Asking for thoughts 7.62 twist rate

[WHSmithIV]

OK, so I have now found two 10.5" barrel candidates for this pistol of mine. Both are carbine length gas systems, that's perfect. One is 1:8 twist and one is 1:10 twist. The 1:8 twist is a nitride treated barrel, the 1:10 twist is a melonite barrel (both are actually basically the same metal treating process, just a small difference in the chemicals used). So, that's equal. The 1:10 twist is just under a hundred bucks, the 1:8 twist 130 bucks. So, my decision now boils down to which twist rate would be better? With a 10.5" barrel, there will be one full rotation of the bullet before it leaves the barrel either way. There won't be a great deal of extra spin in 2" with a 1:8 twist. I don't see that accuracy for 100 yards could be effected much with a 1/4 extra spin on a bullet.

If this was to be a 16" barrel design, yes, I'd go for 1:8 twist over 1:10. But, this is a 10.5" barrel. It is designed to be a short range, accurate gun. I'm not so sure I see an advantage with that barrel length for 1:8 twist vs. 1:10 twist. Bullet weights for 7.62x39 generally are all in the 123 grain range. Bullet weights in 300 blackout are generally in the 140 grain range. Heavier bullets - faster twist. .300 blackout barrels are, for the most part, 1:8 twist.

I was going to get the 1:8 twist barrel and this is not an issue of saving 20 or 30 bucks, but, now I find a 1:10 twist barrel that will do the job too.

So, what do you guys think? 1:8 twist or 1:10 twist?

[alamo5000]

OK, so I have now found two 10.5" barrel candidates for this pistol of mine. Both are carbine length gas systems, that's perfect. One is 1:8 twist and one is 1:10 twist. The 1:8 twist is a nitride treated barrel, the 1:10 twist is a melonite barrel (both are actually basically the same metal treating process, just a small difference in the chemicals used). So, that's equal. The 1:10 twist is just under a hundred bucks, the 1:8 twist 130 bucks. So, my decision now boils down to which twist rate would be better? With a 10.5" barrel, there will be one full rotation of the bullet before it leaves the barrel either way. There won't be a great deal of extra spin in 2" with a 1:8 twist. I don't see that accuracy for 100 yards could be effected much with a 1/4 extra spin on a bullet.

If this was to be a 16" barrel design, yes, I'd go for 1:8 twist over 1:10. But, this is a 10.5" barrel. It is designed to be a short range, accurate gun. I'm not so sure I see an advantage with that barrel length for 1:8 twist vs. 1:10 twist. Bullet weights for 7.62x39 generally are all in the 123 grain range. Bullet weights in 300 blackout are generally in the 140 grain range. Heavier bullets - faster twist. .300 blackout barrels are, for the most part, 1:8 twist.

I was going to get the 1:8 twist barrel and this is not an issue of saving 20 or 30 bucks, but, now I find a 1:10 twist barrel that will do the job too.

So, what do you guys think? 1:8 twist or 1:10 twist?

I had a discussion with a guy at Rainier Arms the other day that shed a whole lot of light on this ongoing barrel discussion of mine... and it might apply here too...

As to the hardening processes... I called in because I bought a stainless barrel from them and I was wondering what the life expectancy of it was among other things... I was considering getting a hardening job done and they advised against it...why? Because it gets TOO HARD. Repeated shock of shooting over time in an over hardened barrel can lead to cracks and breaks and even outright failure. The hardness can mean the barrel metal becomes too brittle.

In Rainier's testing they found that with some of the standard hardening processes it led to a 15% failure rate over the alternatives... so they failed a lot more often and always for the same reasons. Hence they came up with a proprietary hardening formula that found that magical sweet spot for life and durability... In my stainless barrel they have found that magical middle ground that works.

I would assume this same logic applies to several different types of metals and barrels as well. So, beware of generic 'hardening'.... harder can be better but not always. You gotta get to the right spot there. If I would have gone the extra mile to make my barrel a 60 rockwell hardness I would have actually DAMAGED my barrel and cut the life of it by 15-20%.

Hence, overkill sometimes is a bad thing. It would be wise to search out more about what kind of metals they use and their hardness...but then again the barrel will wear out over time... but I figured the information I got from them was useful none the less.

As to twist rate I would think the faster twist would be a better performer in the grand scheme of things for that round at those velocities. With an AK and the notoriously cheap ammo that is often not balanced or whatever... I get keyholes sometimes at 100 yards or less with a 16" barrel. I would tend to think the faster twist would be even more critical in a shorter barrel. But I could be wrong about that.

[alamo5000]

With a 10.5" barrel, there will be one full rotation of the bullet before it leaves the barrel either way. There won't be a great deal of extra spin in 2" with a 1:8 twist. I don't see that accuracy for 100 yards could be effected much with a 1/4 extra spin on a bullet.

I also think you might want to think on your logic on that statement. In absolute terms yes. It will have that physical attribute. But the SPEED of the rotation will be different. These are not the same thing.

[WHSmithIV]

I also think you might want to think on your logic on that statement. In absolute terms yes. It will have that physical attribute. But the SPEED of the rotation will be different. These are not the same thing.

This is very true. And, that is why I have asked this question. I'm working on the mathematics of the ballistics. The faster twist will result in faster rotation. There are many variables with ballistics. The reality is that I have only found the two barrels to choose from. There is a 3rd one with a pistol length gas port, but I would much prefer the carbine length gas port for obvious reasons. Meloniting and Nitriding (the same process actually) do not harden the metal of the entire barrel. What the chemical application does metallurgically is change the alignment of the surface molecules of the metal. It's very similar to what anodizing does for aluminum. Ii spent 2 years working in a metals foundry a mere 7 years ago - so, I studied metals). A melonite/nitride post process of a barrel will definitely give it better life than a chrome lined barrel will have.

Now, that's all fine and dandy, and I doubt that in my lifetime I will put 10,000 rounds through the barrel. My question was regarding ballistics and accuracy over 100 yards basically. Which twist rate would be best? Would there really be any appreciable difference? Granted, accuracy has at least as much to do with the shooter as with the gun, but, is there a real difference down range from the two twist rates at 100 yards for 7.62x39? I guess I may have to build 2 to find out then put them both in bench vices and test it myself if nobody else has done so.

[alamo5000]

I just googled (a lot) and about 1:10 is the standard 'milspec' for an AK. If that means anything.

[WHSmithIV]

I just googled (a lot) and about 1:10 is the standard 'milspec' for an AK. If that means anything.

I know 1:10 is the standard for AK's and that's why 'most' AR 7.62x39 barrels are 1:10. Until I saw Faxon's barrel, the only faster twist than 1:10 I had seen was 1:9.5

I did ask Faxon if their barrel was indeed 1:8 when I mailed them to check on the barrel threading and they did confirm that their barrel is 1:8 twist. This is what I got back from Faxon in answer to the question:
"We opted to run the 1/8 twist since the bullet weights are typically closer to 300 BLK (147 grain+) and 1/10 is on the low end for stabilization.
a. This is critical especially when you choose a suppressor. Low stabilization may lead to baffle strikes, especially if you do not have good concentric threads. "


OK, I guess that makes sense if you're going to run a suppressor... but suppressors work best with subsonic ammo and Russian 7.62x39 steel cased ammo is definitely not subsonic. I'm wondering if 1:8 twist just might overspin a typical 7.62x39 bullet.