Time for a serious discussion of 7.62x39 rifling twist

Let’s start with a short history lesson.

In 1886 the French adopted the 8x50R Lebel with 1:254mm/1:10” rifling and a 232 grain flat nose bullet.

In 1888 the Germans adopted the 8x57 with 1:254mm/1:10” rifling and a 226 grain round nose bullet.

In 1888 the British adopted the .303 Mk 1 with 1:10” rifling and a 215 grain round nose bullet.

In 1891 the Czarist Russia adopted the 7.62x54R with 1:240mm/1:9.45” rifling and a 210 grain round nose bullet.

In 1892 America adopted the .30-40 Krag with 1:10” rifling and a 220 grain round nose bullet.

In 1903 America adopted the .30-03 Springfield with 1:10” rifling and a 220 grain round nose bullet.

Those long, heavy 210 grain to 232 grain bullets needed those fast 1:9.45” to 1:10” twists.

By the end of WWI the Krag was obsolete and all of the others had migrated to spitzers from 147 grain to 154 grain, but without changing the rifling to a more optimum 1:14” twist.

When the Soviets developed the 7.62x39 M43 round it made economic sense, if not ballistic sense, to stick with the bore and rifling specs of the Mosin Nagant so that the same boring and rifling machines could be used.

1:16” would have been much better for accuracy with the short 122 grain to 125 grain bullets of the 7.62x39.

The similar .30BR cartridge is used primarily with 120-125gr bullets, has a 39mm case length, and is usually paired with rifling twist of 1:15" to 1:18" for best accuracy.

Maybe one day Ruger, CZ, Howa, and the producers of AR15 7.62x39 uppers will give us 1:16” 7.62x39 barrels. And while they’re at it they should replace the accuracy destroying C.I.P./SAAMI chamber throat design with an improved chamber throat design like the 7.62x39 Lapua chamber developed by Lapua and Valmet (see JGS Print 3514). This would cause a step change in the accuracy, usefulness and popularity of the 7.62x39 as a sporting cartridge.

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

American Rifleman published an article on handloading the 7.62x39, and was able to achieve right at 1MOA (1.05" average IIRC, at 100yds) for five shot groups with certain loads. That was a LAR-47 with 1:10 twist.

According to the quick guesses I made with the stability calculator, the 123gr .311 bullets are stable.

If anything, quality of the ammo available, and the guns it is being shot out of, are likely the limitations.

Using the Berger stability calculator, or any other accurate calculator, a stability factor less than 1.00 indicates a bullet sure to keyhole. A stability factor between 1.00 and 1.50 will be marginally stable and may become unstable if the barrel crown or throat is damaged or if the barrel is heavily fouled. A factor between 1.50 and 2.00 will give best accuracy at all ranges. Factors over 2.00 are still stable but can cause accuracy issues.

The faster than required spin will accentuate the negative effect on accuracy of any imperfection in the bullet. With close to perfect, high quality bullets, the effect may be minor, turning what could have been a sub .5 MOA group into a 1.0 MOA group. With less well balanced projectiles, as is usually the case with Russian ammo, this over spinning can turn what could have been a 1.0 MOA group into a 2.50 MOA group.

A properly stabilized bullet (SF between 1.5 and 2.0) will travel with its nose following the arc of its trajectory. An overstabilized bullet will tend to travel with its centerline remaining parallel to its original direction at the muzzle. This causes it to travel progressively more and more nose up, increasing drag, velocity loss, and wind deflection. At 100 yards this is not important, but it increases in importance as range increases.

Using the Berger calculator and careful measurement of a number of Russian, Eropean, and US made 122-125gr bullets (no guesses), in a 1:10" twist the stability factor varies from 4.15 to 4.81. This is far from optimum and does adversly affect accuracy.

Whatever accuracy you are getting now with high quality bullets and carefully taylored handloads, you would get better accuracy with a slower twist rate. The accuracy improvement with less than perfect ammunition would be even more dramatic.

Please research the twist rates used by benchrest shooters using 125gr +/- .30 caliber bullets. As I said before, you will find that these guys are using twists from 1:15" to 1:18". If 1:9.45" to 1:10" twists were a good idea, these guys would be using it.

Yeah you could do all of that but why? At 300 yards the bullet is affected by wind and you really have to know your dope to make hits. Mind you I said in the wind.

OP, what in the hell are you thinking????  This is ARFCOM.  More twist is always better here!

Tony

Hi Tony, you're right. I forgot that logic is not allowed on ARFCOM or in the firearms industry for that matter, present company excepted of course ;0). Most 5.56 AR buyers will never shoot anything but quasi M193 55gr .223 Rem spec ammo and would be better served with 1:12" rifling and a SAAMI .223 Rem chamber throat. But all they can buy is 1:8" or faster and 5.56 milspec chamber throats.

