What's the downside to 1:7 in an AR?

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epijunkie67

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I've got an AR lower I was going to build up before all the craziness hit the fan. For now I've got it on hold but I expect eventually all this will die down and I'll be able to buy parts for it again without taking out a second mortgage. So I'm looking for some opinions on barrel twist rate to guide me later.

I know that the 1:7 twist is to stabilize heavier bullets, which sounds like a good thing. And I realize that a higher twist rate might decrease barrel life. But I don't expect to be burning up 10,000 rounds a year in this thing.

So aside from decreased barrel life is there any reason to NOT get a 1:7 twist in an AR?
 
Not much unless you're exclusively shooting <50gr bullets. 55gr PMC from the AR hovers right at 1.25" groups at 100 yards for me.

BSW
 
I prefer a 1:7 twist on an AR. I reload and have loads that use 77 and 80 grain projectiles that may not perform well in a 1:9. I'd say if a 1:7 barrel has the same price/availability as a 1:9 go for it.
 
I dont have any hard data but logic would dictate increased wear , there is more bullet travel per inch than a slower twist barrel.
 
I have had the same experience as Robert.

I have shot 55 grain FMJ and varmint bullets out of a 1-7 twist barrel with excellent results. One of these days, I am planning to try some 52 and 53 grain target bullets in my 1-7 twist AR.

I am not sure if the 40 and 45 grain varmint bullets would hold up at the high twist rate. I am not really interested in shooting them in the 1-7 twist barrel. I keep them for my 221 Remington Fireball and 22 Hornet.

But, it might be worth a try.
 
I seem to remember lower velocity being a side effect.
Click to expand...
just the opposite, you get slightly higher velocity because the bullet is in the barrel longer and higher pressures build behind it.
 
1:8 twist is the best overall for normal shooting however they are normally only offered in match barrels. Your best accuracy comes from the 1:9 & 1:8.
 
just the opposite, you get slightly higher velocity because the bullet is in the barrel longer and higher pressures build behind it.
Click to expand...

How does that work?

If the bullet is in the barrel longer, it is travelling slower. Once the bullet leaves the barrel, it is slowing down.
 
i agree with adelbridge. seems to me that more time for pressure to build up, and maybe more powder burned, would result in more velocity.
 
Seems to me that if the bullet is resisting the pressure buildup because it has to twist more to exit the barrel then it will be leaving the barrel at a higher velocity due to the pressure behind it being greater.

Also, 1:7 twist barrels on military M16's and M4's is so that they could fire the longer and heavier tracer rounds. However, it is an overall advantage as well now that we're moving towards heavier bullets for normal use.
 
I have a 1:9 twist barrel and bought a range of different wt ammo, from 55-72, without question, the 55gr. performed the best.
 
Tight twists and light bullets is not always bad, varies from rifle to rifle, you would think a 1:10 30 cal barrel suitable for 240gr BTHPs bullets would be downright horrible with stubby little flat base 125s but some love them. There is no hard and fast facts about tight twists and light bullets in my experience, every rifle is different. In general I have always been able to accurately use a wider range of bullet weights with tighter twists, but that could very well be just the differences in my rifles.
 
Twist is about bullet length (weight, density, and shape), not just weight. Luke's graph appears to be for conventional lead jacketed bullets. 1:8 is the optimum civilian twist: I agree 100%. I disagree with Luke that it would be better than 1:7 for military use. 1:7 covers the steel tipped 62 gr. SS109 (which is longer than a conventional 62 gr. bullet) but also covers much longer tracer rounds that are ballistically-matched to the 62gr SS109.

Mike
 
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How does that work?

If the bullet is in the barrel longer, it is travelling slower. Once the bullet leaves the barrel, it is slowing down.
Click to expand...

Its not slower when you account for rotational travel. With a faster twist rate the bullet has more travel per equal barrel length as compared to a slower twist hence accelerated barrel wear. Take 2 different 4 inch screws, one with a coarse thread pitch and one with a fine thread pitch. Screw them in with the same drill at the same speed and time it, you will discover that the fine thread pitch actually traveled farther and took longer per the same 4" of linear travel. Drill took on slightly more wear as well.
 
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There's no downside to 1:7 for 55gr and over jacketed. If you were shooting lead (and I can't see many folks casting ww .223) than a slower twist ( 1:12) would be better.
 
And if I were going that heavy I would look at 6.8 spc with 85-95 gr. Solids
 
Honest opinion here, you couldn't get me to argue that the 1:8 is better than the 1:7 but I can argue that the 1:8 and 1:7 have advantages over the 1:9.

But with all that said it really depends on how one is going to use the gun. If just for plinking and having fun, any of them will work. If you're looking for long range accuracy and hunting you will be needing to step up in grain size, which denotes the advantages of the 1:8 or 1:7. But even the 1:9 gives a pretty good range of grain size.
 
I am shooting a 1:8 in a 20" and it loves the 77gr Federal Match load. We're talking 0.4"-0.6" 5-shot groups at 100 yards.
 
No downside. 1/7 stabilizes 55 and 62 grain just as well, and lets you shoot the heavier stuff that 1/9 twist doesn't.
 
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