Hello All - I have a question regarding cartridge pressure limits and the guns they were designed for. I'm specifically looking at the 7.62x39, as I have a Ruger American Ranch rifle that is a really handy little gun. In researching loads for the 7.62x39, most all of them top out at around 50,000 PSI. Is it safe to try to load this round to higher pressures, say 60,000 PSI, for use ONLY in this rifle? I understand that this would be most likely catastrophic if used in an AK or SKS, but the American is a modern, strong bolt action that is chambered in other cartridges that go as high as 65,000 PSI. I know the 45/70 has many different levels based on the guns it's chambered in (trapdoor vs modern lever action), so I'm wondering if the same would apply here.
Is the weakness in the brass, or is the pressure kept lower because of the guns normally chambered for it?
Thanks
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 grainsLapua 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 BrandsThe 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 LoadBullet - 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.