7.62x39 in a .308 Barrel

[405grain]

50 Yard Targets.JPG

Well I'm back from a day at the range. Even though it was 101 degrees I'm not going to blame the weather; I shot more poorly than usual, but that's OK because I shot both types of bullets equally as bad so the comparisons are valid. I shot several groups at 50 yards with both .309" sized bullets and .312" sized bullets from two different rifles. One is a 94 Brazilian Mauser with a 30 caliber barrel that I turned from a hammer forged blank and chambered with a Lapua reamer. The other is a 93 Mauser with a barrel that I got from Numrich. Here are the average groups sizes for these rifles:

1894 Brazilian Mauser - 50 yards - .309" bullets -1.56", .312" bullets - 1.84"
1893 Mauser - 50 yards - .309" bullets - 3.75", .312" bullets - 2.75"

Not surprisingly, the hammer forged barrel shot better than the Numrich barrel. After firing eight groups at 50 yards the results showed that the 94 Mauser shot better than the 93 Mauser. The 93 Mauser shot groups an average of 1" smaller using the .312" bullets, but the 94 Mauser shot tighter groups over all. The difference in group size between .309" and .312" with the 94 Mauser was only about a quarter inch, and this could well have been caused by the shooter as much as the ammo.

At the 100 yard range I really stunk. I was shooting the 94 Mauser and I only shot one group with each lot of bullets. I'm ashamed to say that I shot both groups at around 4.5" in diameter. There was no difference in the accuracy between the .309" bullets and the .312" bullets at this range. In spite of my bad shooting today the results seem to suggest that getting better accuracy by increasing the driving band diameter relies entirely on the gun that you're shooting. I know that this test was a limited data pool, and is semi-inconclusive, but in this shoot one rifle shot better with the larger bullets, and the other didn't. I would have to guess that individual shooters will have to determine which diameter works best in their particular firearm.

[Mk42gunner]

50 Yard Targets.JPG... I would have to guess that individual shooters will have to determine which diameter works best in their particular firearm.

And this is why when you ask ten shooters for a good load for whatever cartridge, you get fifteen answers.

Kudos to you for going to the range when it is 101 out. Way more dedicated than I am now.

Robert

[Driver man]

I shoot a Norinco bolt action rifle and of late developed a very accurate load. 1.6inch at 100 m Boollit 311291 powder coated gas checked sized .314 weight 174 grains ,C.O.A.L. 2.133 inches. powder 28.00 grains W748 . The Norinco likes the fatter boolits

[stubshaft]

A few years ago, I bought an Encore pistol barrel from MGM chambered for 7.62X39 which also had the diameter .312" stamped beside the caliber designation. The guy I bought it from had never gotten the accuracy he believed the barrel was capable of.

Once I got it home, I slugged the barrel and lo and behold it mic'd out at a solid .308". I called MGM to complain abd they stated that "since I wasn't the original buyer there was no warranty"! Until that time, I had nothing but positive resuts from the many barrels I had bought from them. After that, my confidence in them has waned.

BTW - Shooting RCBS 165-SIL bullets sized at .309" gets me 1.25" groups at 100 yards.

[MT Gianni]

I had a Handi Rifle with a 7.62x39 chamber. It measured 0.308". It shot fine with cast. Don't overthink things, just develop a load your gun likes.

[405grain]

MT Gianni: Can't help it. I'm an engineer. Overthinking things to see if they will work is what I do.

[lar45]

Hi 405grains, thanks for posting the info.
I just wondering what would happen if you ignored the COAL and seated the bullets out until they touched the lands?
They might not feed in a semi auto or fit the magazine, but if you are testing accuracy? As long as you have everything setup with .312" and .309"?
Way back when Recluse gave me the go ahead to make 45-45-10 and sell it, I was playing with bulk wax vs the cans of Johnson's and adjusting proportions... I was using the RanchDog 165 in my 30-06 and was getting okay accuracy until I started seating it farther out to just engrave into the rifling.
I've got a picture saved someplace. I still had some WC820 back then...

[MT Gianni]

MT Gianni: Can't help it. I'm an engineer. Overthinking things to see if they will work is what I do.

