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Musings on the best gun & load for CBA competition

Posted: Mon Dec 05, 2016 6:08 pm
by mtngun
I don't compete in jacketed benchrest but nonetheless I try to keep up with the latest benchrest technology because some of it applies to shooting in general. Likewise I try to follow CBA benchrest technology even though I have not yet participated in CBA matches. In particular, I've long wondered what limits the accuracy and the velocity of cast bullets?

Anyway, here is a list of CBA competitors who agg'd 1.100" or better at the 2016 Nationals. I'll refer to these shooters as the "serious competitors." Obviously it is an incomplete list of serious competitors because not all of them attended the 2016 Nationals and some who did attend may have had an off day. Nonetheless the list should serve to give us an idea of state-of-the-art CBA practices.

1) Nearly all serious competitors use a 30 BR. Why not a 6PPC like the jacketed boys? Well, one serious competitor did use a PPC, and he does well at 100 yards, but the 75 - 80 grain bullets he shoots are disadvantaged at 200 yards due to wind drift.

2) The average rifling twist was 12.3 inches. Even though the traditional twist for 30 caliber is 10 inches, serious competitors have moved to slower twists. IMHO, they need to keep moving, to 13 or 14 inch twists, with a Miller Stability Factor of 1.3 - 1.5.

3) The average bullet RPM was 118,000. I can accept that because the laws of physics tell us that our imperfectly balanced cast bullets will wobble more as they are spun faster, and I know from personal experience that it's challenging to get consistently good accuracy with cast bullets at 160,000 - 200,000 rpm.

4) While my chart did not include alloys, nearly all serious CBA competitors use linotype. I would suggest that switching to heat treated reclaimed shot would be just as accurate, if not more so, plus the denser alloy would increase their bullet's G1 ballistic coefficient by about 0.010.

Next step, let's assume that 118,000 RPM is a valid limitation. Let's also assume that a Miller Stability Factor of 1.3 is optimal for the ranges involved (jacketed benchrest favors a SF=1.3. A higher SF requires higher RPMs which hurts accuracy). For now we'll stick with linotype spitzer bullets, even though I prefer reclaimed shot. What combinations of twist, RPM, and velocity will yield 118,000 RPM and SF=1.3?

According to our chart, a 130 gr. at 2950 fps would do the job, but few casters are comfortable with 2950 fps, so let's limit ourselves to a modest velocity that has been proven capable of competitive accuracy -- say, 2300 fps. At 2300 fps, our chart says we should shoot a 180 gr. lino spitzer from a 14" twist.

This Mountain Molds spitzer weighs 185 - 195 gr. depending on the alloy. Stability factors in a 14" twist at standard atmospheric conditions:
-- lino @ 2100 fps, SF=1.28
-- lino @ 2200 fps, SF=1.30
-- lino @ 2300 fps, SF=1.32

-- reclaimed shot @ 2000 fps, SF=1.31
-- reclaimed shot @ 2100 fps, SF=1.33
-- reclaimed shot @ 2200 fps, SF=1.35
-- reclaimed shot @ 2300 fps, SF=1.37

Note that the denser alloy has a higher stability factor -- yet another reason to use reclaimed shot rather than linotype.

The ~190 grain spitzer has a calculated G1 ballistic coefficient of 0.34". I'm skeptical of calculated BC's, in real life the BC is almost always lower. Nonetheless let's look at wind drift using the calculated BC and assuming a 10 mph wind:

2000 fps, 100 yards = 1.6", 200 yards = 6.7"
2100 fps, 100 yards = 1.5", 200 yards = 6.2"
2200 fps, 100 yards = 1.4", 200 yards = 5.8"
2300 fps, 100 yards = 1.3", 200 yards = 5.5"

While unimpressive by jacketed standards, that's respectable drift for a CBA load.

To sum things up, I suspect CBA competitors could gain a little accuracy -- and by "little" I mean a few hundredths -- by replacing their 12" twists with a 14" twist.

All that said, if I am ever able to shoot in CBA matches I'll probably use something different than everybody else just to prove that it can be done. :lol:

Re: Musings on the best gun & load for CBA competition

Posted: Wed Dec 07, 2016 8:08 am
by Paul Pollard
Nice looking bullet. Is it available? Looks like it would be a good fit in 300 Blackout.

Re: Musings on the best gun & load for CBA competition

Posted: Wed Dec 07, 2016 8:59 am
by mtngun
As a matter of fact, I made that bullet for someone who plans to shoot it in a Blackout. :lol: I'll be making a plain base version later this week.

I'm not sure it will stabilize at Blackout velocities, though. The various stability rules don't necessarily work reliably at transonic speeds. I guess he'll find out when he receives the mold and the weather allows him to test it.

Re: Musings on the best gun & load for CBA competition

Posted: Thu Dec 08, 2016 2:30 pm
by mtngun
A plain base 180. This one's BC is only .32 because I deliberately scaled back the weight to get it closer to 180 grains. It dropped out 182 grains in lino.