-16-2014, 06:08 PM
Subsonic BC's are a LOT higher than supersonic, and in particular for boattails.... The drag of a bullet increases 4-5 fold between Mach 0.8 (900 fps) and Mach 1.2 (1300 fps).... If you play with the drag calculator you will see just how dramatic the change is.... Also you can Google "Transonic Drag" and "Drag Divergence Mach Number", which is the point at which the drag skyrockets.... The BC of that 7mm bullet at Mach 1.5 (1700 fps) is only 0.12....

Bob

HARRYMPOPE
06-16-2014, 07:39 PM
I use to shot alot of short pistol bullets at mach speed and at 100 yards found them unreliable wind buckers compared to the heavier longer ones at 1000 or so FPS.it would be cool beans if they really worked.I can’t wait to see a series of groups with your new bullet.

rsterne
06-16-2014, 08:41 PM
I agree 100%, short bullets and pellets are not great at bucking the wind, especially when you drive them close to Mach.... When you consider that even a very good pellet has a BC of about 0.04 at 800 fps, and that can drop to 0.02 at 1000 fps and 0.01 at Mach, virtually any bullet is going to be a HUGE improvement.... All I'm trying to do is the best we can do without going to such a high SD that we can't get decent velocity with the very limited pressures of an airgun....

Bob

melloairman
06-16-2014, 10:00 PM
Bob BC is only one factor that effects bullets and the amount of drift that takes place . Have you considered the reason that your design requires a faster twist rate to be stable as well ? There must be other factors and or reasons why all the suppliers of molds and bullets have not gone to all boat tails . Not saying you might not have a good design .But I do know that some of my 1-20 twist rate barrels do what your calculators say will not work and they do it with sub moa results . GL Marvin

rsterne
06-16-2014, 11:36 PM
You are quite correct, for equal BC, a bullet with a higher velocity will have less wind drift.... Boattails require a faster twist rate, and the smaller the diameter of the base, relative to the bullet, the faster the twist need to be.... I suggest you read Robert McCoys papers on drag and stability that he wrote while at the Aberdeen Proving Grounds, they are quite comprehensive, and his calculations have been tested against real projectiles by the Military....

They are not my calculators.... However, I have not found any better tools for subsonic bullet design.... If you have input your bullet dimensions into the Kolbe Twist Calculator and it says it has a Stability Factor of less than 1.0 at the velocity you are using, please post the graph, or give me the dimensions and velocity as I'm most interested in finding a set of dimensions that his calculator says won't work than in fact does....

No modelling tool is perfect.... but I prefer to use the best ones available....

Bob

HARRYMPOPE
06-16-2014, 11:59 PM
Has anybody gotten a pcp to shoot 1/2 moa on more than one occasion other just a fluke group?I hear of moa airgun groups at 100 and beyond but I haven't seen papers with a few groups,just one screamer group.

It's great to see this technology advance! I have grown board with most other cast bullet stuff.I just need to get away from my springers and get a pcp someday.

melloairman
06-17-2014, 01:13 AM
1/2 MOA. is not what I stated as for my self . But we have both more than likely seen the same post . And the regularity of it I can not state any better than you . As you well know many think this is a new sport and want to be the history maker and the leader of the pack . Marvin

HollowPoint
06-17-2014, 11:28 AM
rsterne:

I really hope this design works out well for you and anyone who uses it.

I have to say, I kind of find it humourous in some ways because a while back I posted my idea of the ideal
cast 22 air rifle slug and it resembled your depiction in certain ways; and I don't have any dedicated
design software. I just did my computer rendering based on extrapolation and what I pictured in my mind as being optimum for this air gun application.

I did do alot of research to come up with my rendition of the ideal 22 air rifle slug. All I really got for posting my idea was alot of polite replies hinting at all the reasons why it most likely wouldn't work and such. I'd like to see your design succeed with flying colors. I think any design that will move us away from the shuttle-cock wasp-waisted traditional pellet design that most are shooting now is a good thing.

I guess great minds do think alike.

