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View Full Version : Will a 13.5 twist stabilize this bullet??



jackie schmidt
05-08-2009, 09:21 AM
My Older Brother is into AR's, and he wants to build a AR in 6x45.

He is eyeing that pile if 13.5 twist take off barrels I have, and figures he could just use one of those as a blank.

Sierra makes a bullet that is a 80 grn BT, with an overall length of about .855. Of course, the lead core runs all the way to the end, as it is a hunting style bullet.

I am thinking that might be a good bullet to shoot in a 6x45 with a 13.5 twist. But, I am not sure that the twist will stabilize it.

That bullet is just a few Thousanths longer than our typical 68 grn Benchrest Bullet.........jackie

Larry Elliott
05-08-2009, 10:32 AM
I had an old Kimber in 6 PPC with a 1 in 14 twist that would stabilize most bullets up to 70 gr and Speer 75 gr HP's. It wouldn't stabilize 75 gr Hornady or Sierra HP's though. A 13.5" twist might stabilize an 80 gr bullet from a 6x45, but I sort of doubt it. The Kimber had a 24" barrel and was pushing it to get 3100 fps from a 70 gr Hornady SX. I doubt that a short barrel AR would be able to push an 80 gr fast enough from a 6x45 to stabilize it unless he really tips up the powder can which isn't the best thing in an AR.

The only data I could find on the 6x45 was in a bolt rifle with a 22" 1 in 9 barrel which probably isn't too applicable. Hornady shows that the 6x47 they shot stabilized their 75's with a 1 in 14 twist, but their rifle had a 27" barrel and they were getting 3000 fps with the 75's.

Donald
05-08-2009, 11:44 AM
I have shot Berger 74 grn bullets thru my 14 twist ppc barrels. They shoot real good, no evidence of instability. My guess is that if he can push them close to 3,000 fps they probably will, if it is warm. In the cold, probably not. Using the old Greenhills formula this bullet would need a 12 twist at 2800 and up. With his probable cost of machine work it would be worth a try.

jackie schmidt
05-08-2009, 12:40 PM
Most of my take off barrels are at 21 1/2 inches, so a 19 incher is about what one would yield for a AR, somewhere between 2700 and 2800 fps is about all you could ask for with that 80 grn bullet.

There is really no cost involved, I have the barrels, and my brother is just as skilled a machinist as I.

Maybe he needs to look at shooting a lighter, shorter bullet if he wants to make use of these barrels.........jackie

jackie schmidt
05-08-2009, 12:50 PM
I will probably go to the range tommorow, I could take some of these bullets, load my 6PPC down to about 2750, and see what they do..........jackie

brian roberts
05-08-2009, 01:56 PM
on some AR-180s in 6X45, the barrels were 18" I think, got one sitting, but never measured it; Those barrels all had a 1-10" twist. Just like the fireball, unless you can put it in a 22-24" barrel, a 14" twist will do it no good, that's why it had a 12" twist in the short Rem. barrel. On paper, some combinations may look great, but if one is stuck with 2 or 3 powders and the rifle doesn't happen to "like" those powders, well, then its NEW barrel time("I'll get this sucker to shoot THIS time") :D

Rustystud
05-08-2009, 02:45 PM
I once owned a Model 7 Remington with a 20" barrel in .243 Win. It had a 1:12 twist and would not shoot anything over 70grains worth a hoot. I rebarreled with a 1:8 twist Broughton 5C. It now shoots 85-105s in one hole.

Nat Lambeth