Is there an improved accuracy version of either the SAAMI or C.I.P. 7.62x39 chamber? Something with a more accuracy inducing throat design and a tighter chamber neck?
I’ve got a couple of AR upper and a CZ527 in 7.62x39. If a chamber more conducive to maximum accuracy exists, I’d like to have the CZ barrel set back and re-chambered.
So, has anyone designed such a chamber and if so, who has a reamer and can rework my barrel?
Yes, it’s called 6.5 Grendel. 
You can even fireform with 7.62x39 brass.
Beyond that, use decent bullets, Etc. Play close attention to actual rifling land diameter relative to Bullet.
I’ve not seen different chamber specs for 7.62x39.
Back in the early days of ARs in 7.62x39 I did see quite a few AR barrels that had tight chambers which was great for accuracy with brass cased ammo but terrible for reliability with steel cased. Also most US made barrels were made for .308 bullets which is what most domestic made ammo used while overseas barrels were made for .311 bullets. The new barrels & rifles seem to mostly be chambered loose for reliability.
Are you looking for improved accuracy with imported steel cased ammo, domestic, or hand loads?
And I agree with the above, if you’re looking for sub MOA accuracy you’ll probably like 6.5 Grendel a lot more, and still have the option for cheap steel cased ammo. Or
AK barrels I’ve sectioned don’t even have a step down in diameter from case neck to freebore… it’s like there is no freebore, just whole lotta throat. Don’t use that one:p
I’ would guess your CZ already has the optimal accuracy chamber for this caliber, what makes you think there is room for improvement, is it inaccurate with good ammo? My approach would be, shoot some Lapua in it, anything else is not telling you much.
Yes, the SAAMI chamber for 7.62x39 has no freebore, just .395" of throat that starts out nearly .004" larger than bore diameter.
By way of comparison, the modern 300BLK chamber has .174" of freebore and .172" long throat that starts out at .001" larger than bore diameter.
Interestingly, the SAAMI 9x19 and 45ACP chambers are designed the same way with huge funneling throat, while the .40S&W/10mm have freebore.
It seems most older cartridges/chambers weren’t designed with freebore, which didn’t become popular until after WW2, perhaps due to the influence of .300 Savage and .300 Weatherby Magnum designs.
The cr@ppy SAAMI chamber design was copied from the cr@ppy C.I.P. chamber design. I’m sure CZ uses the C.I.P. chamber spec. Very sloppy chamber neck giving poor guidance to the cartridge and the aforementioned cr@ppy throat design.
I’m sure a tighter chamber neck coupled to a tighter and better throat design similar to the .300 Blackout/.308 Win/.223 Wylde/name any US catridge designed since the great depression, would enhance accuracy with all ammo.
I understand why the lawyers at CZ, Ruger, etc force the use of these extremely poor industry standards. I do not understand why aftermarket barrel makers don’t start using a “7.62x39 Spec II” or “7.62x39 Wylde” chamber of their own design. Surely it would improve their market penetration.
KIK claims to have a proprietary “match” chamber design, but from the reported accuracy I’ve seen, it looks like they failed miserably.
I’m going to hazard a guess that the US demand for match-grade barrels or rifles in 7.62x39mm is in the low single digits and is met by CZ’s bolt action. Likewise, I’ll bet >99% of 7.62x39mm buyers in the US are planning to shoot quantities of imported steel-case ammo, and are not expecting MOA accuracy.
I understand your point. But I doubt that the .300 Blackout would exist if the original Colt 7.62x39 AR’s would have been capable of 1" or better groups with quality brass cased American ammo and and 1-3/4" or better groups with the better brands of Russian steel cased ammo. I believe this is not that difficult to achieve with a quality barrel with .3105 groove diameter, .3045 bore diameter, 1 in 12" twist (certainly no faster than 1 in 10"). In addition they would need to make the the chamber neck parallel like our cartridges - the same diameter at the case shoulder as at the case mouth. The cartridge has a straight, parallel neck, the is no reason the make the neck of the chamber a loose cone. Also, as stated earlier, it should have a throat/freebore design similar to the .300 Blackout but with all diameters increased by 0.0035".
Such a barrel should be very accurate with quality .310-.311 bullets.
My Sabre barrel has a normal-looking chamber and given the gun it’s in, it shoots pretty well with Lapua ammo. I will try to find it and get a pic up by I may have remelted it. I don’t know that it shoots fodder-grade 7.62 much better than an AK.
I found out that JGS has a reamer design they call the “7.62x39 Lapua”. It’s their print # 3514. It has a cylindrical .3114" dia. X .078" long throat transitioning to a 1 degree leade. Very similar to the .223 Wylde design.
Now if I can just find someone making .310"-.311" barrels with this chamber design. Any suggestions on what barrel supplier might be using this?
I found out from Kurt at KAK that they use their own chamber design in their 7.62x39 barrels and it is very similar to the JGS 7.62x39 Lapua chamber. The KAK has a cylindrical .3115" dia. X .088" long throat and a .311" groove diameter.
Do any of you have experience with KAK 7.62x39 barrels? What sort of groups do you get with good U.S. made ammo or handloads. What kind of groups do you get with steel case Russian ammo?
Good short article on the importance of chamber throat design to accuracy:
http://www.lasc.us/BellmChamberThroatWhatIs.htm
I’m still researching and found that both Colt and Ruger made their x39 reamers using Lapua’s chamber print. A further advantage of the Colt barrel would be its 1:12 twist which would reduce the radial group dispersion on target with less-than-perfect bullets, like in bi-metal Russian ammo. Now if I can only find some of those old Colt Sporter barrels.
McGowen Precision Barrels makes .303 cal./.311 groove dia. barrels with 1:16 twist and a 7.62x39 “Match” chamber that is the same as the 7.62x39 Lapua chamber and the KAK 7.62x39 “Match” chamber.
1:16 twist sounded slow to me until I ran the numbers through a sability calculator, 1:16 provides the optimum stability for 122-125gr .311 bullets. The stability calculates to be the same as a .5.56 77gr Sierra Matchking in a 1:7.7 barrel.
The only box on my checklist that McGowan doesn’t check is that they don’t offer their barrels with a nitride/QPQ/Melonite finish.
Never-the-less, McGowan just became my barrel maker of choice for 7.62x39 AR15 barrels.