Projectile design
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Projectile design
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Topic: Projectile design (Read 1654 times))
Mark Davis
Plinker
Posts: 298
yes
Real Name: Mark
Projectile design
«
on:
May 10, 2017, 10:45:23 AM »
This is really directed at rsterne, but all comments are welcome in the interest of enlightenment.
I'm new here on GTA, and am aware that you are "Bob" of Bob's boat tail bullets available from NOE bullet molds.
I have purchased a few of Swede Nelson's molds and find them to be high quality, reasonably priced, and all around good stuff.
But I want to make my own .25 cal air rifle mold for a stock marauder. I have a CNC mill and plan to drill and ream cylindrical cavities undersize, then go back in with a homemade "D" shape cutter and finish the cavity with circular interpolation.
What angle of taper is close to optimum for boattail bullets at 800 fps?
Is there a ratio between bullet diameter and boattail length I should be aware of?
Is the radius at the back of the boattail a strong factor?
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Bozeman, MT
rsterne
Member 2000+fps Club
Bob and Lloyd
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Posts: 26956
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Projectile design
«
Reply #1 on:
May 10, 2017, 01:27:31 PM »
Typical boattails seem to be about 8* per side, but boattails are seldom used on short stubby bullets like we use in airguns.... Therefore, conventional wisdom for boattails really doesn't really apply, for taper, length, or base diameter.... A stock MRod has two huge limitations to the power, the 20" barrel length, and the porting diameter.... Even if you hog the exhaust, transfer and barrel ports out as large as you can go, you cannot get beyond about 0.20" diameter, because going larger will have the exhaust port break into the valve cavity, ruining the seat.... There is simply not enough metal there.... At 3000 psi, for a single, full power shot, I don't think you can get to 100 FPE, more likely much less than that.... You are probably aware that at 950 fps, the bullet weight in grains is half the FPE, so you are looking at a bullet of 40-50 grains.... Have a look at my two NOE designs of those weights, and you will get an idea of the proportions you are dealing with....
Like I said, short and stubby.... Those both use a 10* rebated boattail.... At 800 fps, you may be able to use a slighty longer, heavier design.... The new 62 gr. BBT would be 88 FPE at 800 fps, and you won't get there with a stock MRod, IMO....
HTHs....
Bob
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Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
Mark Davis
Plinker
Posts: 298
yes
Real Name: Mark
Re: Projectile design
«
Reply #2 on:
May 11, 2017, 10:25:13 AM »
Thanks for your reply.
This will be my first attempt at bullet design. After rejecting the idea of a pin to form a hollow skirt for speed, ease, and keeping the temp stable for casting. Your boattail design Looks like something I could simulate.
My shooting is mostly in sling supported positions, so an endless supply of good pellets has more appeal than a small supply of perfect pellets.
The .25 grain JSBs are pretty good, and I envision my pellets weight to fall somewhere between 25 grain and your 41 grain design.
I might even leave the boattail off, it would make for a stronger reamer.
When I cut this mold it will be 11 cavity, in blocks the same length as Lee six cavity mold blocks.
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Bozeman, MT
Mark Davis
Plinker
Posts: 298
yes
Real Name: Mark
Re: Projectile design
«
Reply #3 on:
May 31, 2017, 08:47:07 AM »
Finally getting to reamer design.
Thinking along the lines of a wasp waist, stubby truncated cone, with flat base.
For first attempt, the shape seems easy.
rsterne,
Is a shape with more weight ahead of center a desirable trait?
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Bozeman, MT
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26956
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Projectile design
«
Reply #4 on:
May 31, 2017, 12:34:25 PM »
Nick at Nielsen Specialty Ammo is the expert on CG position.... and he always seem to be pushing the CG as far back as he can.... I would ask him....
Bob
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
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Projectile design