Sorry I spent 45 working years explaining the real world to engineers.

[405grain]

I sent in an order for a revised bullet design to Accurate molds today. Learn from the mistakes & make the improvements. If it works out, the revised design should address the problems associated with the driving band diameter vs the bore riding nose.

lar45: I chose the RCBS 30-165-Sil for testing specifically because the nose would be forward and in contact with the lands. This bullet is a bore riding design, and I was seating them longer than the COAL. Because the cartridge length was too long to load them into AK magazines, I had to single load them into the rifle that takes removable magazines. Going over some old target photos I see where that particular rifle was shooting 1" groups at 100 yards with jacketed bullets, so something is decidedly wrong with the groups that I shot during these last cast bullet tests. I was shooting two different rifles and both of them shot poorly. Either there was something wrong with my ammo, or else I was way off my game that day.

MT Gianni: I don't just give up because things get hard. If you want good results you have to put in the effort for them. There's no guarantee that any of this will work, but there's always a guarantee that they won't if I quit without trying.

[lar45]

405grain: In your notes you are talking about keeping the gas check inside the neck. I understand this for a wax based lube, but for powder coating is it necessary?
Could the pressure from the gases knock the gas check off? Or is it potential damage to the sides of the bullet at the base from the pressure?
With the 30XCB project it was noticed that as velocities got over 2800-2900fps that the gas checks were falling off. My thought is that the lube was building up at the gas check going down the barrel and hydraulically swaging the base of the bullets smaller. We shot some bullets into a 6' long stack of saw dust in cardboard boxes, then Tim took pictures under his microscope and measured the bases, they were smaller.
just trying to understand more of the process.

[405grain]

It's a generalization, but at muzzle velocities above 1600 fps it's beneficial to use gas checks. It's not so much an issue with bullet base deformation as it is with gas blow by. When hot high pressure gas (basically a plasma) gets past a bullet the flow will vaporize material off the bullets surface. We know that some gas can flow past a bullet, even under perfect conditions, because of high speed imagery. If you watch a high speed video of a bullet exiting the muzzle of a firearm you will see that there's always a small cloud of gas that exits the muzzle before the bullet does. An excess of this escaping gas at elevated pressures can and does erode the surface of the bullet, plus when the vaporized materials from the bullet cool they will condense and begin plating the inside of the bore. Once enough material accumulates on the inside of the barrel it will begin galling against each passing bullet and will cause a build up of barrel leading.

The main purpose of a gas check (as the name implies) is to create a stronger gas seal to prevent the negative effects of gas blow by. If a bullet is seated with the gas check below the case shoulder there will be a section of the bullets driving band (or shank if you prefer) that is going to be exposed to these high pressure gases until the bullet moves forward enough for the gas check to enter the chambers leade and begin forming a seal. This may not present a barrel leading issue, but will erode material on the bullets driving band near the bullets base, and this may have a detrimental effect on accuracy. It is unlikely that a properly crimped on gas check will come off in flight or inside the barrel.

Cast bullets can have semi-predictable behavior up to velocities around 2100 to 2200 fps. At jacketed velocities of 2800 to 2900 fps all bets are off. Weird stuff is going to happen when you push the envelope. When pressures get up to around 50,000 psi lead alloys can start to deform. Even the accelerations at that velocity can have a hand in bullet deformation. There's so many possible things that could happen that prediction would be folly, and the best indicators of what's occurring with your loads would be through observation of the results. Even the textbook remedies like "just use a harder alloy" don't take into account that at some point harder alloys are going to become brittle. Accuracy at jacketed velocities with cast bullets is a tough nut to crack, and is beyond my scope of knowledge, but I'm certain that there are folks on this site that are way more knowledgeable than me on that subject. I suggest an internet search of "cast bullets at jacketed velocity" and see where it leads to. There should be information on this site as well as others. As for me, if I can shoot a 1 MOA group at any velocity I'm smiling ear to ear.