HollowPoint

rsterne
06-17-2014, 11:40 AM
The tightest consistent groups are being shot in the 100M benchrest matches in the Netherlands.... or at least they are the ones that are publishing their results regularly.... They have 100M indoor ranges, and in those the top rifles are a mix of bullets and pellets, in calibers from .22 to .30.... However, they shoot their major 100M contests outdoors, and if there is any significant wind, the top rifles and bullets tend to be the Sinner by Drummond shooting the swaged RWS 41.3 gr. bullet, which is a .224 version of their R-10 .22LR match bullet.... Here is a photo of the bullets....

http://i378.photobucket.com/albums/oo221/rsterne/224%20Long%20Range%20PCP/224cal41_5gr.jpg (http://s378.photobucket.com/user/rsterne/media/224%20Long%20Range%20PCP/224cal41_5gr.jpg.html)

They often use a .22 Hornet barrel with a 14" twist, although I have heard that the Sinner is 15".... Velocity is typically in the 1000-1050 fps range, and many of the rifles are tethered to a large tank, running on a regulated 3000-3500 psi.... I have seen multiple groups at well under 1" at 100M (110 yards), many in the 15-19mm range, and the smallest I have seen is 11mm (0.433" C-T-C), which works out to 0.4 MOA.... In the wind, it appears that bullets are outshooting pellets on a consistent basis in these 100M contests, as would be expected.... I view this as the road to the future in PCPs, and that is why I'm working on my Bob's Boattails, in the hopes of pushing subsonic bullet development in a direction that so far seems to have been neglected.... I may not get it right, but hopefully somebody will, and eventually that may result in the availability of high quality, swaged bullets specifically designed for modern PCPs....

Bob

melloairman
06-17-2014, 03:28 PM
Bob things some times do not work out as they should and defy all odds . And sometimes they work out as planed . GL. I have a .452 bullet that in two rifles when loaded with the mel plate forward can not find their way to the paper at 60 yards . But when I load them base first and use them as a boat tail wad cutter they will stack at 60 yards . Marvin
http://i301.photobucket.com/albums/nn55/melloroadman/IMG_6969A.jpg (http://s301.photobucket.com/user/melloroadman/media/IMG_6969A.jpg.html)

rsterne
06-17-2014, 03:44 PM
I wonder if the chamber is too short and when loaded backwards the base engages the rifling before firing?.... just a thought....

Bob

melloairman
06-17-2014, 05:27 PM
No that is not the case . Some things just don't work as planed . By the way it is a 165 grain .452 . I have had a couple of bullets respond this way after many different bhn levels and sizes were tried . As well as a 1-16 and a 1-20 twist . Marvin

Larry Gibson
06-17-2014, 06:35 PM
Subsonic BC's are a LOT higher than supersonic, and in particular for boattails.... The drag of a bullet increases 4-5 fold between Mach 0.8 (900 fps) and Mach 1.2 (1300 fps).... If you play with the drag calculator you will see just how dramatic the change is.... Also you can Google "Transonic Drag" and "Drag Divergence Mach Number", which is the point at which the drag skyrockets.... The BC of that 7mm bullet at Mach 1.5 (1700 fps) is only 0.12....

Bob

I don't play with drag calculators; I actually measure the TOF and the BC via screens at muzzle and in front of the target (25 - 100 yards) with an Oehler M43. Sorry, but with numerous cast bullets from SWC nose shape to RN I have not see a "4-5 fold" increase in the BC below Mack .8 as compared to Mack what ever upwards of 2600 fps. Yes the BC does get lower at higher velocity but there is no drastic increase as indicated by the "calculator".

Have to concur with HARRYMPOPE; "many of the things that made sense on paper didn't always show up on the target"

Larry Gibson

GARD72977
06-17-2014, 09:58 PM
if I remember right you were going to shoot this form a magnum springer. The replies you were referring to suggested that the twist rate of your barrel was a problem. I doubt a springer would get a bullet with that much bearing surface and weight to the intended target.





rsterne:

I really hope this design works out well for you and anyone who uses it.

I have to say, I kind of find it humourous in some ways because a while back I posted my idea of the ideal
cast 22 air rifle slug and it resembled your depiction in certain ways; and I don't have any dedicated
design software. I just did my computer rendering based on extrapolation and what I pictured in my mind as being optimum for this air gun application.

I did do alot of research to come up with my rendition of the ideal 22 air rifle slug. All I really got for posting my idea was alot of polite replies hinting at all the reasons why it most likely wouldn't work and such. I'd like to see your design succeed with flying colors. I think any design that will move us away from the shuttle-cock wasp-waisted traditional pellet design that most are shooting now is a good thing.

I guess great minds do think alike.

