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1 in 9 twist with >70 grain bullets

When do the bullets begin to destabilize? Right out of the barrel or do they fly fine for a while and then begin to destabilize?

I'm wondering because I bought some 75 grain bullets and at 50 yards they seem to make the proper holes in paper (no keyholes).

Thanks.

-Crashy.
 

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Re: 1 in 9 twist with >70 grain bullets

To answer your question, both, it just depends. You need to shoot them at different distances 100-200-300-etc,. This will provide you with more info and you can determine at what distance they start to become unstable, if at all.

Mero
 

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Re: 1 in 9 twist with >70 grain bullets

it really does depend on a great number of different criteria. some bullets need some distance to settle down while others are ok right at the muzzle. once a bullet's velocity drops below supersonic speed, there are aerodynamic forces that affect the stabilization of all bullets. really vexing since there are so many different factors to contend with when not only selecting a bullet, but think about somebody trying to design the little suckers.
 

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Re: 1 in 9 twist with >70 grain bullets

In general (and it's not cast in stone)

.223 Remington / 5.56 NATO rule of thumb.....

1:12 - 55 grain max, best with 50 grain or less

1:10 - 60 grain max, best with 55 grain or less

1:9 - 69/70 grain max, best with 65 grain or less

1:8 - 77 grain max, best with 69/70 grain or less

1:7 - made pretty much for the 77 - 80 grain VLD (Very Low Drag) bullets

With a fast twist (1:9 or faster) and a light (like 40 grain) bullet pushed to a high velocity, depending on bullet construction, you can cause the bullet to self-destruct from centrifugal force. Those Varmint Grenade 40+gr out of a 1:8 or 1:9 tend to go "poof" within about 50 feet of the muzzle, pretty/funky to watch but not practically useful.......:cool:
 

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Re: 1 in 9 twist with >70 grain bullets

1:9 should be fine up to 69gr or so. Thereafter a 1:8 or 1:7 is good. Go 1:8 if you can as 1:7 can wear down faster. Also speak to the manufacturer of the bullets you shoot. Nosler, Barnes, Lapua and Sierra, for example, really do know their bullets well.

Note that it's not bullet weight that determines rifling, but bullet length. We just assume that a heavy bullet is longer. Longer bullets need faster rifling rates to stabilize the tips. Go too tight however and you reduce your ability to shoot lighter bullets as with a 1:7 twist 40 grain varminter tips will fragment almost as soon as they exit the barrel. A bullet passing out of a rifle at 3200 fps, with a 1:7 twist is doing close to 330,000 rpm. In a 1:14 barrel it will only do half that, hence you need to be aware of the bullet design limits. Additionally, tracers go against the general weight length assumption as they are usually long for their weight. A 145 grain Orange tip 308 tracer is about the same length as a 168 grain HPBT tip.

My 1:9 XCR shoots 77 grain ok, but my 1:7 AR gets tighter groups and is better out at 200-300 yards. At 50 yards you can't tell the difference and at 100 you need to shoot several groups back to back to see a small group size difference. At 200-300 yards the group difference is more apparent. So, beware of choosing bullets based on 50 yard performance. Depending on the caliber/load/barrel combination the bullet may not have stabilized at 50 yards. Most of my 308s cavitate until about 50 yards and my 338 is not stable for at least the first 100 yards of it's flight path. As an example of this, it shoots a 0.5" group at 100 yards, but does the same at 200 yards. At 100 yards the bullet is still cavitating and this affects ultimate grouping.
 
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Re: 1 in 9 twist with >70 grain bullets

your rifle will shoot differnt than anyone elses. you need to go out and find out for yourself. NO-ONE here can answer that question with any measure of certainty
 

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Re: 1 in 9 twist with >70 grain bullets

I have a 1:8 20" Douglass barrel on an AR and it shoots 75gr's with as good of accuracy as the shooter can muster (need to drink less coffee :duh:).
 

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Re: 1 in 9 twist with >70 grain bullets

You'll just have to try it, just like everyone else. But, and adding to the comment to speak with the bullet manufacturer, the box of Hornady 75gr I have says 1:9 or faster. I think you might be on the end of the spectrum of stability though.

Go to patsreloading.com and get some 68gr BTHP w/ cannelure. You'll be able to load a BA round and still get a good crimp for just in case....just promise to leave a few boxes for me, I'm running low.
 

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Re: 1 in 9 twist with >70 grain bullets

I have some 77gr SMKs coming in tomorrow. I bought them to shoot in 1/7 ARs, but I have read a couple isolated reports that these bullets are short for their weight, and that a 1/9 barrel will stabilize them. Should I give some of them a try in the XCR? Has anyone tried this exact bullet?
 

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Re: 1 in 9 twist with >70 grain bullets

They shoot fine but at longer ranges the groups are better out of a 1:8/1:7 barrel.
 

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Re: 1 in 9 twist with >70 grain bullets

I shot the 77gr SMK's out of a 1/7 AR, 20" and 14.5", and they shot like nobody's business! Didn't try them in the XCR though. Hornady's 75gr A-MAX is listed for 1/9 or better but I think Mickey's right on this one, you'll hit paper but the faster twist will win.

Did you get the Mk262 slugs w/ cannelure or the plain 77's? That cannelure is pretty non-existent but I guess it helps in jacket fragmentation on impact.
 

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Re: 1 in 9 twist with >70 grain bullets

When do the bullets begin to destabilize? Right out of the barrel or do they fly fine for a while and then begin to destabilize?

I'm wondering because I bought some 75 grain bullets and at 50 yards they seem to make the proper holes in paper (no keyholes).

Thanks.

-Crashy.
You'll need to try it and experiment and see what your barrel likes.

Though, I recall awhile back one of dealers on ARFCOM (Denny from Global Tactical I think) was using 1/9 barrels and >69gr bullets and said there was no reason for concern.

Try it, see what happens, and record results.
 

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Re: 1 in 9 twist with >70 grain bullets

My SMK 77s use both canelure and smooth. The cannelure helps the round split into two on impact at lower velocities/longer range.
 
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