1:9 or 1:7 Twist in AR15

[hags]

I've shot 50 gr Hornady Vmax bullets in a 1:12" barrel that wouldn't stabilize at all.
Sideways at 100 yards.

I've shot from 55gr upt to 77 gr in my 1:7" barrels with zero problems. The 75gr and 77gr bullets group really tight.
I've shot low .2s with a match grade 26" barrel and match grade Hornady 77 grain. Of course they were all 5 shot groups, none of this 3 shot nonsense.

 

[1911Gunslinger]

1 in 8 or faster is needed to stabilize bullets in the 77 gr. class. Most 1 in 7 barrels don't do a good job on bullets lighter than 62 gr. though. I have tried several from Bushy, Colt and others whereas the 62's shot well the lighter stuff was minute of man.

 

[P-32]

I shoot NRA High Power and hold a "Master" classification. I do not claim to know everything about AR's. I can only pass on what I have observed.

I have 2 AR's plus a couple of other rifles I use. One of the AR's has a Nor Pac barrel with a 6 1/2 to 1 twist. The other is a Kreiger with a 7 to 1.

Both shoot 77 and 80 gr. SMK's very well with the 80's loaded long for the 600 yard line.

If I shoot a 100 yard match with reduced targets, I use the Pac Nor with 53 gr SMK's and have not had a bullet come apart yet with it's 6 1/2 to 1 twist. I know RPM's are pretty freaking high but the rifle shoots them pretty nice.

Both AR uppers where built by White Oak Precision.

My next project is to get a 90 gr. Berger out to a 1000.

 

[ugaarguy]

I'd say that 1:7 stabilizes 55 grain just fine and it does not "disintegrate" (where do people come up with this nonsense?).

Bullet weight alone doesn't tell the whole story. M193 and similar heavy jacket 55 grain FMJ will, as you've seen, run fine in a 1:7 twist bbl.

From Speer Reloading Manual Number 13 p. 137

Although the fast twist works well with military ammo, handloaders ran into a problem. Most 22 caliber centerfire rifle bullets are of light construction for varmint hunting. When fired at 3200 feet/sec in a 1-in-7 twist rifle, the bullet is rotating at over 300,000 rpm when it leaves the muzzle. This rotation is more than most varmint bullets can withstand so they are literally ripped apart as they leave the barrel.

If you have a rifle with the faster 1-in-7 twist, you should limit the muzzle velocity of sporting-type bullets to around 2800 feet/sec.

Edit to add:

Finally, you need to test YOUR bullets in YOUR barrel. There are guys that shoot 1:9 barrels with 77 grain all day long and get sub-MOA accuracy out of them. There are also guys that shoot 45 grain out of 1:7 barrels and they don't vaporize on leaving the barrel.

My suggestion is to figure out your application and choose accordingly.

That's the best advice in this thread. No amount of experience nor data from industry standard reloading manual can account for all variables in your rifle. Your bore may be slightly rougher or smoother than another, your chamber throat may be slightly looser or tighter than another, etc. These are going to have minor effects on velocity and friction; as many of us have seen some 1:9 twist bbls. will shoot 75 grain bullets very well, and others won't shoot them well at all.

 

[rob_s]

I invite anyone to send me their "light" .223 loads and I'll be happy to shoot them in my 1:7 to see if they disintegrate.

 

[peck1234]

hahahhahah

 

[ugaarguy]

I invite anyone to send me their "light" .223 loads and I'll be happy to shoot them in my 1:7 to see if they disintegrate.

Rob, are all your 1:7 bbls chrome lined and 16" or less? I'm wondering if even Speer's max loads with a 40 grain bullet would hit 3200 FPS from a 16" or less barrel.

Caveat: I have NOTHING to back up the following. However, hard chrome is slicker than plain steel. I'm curious if the increased friction in a longer, non-chromed, steel barrel (typical in traditional bolt rifles) has an affect too. Speer's test rifle was a stock Ruger M77 with a 22" long 1:12 twist barrel.

 

[hags]

I shoot NRA High Power and hold a "Master" classification. I do not claim to know everything about AR's. I can only pass on what I have observed.

I have 2 AR's plus a couple of other rifles I use. One of the AR's has a Nor Pac barrel with a 6 1/2 to 1 twist. The other is a Kreiger with a 7 to 1.

Both shoot 77 and 80 gr. SMK's very well with the 80's loaded long for the 600 yard line.

If I shoot a 100 yard match with reduced targets, I use the Pac Nor with 53 gr SMK's and have not had a bullet come apart yet with it's 6 1/2 to 1 twist. I know RPM's are pretty freaking high but the rifle shoots them pretty nice.

Both AR uppers where built by White Oak Precision.

My next project is to get a 90 gr. Berger out to a 1000.

I use John almost exclusively for barrels on my builds. I have spent a good amount of time talking about this very subject.
Although I have never had this happen to me personally, WOA has stated that the reason they switched their 26" match barrel to 1:8 from 1:7 is because of comments and "complaints" about bullet break up.

Good luck with that next project. Have you made any progress towards that end so far?

 

[Heck]

Thanks for all the info guys. Think i will go wit th 1:9 when i get the cash and stick to 62 gr bullets as this will be just a paper puncher.

 

[P-32]

hags,

Yes John does build one heck of a nice upper. When I decide to go to a space gun it will be one of his.

The project of getting the 90 gr Bergers to a 1000 is in the thinking stage right now. I have the bullets but I'm still pondering the load as there isn't much data. I will more than likely use Reloder 15. It works very well with the 77 and 80 gr. SMK's for me.

I bought the 6 1/2 Nor Pac to shoot 90's but after a few years of hard use the throat has moved down the bore making 80's really long. It still shoots though. The 7 Kreiger has had alot less use as it was built last year and should launch the 90's without problems.

The Pac Nor shoots well but does make more copper than the Kreiger ever thought of.