7.62x39 performance thru 12-14.5" barrels?

[Sixgun_Symphony]

looking for an understanding of the terminal ballistic performance of the x39 say the Corbon 123gr DPX and Federal Fusion 762x39 loading.

[GJM]

Might be info in this thread, which I recalled from times past.


http://www.m4carbine.net/showthread.php?140969-barnes-expansion-threshold


Sent from my iPad

[Luke]

There is another one on here. If I remember right BES started it, Doc posted in it, I'll see if I can find it.

[Sixgun_Symphony]

There is another one on here. If I remember right BES started it, Doc posted in it, I'll see if I can find it.

https://pistol-forum.com/showthread.php?20828-SBR-Lethality-AK-vs-AR

Is this the one?

Good thread. Doesn't completely answer what I'm looking for but somewhat.

I'm looking at a little project of taking a SAR1 and cutting down the barrel to around 12"...I just wanted to make sure that if I cut the barrel down I wasnt completely neutering the 762x39. Mostly concerned with how two of the top loads (123gr TSX & Fusion) I'm likely to use for anything serious (hunting, SD etc etc) would perform. No so much concerned how the various Russian/Eastern European FMJ/JHP etc perform so long as on the range I'm hitting what I'm aiming at 0-300yrds

[TiroFijo]

I've seen this posted many times over the net, can't find the original post now.

http://i810.photobucket.com/albums/zz28/pogi_pogi_pogi/762x39Velocities1.png

[Sixgun_Symphony]

I emailed Barnes the other day to get and idea on the expansion threshold of the 123gr TSX

reply:

The .310 cal 123gr TSX requires a minimum impact velocity of 1500fps for beginning yet reliable expansion in our water tank test. We suggest an additional 100fps to 200fps for good bullet expansion and subsequent good wound channels.

Thanks, Ty

[Randy Harris]

7.62x39 does not lose much velocity when shortening the barrel. Typically around 100 fps when cutting from 16" to 12" (like cutting an Arsenal SLR 107CR from 16 to 12.5" AK104 configuration). Not enough velocity loss to really matter. Paul Gomez and I used to discuss why SBR AKs were maybe a "sleeper" or "unknown cool setup" because they could be truly tiny with a folder, still run reliably, give up little in ballistics (especially compared to going to a 10" AR from 16") if you could just stand the muzzle blast.

The bigger issue MIGHT be the twist rate. They typically have a 1/10" twist rate. If we look at the 8.5" barreled "Krink" style SBRs (like an Arsenal SLR 107UR shortened to 8.5") the bullet is not getting even a full turn before it leaves the muzzle. The 11" Draco is getting 1 whole rotation before it leaves the barrel. So the accuracy in different weather conditions/altitudes might be an issue but typically these guns are not used in the DMR role but more in the 50 yards and in PDW role so it might be a moot point any way.

[TiroFijo]

What does getting a full turn of the bullet before it exits the barrel has to do with gyroscopic stability?

A 1/9.45" twist (1 in 240 mm) is plenty for any 30 caliber bullet that might be used in the 7.62x39. even with reduced velocities and adverse environmental condition (low temperature, high altitude, etc.).

Using this calculator by Berger, and data for a typical 7.62x39 bullet (122 gr, bullet 1980 fps MV, 1.05" bullet length, G1 BC = 0.310, 1/9.5" twist, 0ºF, 7000 ft altitude) the SG = 3.17, double than what is recommended.

http://www.bergerbullets.com/twist-rate-calculator/

[Randy Harris]

Jim Fuller (Rifle Dynamics) tells me that from test firing builds the 8.5" 7.62x39 guns are barely stabilizing the bullets ....the assumption is probably because the barrel length is shorter than the rate of twist.

[TiroFijo]

Well, respectfully to Jim (and all) it may behave like that but perhaps the real reason is something else...

A S&W revolver in 38/357 has a twist of 1 in 18 3/4", the bullet only does about 1/5 of a complete turn in a 4" barrel, yet it shoots perfectly.

A real H&K G3K has a 12.4" barrel with a 1/10 twist, the chamber+leade make for a bit more than 2" so the bullet only barely makes one turn, but with a MUCH longer and heavier bullet than a 7.62x39 russian. You can shoot up to 175 SMK in them with no problems. Many people have cut normal 1/12 barrels (this would be less than one turn) with good accuracy in clone rifles.

I've shot 10" TC 1/12 barrels in 223 and 22 hornet (less than one turn) no problems.

[Randy Harris]

Fair enough and you may very well be right.

I assumed (again very dangerous sometimes) that the difference in velocity might account for 9mm pistol bullets being stable out of a 1/9 twist barrel that was only 3" long but 7.62x39 not being stable out of a 1/10 8.5" barrel rifle.

I'm just going by what Jim told me in regards to barrel length and the bullets being barely stable.... that was why I capitalized the word "might" in my initial post.

I have not seen any key hole issues with either my 8.5" (107UR SBR) or 12.5" (107CR SBR)...on paper at 50 yards. But a bullet can be "barely stable" or "just stable enough" to fly straight but still yaw like crazy when it hits something. Kind of like the 5.56 in the original 1/14" twist M16s. But then again they supposedly didn't see any accuracy issues with 55gr 5.56 out of the original 1/14" twist M16s until they shot them in arctic conditions....