questions about reloading for a 1:7 twist 11.5" ar15

[anaxagoras]

I've been reloading for a while, but mostly pistol and a little bit for my 20" 1:8 twist ar15 with 68 or 75gr pills while shooting longer ranges prone or off bench.

I want to do some loads for my 11.5" 1:7 twist ar-15 sbr for some tactical shooting. A shoot house style range was built near me recently and they will be doing some tactical carbine classes. 100 yards max. Most engagements <50 yards. I don't need crazy accuracy, but i don't want something that shoots 5moa either.

How important is bullet weight? since it's a 1:7 twist should i really be avoiding 55gr pills and aiming for 62-68 gr pills? I don't think there's any point in heavier pills and the associated cost for relatively CQ. I've heard that heavier pills take longer to stabilize and may suck at shorter distances? True or BS?

I was looking at some 62gr armscor and 62gr Prvi, but i've read mixed reports on them of anywhere from 2-5moa.

I've got some hornady 68gr bthp that might be up to the task, thoughts?

Since it's a shorter barrel, i'd think i should aim for the fastest powder i can find that's recommended for this load?

What do you think would be a good load for my uses? Am i just completely over thinking this?

 

[243winxb]

A 11.5" barrel is not going to produce high RPMs/velocity in a 1-7 twist. I dont think Blitz/varmint type bullets will blow up. A 55 gr bullet should work well, but not a FMJ type, if you want better accuracy. With fast powder like 4198, you may have to match bullet weight to port pressure to work the action??

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[redbullitt]

I don't see why a 55g pill would not work. I don't think you could drive it fast enough to shread it.

bullet rpm= 720x velocity / inches of twist (i.e. 7 in your case)

Hand loading should make it relatively easy to find what your after, especially with a bit of experimentation. Do a bit of testing and have at it!

Personally I would shoot the cheapest I could. Most anything you load right should be passable for "practical" purposes within 100 yards. Softball sized groups at 100 yards is plenty deadly with regular old nasty pull downs if you are shooting practical or tactical style targets.

FWIW my 80g amax, 77smk, 69smk, and pulled SS109 all shoot fine close and far. No weird stabilization things going on at all from 1/7 to 1/9 twists. Barrels from 14 to 28 inches.

 

[CMV]

My 11.5" SBR shoots 55gr Hornady SP & VMax with no complaints. Carbine gas.

 

[anaxagoras]

Thanks, guess I was just over thinking things, going to try some of the 55gr hornady sp's. @ $0.09 per round shipped it doesn't look like a bad choice for my uses.

 

[Jenrick]

A 1/7 10.5" will produce a higher RPM at the muzzle then a 1/9 20" barrel. So if you've got a round that will hold together in a 1/7 16" or 20" gun, then you are good to go with the shorter barrel.

As far as accuracy goes, a 5 MOA gun/ammo combination will shoot in about .5" at 10 yds which is a dang big interior room, and even a pretty long interior hallway. Pretty much any ammo which doesn't key hole will have an acceptable degree of accuracy for true CQB work/training.

Math:

MV x (12/twist rate in inches) x 60 = Bullet RPM

1/7 10.5" - 2440 fps * (12/7) * 60 = 250,971

1/9 20" - 3100 *(12/9)*60= 248,000

1/7 16" - 2900 * (12/7)*60 = 298,285

1/7 20" - 3100 * (12/7)*60 = 318,857

1 MOA @ 100 yds ~ 1"
1 MOA @ 10 yds ~ .1"
5 MOA @ 10 yds ~.5"

-Jenrick