[lar45]

This is 10 shots at 2490fps into less than 1", this includes the cold bore 1st shot. The bullets are Linotype.
I can get up to around 2800fps with my 14 twist Krieger barrel. Anything less than Linotype can slump to a full wad cutter in the barrel with enough acceleration at extreme velocities.

cblue-3006xcb-02.jpg

[ACC]

My Ruger Bolt Action 7.62X39 has a .308 bore and it shoots .311,.310, and .313 bullets just fine, although I prefer .308 myself.

ACC

[atfsux]

My only experience with rifles chambered in 7.62x39 with .308 bores is all jacketed and not cast. I think your 2 Mausers are strong enough that there should be no danger if someone later after you are gone try shooting .311 jacketed bullets through them.

When Ruger first started making their Mini-30 in the 80s, they did so specifically from the viewpoint that there wasn't a wide variety of .311 projectiles for handloaders and decided to put .308 bore barrels on their rifles so that loaders could use the more readily accessible .308 diameter pills. But Ruger didn't anticipate how Bubbas would blow cases of cheap full-pressure ChiCom surplus ammo through their rifles, and they started getting a lot of warranty returns from over-pressured instances. After about 3 or 4 years, Ruger changed over to .311 barrels to stave off such a high rate of returned rifles. I think the problem was more related to ChiCom quality control practices than anything else. But that one change did seem to reduce how many rifles came back for service.

[405grain]

atfsux: Some of that Chinese milsurp ammo was made with copper plated mild steel bullets, so I'm not supprised that Ruger had problems. You are correct in that small ring Mausers can handle the pressure from shooting .311 lead core jacketed 7.62x39 bullets through a .308 barrel. Tons of guys that did a small ring Mauser conversion shot even more tons of steel cased com-bloc ammo through them.

lar45: that's super impressive. I have a 7x57 Mauser that can shoot groups like that with the RCBS 7-168-Sp and IMR-3031, but I've never shot it at velocities above 1850 fps. When I say "groups like that", I mean that I've been able to do it on three occasions out of at least a dozen range sessions. Folks that can shoot cast groups at high velocity like yours are mystical wizards who are keen to knowledge that us ordinary shooters don't possess.

[OTShooter]

It wasn’t the bullets - copper washed (not plated) steel bullets are essentially as soft as the brass used in “traditional” bullet jackets. The Ruger issue was Chinese hot ammunition and variable gas pressure (often well above the CIP standards). The Chinese didn’t grace us with surplus ammunition, they dumped out-of-spec stuff that AKs would handle without anyone noticing issues.

[405grain]

It took a few months but I've finally received the bullet that I designed to see if I could get better performance shooting the 7.62x39 with a Lapua spec chamber and a .308" bore. This design (Accurate is calling it the 31-161A) is made specifically for powder coating, and has a driving band that once powder coated should be at .313" diameter, while the bullet nose should be at .300". It will be a little bit before I begin testing with this bullet as I've got my hands full testing another design that's made to be a 30 caliber powder coated bore rider. Experimenting with the 7.62x39 in a .308 bore is basically a side project at this time.

After the last session that I had at the range (which resulted in some miserable groups), I think that I might have discovered why I was shooting so poorly (or at least more poorly than normal) After I got back home and was cleaning both of the rifles used in that test I noticed a film on the inside of the barrels. This wasn't typical barrel leading, but was instead a very thin blue/grey film that was in the rifles bore. No smeared or streaks of actual lead, just a very thin film.

When I shot those groups I used the .309" sized bullets first, then shot the .312" sized bullets afterwards. It is possible that because all the bullets in the first session had driving bands that were .003" less than the chambers throat, I might have been getting hot high pressure gas blowing past the bullet until it sealed in the rifling, and this could have been vaporizing lead and PC off of the bullets that was condensing and plating in the bore. The bullets sealing the bore once they got passed the leade might have prevented full on barrel leading. It sounds far fetched, but there were two things that did happen that day; 1. my shooting was awful. 2. Afterwards I found that there was some kind of film left inside the bore on both rifles. Either way, no alibis - I just didn't shoot good that day.

After I get done testing the other bullet that I'm working on (the Accurate 31-159A) I'll start trying out the 31-161A and see how it works in the .308 barreled 7.62x39's.
31-161A.png