HollowPoint

HollowPoint
06-17-2014, 10:36 PM
if I remember right you were going to shoot this form a magnum springer. The replies you were referring to suggested that the twist rate of your barrel was a problem. I doubt a springer would get a bullet with that much bearing surface and weight to the intended target.

If the slug I was shooting weighed in at the estimated 96 grains that the OP has listed in his graphic, I believe you're absolutely right; out of my springer. They most likely wouldn't even make it all the way down the bore before stopping.

Although they have a pronounced arch to their trajectory, I am able to shoot the 25.4 grain monster pellets accurately enough for rabbit and feral pigeon work out to 50 yards. The estimated weight of my design only came in at 28-point-something grains. (mine were intended to be hollow pointed to bring down the weight) With the power mods I've tested in my springer I'm more than sure that my design would have no problem making it to "The Intended Target." Making it there accurately is something else all together though.

I have shot the sized-down NOE 35 grain cast bullets out of it, indoors; just to see if my Umarex Octane had the balls to get them out the barrel. It seemed to do just fine but, admittedly I was shooting into a ball of old rags, not at any distance or target. When I sized them down, it completely shaved off all the drive-bands so I was essentially shooting a solid slug of lead.

I've acquired another toy to play with for now so I'll look forward to the progress that rsterne achieves with his design. I did have plans to to buy an AirForce Condor with my tax return but I opted to spend it on a new Tikka T3 instead. That's the toy I'm playing with for now.

Incidentally, I think my barrel twist is 1 in 16. I believe that would be plenty of spin to stabilize pellets up to 30 grains out of my gas springer, regardless of what the computer generated calculations estimate.

HollowPoint

rsterne
06-17-2014, 10:37 PM
Interesting results.... I take it the "sound barrier" doesn't really exist, and the difficulty in breaking it was a figment of the imagination.... I wonder what Chuck Yeager would think of that.... I guess the "Drag Divergence Mach Number", as shown in the diagram below at about Mach 0.85, doesn't really exist....

http://i378.photobucket.com/albums/oo221/rsterne/Important/TransonicDrag.jpg (http://s378.photobucket.com/user/rsterne/media/Important/TransonicDrag.jpg.html)

I didn't state that the BC decreased by 4-5 fold when you go supersonic, I stated that the drag increased by that amount.... In fact the actual drag increases much more rapidly than the Cd, and to calculate the total drag, you have to multiply the drag coefficient by the square of the velocity.... The G1 Ballistics Model (which has a BC of 1.000) takes into account a drag curve like the one below.... In other words, a bullet is expected to behave like that.... BCs are a measurement of how the bullet being tested compares to the G1 Model....

http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/G1BallisticsModel_zps5e9e7901.jpg (http://s378.photobucket.com/user/rsterne/media/Ballistics/G1BallisticsModel_zps5e9e7901.jpg.html)

I have also done quite a bit of velocity testing with diabolo airgun pellets (which admittedly are not bullets), and I have found that the BC drops off rapidly as you exceed a certain velocity.... That actual speed depends on the shape of the pellet, and a wadcutter starts it's decrease in BC at about 700 fps, whereas a round-nosed pellet starts a drastic decrease in BC at around 850-950 fps depending on the SD, with heavier pellets closer to the higher figure.... I have numerous measurements that show a BC in the 0.04 range up to about 900 fps, dropping to 0.02 by 1050 fps, and down to about 0.01 at Mach 1.... Those are done by measuring the muzzle velocity and the downrange velocity with a Chrony, and then calculating the BC....

I have very little experience with cast bullets, nor have I done any velocity measurements downrange to test the numbers predicted by the calculator (yet).... I wouldn't be at all surprised that the actual BC is lower than calculated.... However, considering the credentials of the author of the program, Robert McCoy, and the fact that the program has been checked for accuracy at the Aberdeen Proving Grounds, I feel it is a worthwhile tool.... The bullet maker Sierra must trust his work, as they list his .22 rimfire BCs on their website.... http://www.exteriorballistics.com/ebexplained/22rimfire.cfm .... Note that just between 905 fps and 1047 fps the BC drops by about 25%....

I fully understand that airguns are looked at as toys by the majority of the PB community.... so rather than continue to argue the point with no measured BCs to provide, I'll just let the thread die.... Thanks to all for your comments....

