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Posted: 2/3/2006 4:56:25 PM EDT
Will a 1/10" twist barrel effectively stabalize 68-69gr bullets? I plan on using 55-69gr bullets, so will the 1/10" twist be OK for my needs?
Link Posted: 2/3/2006 4:59:30 PM EDT
[Last Edit: 2/3/2006 5:00:22 PM EDT by Minuteman419]
Only one way to find out truthfully.

ie: some 1X9" barrels will shoot 75's, some won't. Same with other weights/twists.

Danny
Link Posted: 2/3/2006 5:05:56 PM EDT
The answer..... "maybe".
Link Posted: 2/3/2006 5:38:45 PM EDT

Originally Posted By FALARAK:
The answer..... "maybe".



I would say... maybe not. But maybe. The M16A1, a 1/12 would not even stabilize 62 grain bullets.
Link Posted: 2/3/2006 5:40:49 PM EDT
It should, according to Sierra Bullets. They tend to know what they are talking about.
Link Posted: 2/3/2006 5:51:34 PM EDT

Originally Posted By SWO_daddy:
It should, according to Sierra Bullets. They tend to know what they are talking about.



Sierra can really only speak to the 69gr SMK. He said 68-69... but didnt comment on the maker. The Hornady 68's are a different bullet design.... and *might* or might not stabilize as easily. It also matters on the load use, muzzle velocity plays a factory on this. Which is why... the eight answer, is maybe. Maybe it will be stable to 500 yds. Maybe it will shoot like a laser at 100, then start keyholing at 200. Maybe.
Link Posted: 2/3/2006 6:51:09 PM EDT
IIRC, the 68 is longer than the 69 by a tiny bit. This might play into stabilization as longer projectiles need faster twist.


Sierra lists the minium twist you can use to stabilize the bullet. That doesn't mean it will always work. The key is RPM's...how do you get more RPM's on the bullet? By having a tighter twist OR by traveling through the twist faster. Or both.


This is precisely why a 24" 1-9 can stabilize 75's while a 16" 1-9 will probably not be able to..all things being equal. It isn't the barrel length itself, but the barrel length leading to greater velocity through that very same 1-9 twist.


Sierra says their 69 will stabilize in a 1-10, but that doesn't mean it will stabilize out of a 1-10 barrel that's 10" long.
Link Posted: 2/3/2006 9:32:44 PM EDT
i was under the understanding that for the most part 69gr was the max for a 1 in 9". so id figure 62gr for a 1 in 10". you never know till you try though.
Link Posted: 2/4/2006 5:19:17 AM EDT
Hornady 75 grain Amax's say 1/9 on the box. Sierra 69 Matchkings and 65 Gamekings both specify 1/7 to 1/10.

And my 1/10 twist Ruger Mini-14 16 inch barrel shoots 62's and 69's just fine.

That said, I NEVER got superior results using 68/69's in 1/9 twists compared to 55 and 50 jhp match bullets.

Paladin
Link Posted: 2/4/2006 6:20:27 AM EDT
[Last Edit: 2/4/2006 6:20:38 AM EDT by SWO_daddy]

Originally Posted By Dont-Tread-On-Me:
IIRC, the 68 is longer than the 69 by a tiny bit.


The 68 is longer overall, but the bearing surface (the length of the bullet in contact with the rifling ) is longer in the Sierra.
Link Posted: 2/4/2006 9:13:06 PM EDT
[Last Edit: 2/4/2006 9:19:41 PM EDT by HotIce]
My Bushy 1/9" does NOT stabilize the Hornady 68gr BT-HP bullet. The bullet is .984" long, so it was no surprise since GreenHill already told me that Using 165 as constant (given the .223 muzzle velocity), it gives 1/8.14" required to make that bullet happy.
Time to add a 1/8" RRA Varmint to my collection


- Ice

Link Posted: 2/5/2006 5:41:12 AM EDT
All this is why I prefer 1/8 as a minimum. Then I can shoot anything I want up to 80 grains without wondering if they are twisting fast enough.

I've had 1/8 and 1/7 and never had an issue with bullets ranging from 45 to 80 grains.
Link Posted: 2/5/2006 8:17:51 AM EDT
Who makes accurate (and nice) 1/7" twist barrel+upper for AR15 (20" or 24" length)?
So far I Like the RRA Varmint 1/8", but I don't see them offering 1/7"


- Ice
Link Posted: 2/5/2006 8:30:23 AM EDT

Originally Posted By HotIce:
Who makes accurate (and nice) 1/7" twist barrel+upper for AR15 (20" or 24" length)?
So far I Like the RRA Varmint 1/8", but I don't see them offering 1/7"


- Ice



WOA barrels from Adco are available in 1:7. Kreiger match are 1:7.7 There are plenty of custom barrels out there that are rifled 1:7.

Let me ask you this.... why would you want 1:7, when 1:8 will stabilize any target round, up to the 77gr SMK, and even non-mag length 80gr pills?

Also - spend some time doing some research on 24" barrel lengths. It doesnt make any sense in .223 (IMHO) except for limited circumstances, shooting light varmint bullets and trying to squeeze every ounce of velocity.... and those would be 1:9 or 1:10 twists typically.
Link Posted: 2/5/2006 8:59:50 AM EDT
I've never used the 77gr SMK, but from pictures it looks quite longer than the Hornady 68gr BTHP.
And my 1/9" Bushy is not even close to stabilize that. Using a 150 constant, the longest bullet a 1/8" can stabilize is .9408, to go up to 1" using 165 (that I find to work fine with 223 velocities). The Hornady 68gr BTHP is .984, and it's a 68gr bullet.
I expect the 77gr SMK to be over 1".
Leaving the math apart, and to answer your question, I always shot 62gr bullets with my ARs (all 1/9") and I've no plans in going down with the grains, and I do not believe in over-stabilization and/or disintegrating bullets.
A 1/8" barrel might do it (will definitely work on the Hornady 68br BTHP) as well, but I'd be slightly happier with a 1/7".
Is it known for a fact that 1/8" is dead-accurate with the 77gr SMK?


- Ice
Link Posted: 2/6/2006 7:32:18 PM EDT
To determine the twist required for a given bullet, the twist in calibers = 150 / lenght of bullet (in cal.

in the 5.56 this would be the length/.223 = X

150/X = Y calibers

Y /.223 gives you the rate of twist for your barrel

(this assumes the jackets lead bullet which would have a average specific gravity of 10.9

Remember use your bullet lenth divided by .223 to find X

Divide 150 by X to give you Y and Y devided by .223 gives the requird barrel twist.


This is the Greenhill formular from the British Textbook of Small Arms
I made a lot of across the course and 1000 yard guns using this formular and have never went wrong.

KraigWY
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