Years ago Marty made me a 1:18" .458 SOCOM barrel instead of the 1:14" that is common. It shoots great with 300-405gr bullets. We seem to forget that the original .45-70 was carefully optimized  for its 405gr bullet with 1:22" twist. Later they successfuly switched to 500gr bullets in the same 1:22" barrel. The stability was adequate if not really optimum for best accuracy. Current BPCR shooters use 540gr bullets in their .45-70s at 500 meters with 1:18" twist barrels.

Fascinating read.

I certainly would like to know more.

What are your accuracy goals and at what ranges with 7.62x39?

It runs out of gas by anywhere between 400-500yds.  Wind drift is substantial, to the point that hit probability suffers past 200-300yds.

You end up with 2 mils already at 350yds, so you won't be threading any needles with it even at 200yds, where you already have 1 mil.

Making it more accurate doesn't really do any practical favors for the cartridge, unless just wanting to shoot groups at 100yds.

Quality 7.62x39 ammunition from a good barrel and tight build will result in sufficient accuracy.

The Finnish Rk 92s and 95s I've shot shoot better than any issue M4 I ever had, but they use high quality brass-cased ammo, bullets made by Lapua, and accurate barrels in billet receiver rifles that I would never want to carry.

The Finnish RK does use a 1:9.5 twist but it is not chambered like other 7.62x39 rifles. It is made by Valmet and Sako and uses the "7.62x39 Lapua" chamber. This chamber has been closely copied by McGowan Barrels as their "7.62x39 Match" chamber and by KAK Industries as their "7.62x39 Semi-Match" chamber.

This much superior chamber design, along with excellent quality control, gives these "Finnish AK's" better accuracy potential. The sloppy over-size funnel chamber throat in C.I.P. and SAAMI 7.62x39 chambers (think "pre-eroded") makes really fine accuracy difficult to achieve even with high quality bullets. With good bullets, the too fast twist, but a proper chamber design, only degrades group size potential with high quality bullets maybe 50%-100%. So 1 MOA may be doable, but probably not .5 MOA.

The group size enlargement caused by overly fast twist gets progressively worse as the bullets center of gravety deviates futher from its geometric centerline.
http://bisonballistics.com/articles/barrel-twist-and-bullet-stability.

Why do I care about accuracy? As Townsend Whelen said "Only accurate rifles are interesting." My question is, why not make these 2 simple, obvious changes to substantially raise the performance of 7.62x39 rifles at essentially zero cost?

If you want an honest answer, because there is little to no market for it and it is far from a zero cost operation.

The vast majority of 7.62x39 sold is cheap steel cased ammunition with the velocity consistency of a drunk pigeon.  And most people buying it simply want it to go bang in their old sks, or cheap psa ak. Limited quality ammuntion availability leads to less companies seriously looking at chambering the round in any firearm with an orientation toward precision shooting.

Additionally you would likely be quite shocked on the actual expense involved in any reputable large company making a change to a product in one of their lines.  It's not as simple as, hey bob in r&d, some dude on the internet said our twist should be slower and our chamber dimensions should change.  The reality is you would first need someone in house to propose the change, followed by meeting with a sales and marketing side to decide whether the change was marketable, or whether it would increase sales in any meaningful manner. Then engineering would need to have prototype tooling and samples made, followed by a rigourous testing series involving 10s of thousands of rounds being fired of multiple different types of ammunition to study and compare and contrast accuracy and reliability against the existing product bench mark.   And then if all of that goes well, production tooling will be made and likely a first run test series will be made to confirm that the findings from the prototype run were accurate.

That all adds up to a whole bunch of money in man hours, ammunition costs, prototyping, and tooling. And it's going to be mighty hard for just about any major company to justify these costs if all you really get out of it in the end is a 7.62x39  precision rifle that the majority of purchasers will still only be shooting brown bear through.

Eta: oh and God forbid anyone suggest a price increase to justify the development costs, because the consumer will think you are literally Hitler

I don't follow the logic of a slower twist. I thought some people even wanted a faster twist for heavier bullets. From what I've seen 1-10 is about perfect. Guys are shooting close to moa with cheap steal cased ammo. You can't do that with 223 or 6.5 grendel.

Oh really? Poppycock. The superior accuracy and safety of the Lapua style chamber throat with ALL 7.62x39 ammo was fully tested and well documented decades ago. All Valmet AK style 7.62x39 rifles have this chamber.

