Home Forums Hunting & Shooting Ask The Gunwriters Cartridge Pressure Limits

I hope this helps with your question. As you are no doubt aware, operating pressures have been steadily increasing since the introduction of smokeless powder. The evolution of cartridges and the pressures they develop are influenced by several thing - the bullet (cast or jacketed, RN vs pointed), the case, the mechanism strength (93 Mauser vs the 98, for example), and propellant makeup.

In the old days, we had weaker actions, weaker steel and the chemistry of smokeless powder was primitive. Maximum pressures were lower - determined by the technology of the day. But we kept experimenting and we progressed. We made stronger firearms, better metals, put jackets on lead, and improved propellants. To push a bullet faster, we needed more pressure! Look at the pressure generated by older cartridges like the 30-30 or the 30-40 Krag, and compare them to what the 6.5CM is loaded to.

When smokeless arrived, engineers understood what made BP cartridges function best, but they were forced to think outside the box for the new smokeless powders. They had to create different bullets, better case designs to burn the new propellant, different primers and stronger firearms. BP didn't burn in the same way as smokeless. You can tell by looking at the cases. Straight walled, large bores, or droopy shouldered cartridges like the 30-30 really tell us what the thinking was back then. What was good for BP wasn't always the best for smokeless.

Leaping forward about 50 years, we have the 7.62x39mm. It was developed during WWII. The pressure level was set at 51KPSI because of the time in which it was produced and the mechanisms that fired it.

Can you load the pressure up? Yes. Especially in the American because the mechanism can take more pressure than what the 7.62x39mm is rated for. Should you? I'm going to say no, and here's why. A few years back, I did a comparison of six 7.62x39mm cases.

Smaller cases will see faster, more aggressive swings in pressure when you light the propellant. It's made more problematic when you consider that there are noticeable volume differences between the various cases. These differences can take a safe load to unsafe, just by switching the brass you use. You will get a large change in pressure for not much of a velocity increase.

Case Capacity – water weight in grains

Lapua 33.6 grains
Prvi Partizan 35.2 grains
Igman 36.0 grains
Federal 37.6 grains
PMC 37.6 grains
Winchester 37.9 grains

Pressure Differences Caused by Changing Case Brands

The powder charge, bullet and primer remained the same, but look what happened to the pressure when the case capacity was reduced. The 'Pressure by Case' goes from the most to the least internal volume. As case volume decreases, pressure increases.

These volume differences show why you should always start at the minimum load listed!!

The Load
Bullet - Lapua FMJ
Primer - Winchester Large Rifle
Powder - 28.5 grains of AA1680
Cartridge over all length of 2.200 inches

Maximum Safe Pressure - 51,488 PSI

Pressure by Case

Brand -- Pressure (PSI) -- Percentage of case filled
1. Winchester -- 50,264 -- 86
2. PMC -- 51,150 -- 87
3. Federal -- 51,150 -- 87
4. Igman -- 56,522 -- 91
5. Prvi Partizan -- 59,701 -- 93
6. Lapua -- 67,921-- 99

Is it worth dumping an extra grain or two of powder into the case? Not really.

Thanks all for the comments and past experiences - that's why I asked the question, as I knew someone that was smarter than me could explain the do's and don'ts.

With that said, I'm not really trying to hot rod the 7.62x39, just trying to determine if its safe to increase pressure a bit based on firearm strength vs brass strength. As reloading loonys, we all like to tinker with things - that's why we reload :-). I have plenty of other guns in more powerful chamberings, so I completely understand if I really want more power, get a bigger stick!

In the end, think of the example I mentioned before - the classic 45/70. If someone didn't "go off script", we wouldn't have 3 different load levels for it, we would be stuck with the Trapdoor loads (not that there's anything wrong with those!) I could have used any cartridge that was deliberately loaded to lower pressures due to the firearm it was designed for....say the 280 Rem, 7x57, etc.

Thanks again for all the comments and information - that's why I'm here - to learn for those who have "been there, done that" :-)

Merry Christmas and Happy New Year to all!

Your question is perfectly reasonable, and the short answer is - yes, within reason.