Bob

HollowPoint
06-17-2014, 10:50 PM
Threads come and go. Even if you had all the data to prove all your points there would still be those well meaning souls just chomping at the bit to inform you otherwise.

Let the thread die but, please keep the project alive. At PCP velocities, the possibility for success is much greater than those of present day Gas Spring velocities; although I do believe that may change given enough time. The need for more efficient projectiles in both Air Rifle designs will never change.

Give them hell Bob; and thanks for posting your idea.

HollowPoint

GARD72977
06-17-2014, 11:19 PM
Do not misunderstand my post. I think rsterne is making a case for why his bullets will work. I like the idea and will follow this thread. Your post puts you in the same situation that rsterne and that is not the case.

K.O.
10-05-2015, 06:52 PM
http://www.gatewaytoairguns.org/GTA/index.php?topic=96713.0

Tom_et
10-06-2015, 08:27 AM
Just a thought?
You inventors should try a sabot that does not separate in flight
using a bb or similar

rsterne
10-06-2015, 11:20 AM
Not sure what would be the point?.... If the goal is to reach a higher velocity from lighter weight for a given caliber, then you will sacrifice Ballistics Coefficient for exactly the same reason.... While there is a practical limit on the Sectional Density for any give air pressure and barrel length, a sabot that doesn't separate would have a low SD, and hence a low BC, throughout its flight.... losing velocity and energy too rapidly to be useful, IMO....

Bob

LongRangeAir
10-06-2015, 06:56 PM
For all the Nay Sayers out there, Rstern's design, just won 1st and 2nd place at the Extreme Bench Rest comp In Arizona. A 200 yard match for high power AirGuns!

As far as whether an AG can shoot Sub MOA regularly or not, LOL, seems many are ignorant here of what is going on in AG's these days. On a good day, I can shoot that at 200. yards. And usually under!

A fellow who use to frequent here, shot long range with Sharps for years. He stated that at 200 yard, and under, his High Power Air Gun was more accurate than the Sharps. (He won a LOT of Comp's with his Sharps, including his State Championship) He managed to shoot Pepsi cans at 600 yards with his .257 J. Haley Air Gun.

My custom .257 built on a Cheap Air Force Frame will shoot 1/2 to 3/4" at 100 yards all day long. And often, well under 3/8" for five shots.

Say that under MOA on a regular basis isn't what an ag can do? No, what it can and does do, is every once in a while, it will shoot under one tenth MOA or less, as in .009". THAT is what it does on a NON Regular basis. Best Group to date was .027". This is far beyond what a typical .22 long rifle can do, and it does it at 200 fpe. In the world of High Power AG's, it's a light weight!

For plinking fun, I shoot PB hulls off a perch at 100 yards. Shot gun hulls? Nope, .22lr hulls. And have run many a 10 for 10 at that range! Show me a .22 long rifle Powder Burner that can do that! And I'm pushing a Lot more Energy, and enjoying casting for it. Where .22 LR is hard to come by now days, With casting and reclaiming my lead, I shoot darn near for free! Well, except for the RCBS Pro Melt, and NOE Molds. LOL!!!

Knife

HollowPoint
10-09-2015, 10:58 AM
This only verifies what I've believed for some time now. I knew it would work.

How did I know?

Because; unlike some of the arm-chair-experts we encounter in some of these chat forums, I
(and many others; like rsterne) actually take the time to do the designing and testing.

A similar phenomenon also occurred when I posted a subject on Boat-Tailed Gas-Checks placed on Boat-Tailed
cast bullets. I got them to work and outlined how I did it but it only received mild interest. It's only recently
that others with a greater post-count and more recognizable name within this cast bullet forum have started mentioning
this same subject of Boat-Tails that it seems to be more plausible to us all.

No matter; as long as we move forward with our dreams and designs. If we stagnate, we loose interest
and loose out on what could have been.

For the most part, I quit posting my ideas because there always seem to be those well meaning souls who's life mission is to
stifle creativity as politely and courteously as possible.

Congrats to the guy who proved the validity of the guys who came across as invalid to so many.

HollowPoint

LongRangeAir
10-11-2015, 07:54 PM
Yep! See a LOT of that!

The greatest minds in History have all been plagued with the short shortsightedness of others. (Know-it-All's). LOL

PAT303
10-11-2015, 07:58 PM
I find your results staggering,my absolute hats off to all of you,brilliant work. Pat