The 122-125gr bullets in Russian steel case ammo are all less than .90" long. It can be mathmatically shown and has been proven in Benchrest competition that .30 caliber bullets up to 1.00" long will be completely stabilized with 1:18" twist, much more so with a 1:16" twist. Even 1:12" twist will stabilize 180 gr .311 bullets accurately. If any company cared to test the viability of a 1:16" twist with Lapua style chamber throat, they could order a custom barrel from McGowan Precision Barrels for less than $300. Rifling tooling and chambering reamers regularly wear out and get replaced. Otherwise we'd all still be shooting 1:12" twist barrels in our .223 AR's.

So please don't delude people here about the mythical "insurmountable cost" of these kind of changes.

The poor quality, usually exhibited as out of balance, of Russian bi-metal bullets is why going to a slower, more optimum twist, is so valuable. The poorer the ammo, the more its accuracy will be improved by not spinning it any faster than necessary for gyroscopic stability.

These changes would "raise all boats" - high quality ammo and low alike. But the more imperfect the bullet, the more the group dispersion would be reduced by going from 1:9.45" to 1:16" twist.

Don't take my word for it. Do the research yourself. Nothing I've said is any kind of deep, dark secret.

Let's not get too testy here.

Most people that shoot 7.62x39 are just blasting away with AKs, SKSs, and now AR15s.

It's the most common spent (steel) case I see in local shooting areas, followed by .223 steel case.

For the people that care about accuracy and precision, they normally shoot better cartridges that don't go subsonic at 500yds.

The only thing 7.62x39 needs is a faster twist and factory subs.

If the bullet were available in the correct diameter, the normal 1:9.45" (1:240mm) twist should stabilize the equivelent of a 220gr Sierra Matchking down to sub-sonic muzzle velocity.

Bullet wt: 220gr
Bullet length: 1.489"
Ballistic coefficient (below 1700fps) .608
Muzzle Velocity: 1050 fps
Twist: 1:9.45"
Calculated stability = 1.50
http://www.bergerbullets.com/twist-rate-calculator/

This is a tech forum so I won't start a pissing match with you.  I'm simply giving you the reality of product line changes for large manufacturers, but you are more than welcome to see how far you get trying to convince one to take up your cause.  Or if there is such a large unfulfilled market for a slow twist, match chambered 7.62x39 why not just fill the demand yourself, I mean it's only a $300 barrel, some rifle tooling and a reamer right?

When time allows I'm going to try and work down to a subsonic load with AA1680 and the 200gr Lapua D166. I believe this is the heaviest bullet available in the .310-.312 class and am hoping the load will eventually be something similar to a 300BO with ~200gr bullets.

Whether the final 1,000fps load will cycle an AK-103 or AK-104 clone or my 7" AR upper will remain to be seen.

ETA: as to your original question of why did the Russians keep the fast twist rate...could it be akin to our military using the faster twist rate to ensure stability with the "long for weight" tracer bullets? I've no experience taking apart and measuring/weighing a Russian tracer round, but this might be a real world reason for why they kept the sub-optimal twist rate for the stubby 123gr FMJ bullets.

I think magazine length will be your greatest hurdle. The x39 case is alot longer than the Blackout case. The AK mag is around .100" shorter than an AR mag. So you'll be able to use longer bullets in the AR. Still, you can only seat as deep as where the ogive starts. Look for the longest, heaviest bullet whose nose section beyond the full diameter shank is less than .770" long.

I believe that the reason for the fast twist was a desire to use the same rifling equipment as the Mosin Nagant, which originally needed the fast twist for its original 210 gr bullet.

I agree, lets keep it technical. The "match" chamber throat is no more than the throat design used on almost all American cartridges developed since 1940, .308, .243, .223, etc.

I ordered a custom AR barrel to these specs the first week in Novemeber. I should have it by late spring. If I like this barrel maker's execution, I'll order myself a couple of more AR barrels and one for my CZ 527.

I think even the most dull witted blaster would pick the rifle that grouped better with all ammo, all other things being equal. Perhaps, if we are lucky, in a few years some small barrel maker will decide to make these barrels a regular product instead of a custom proposition. John 1:23

While your twist rate:bullet weight:cartridge comparison in the OP is an awesome reference, it doesn't mean your 'belief' and MK's decision making process are congruent, haha.

While on the surface it might seem that simple, I find it unlikely a nation which decided to undertake the army-wide manufacture of a a very different rifle would cut corners when it comes to rifling "because we already have machines that do that twist rate".

I mean, it's not like they just took the Mosin or SVT-40 barrel and chopped it here and there to make it fit an AK47 front trunnion.

You may very well be right, but I find it unlikely that an entirely brand new rifle design would skimp on what is arguably one of the most important parts, especially since they went from MK's stamped receiver design to a milled receiver for the type 1-3 rifles once they figured out their sheet metal production and stamping processes were not up to the specs needed to achieve his initial design.