Home Forums Hunting & Shooting Ask The Gunwriters Cartridge Pressure Limits

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 have worked up slowly for other rifles and have not gotten much more speed.

The boiler room is just not big enough to get large boosts.

Part might be the rifles that were chambered when introduced.
I think that the shape of the case might have something to do with pressure limits.

You could try, but I don't think the case capacity will allow you much gain. You can certainly experiment with some Quickload data. I would think your Ruger American is also a .308 ( not a true .311) and the 110 Barnes would get you some speed?

Two comments:

First the SKS's and AK-47 locking systems depend very much on rounds developing approximately the same pressure. If they don't, then the rifle won't function correctly. This is generally true of semiauto and full-auto rifle actions, and since pressure often varies both due to amibient temperature AND a heated-up barrel, it's better to have a lower pressure standard to insure correct function.

One general rule of internal ballistics is that muzzle velocity increases only about half as much as pressure, though that depends to a certain extent on whether the powder is single- based or double-based. As a result, increasing the pressure from 50,000 PSI to 60,000 won't generate a proportional increase in velocity.

GunLoony - your question makes sense, and with some cartridges that's a reasonable thing to do. You can definitely make more velocity with higher pressure, obviously, and your rifle can obviously handle it given the other cartridges it's chambered in.

However, in 7.62x39 the weak spot is the brass. You can turn up the pressure a little bit and should see some gains over factory ammo, but won't get to full pressure capability of the rifle before you see brass damage. The internal web of this case is very short, so the brass will swell around the case head with high pressure, and if you go far enough it can look like a belted magnum. Obviously that's very bad and if you see that, back off a bunch. The same thing is true for the Grendel, since it was made from the 7.62x39 (with a couple other cartridges in between along the way).


Now if you want to be a true rifle loony - run a Grendel body reamer into that chamber to create a 7.62x39 Improved for more case capacity to go along with your higher pressure loads. (Seriously though - this is possible, but research reloading dies for this chamber first.)

Thanks JB - I'm using CFE BLK, which is a spherical powder, which I'm assuming is double-based. Not looking to really hot-rod too much, just would like to get a 150gr bullet to around 2300 - 2400 fps.

I'm using 24.5gr behind a 150gr Sierra FP, and I'm getting about 2000 fps out of the short 16 inch barrel.

Thank Yondering, I'll keep that in mind. I knew the Grendel was based off the 7.26, but didn't know about the web being short.

I've checked with Ruger and they say it's .311.....

It shoots pretty well with .308 bullets, but noticeably better with .311s. I'm using a .308 Sierra FB designed for the 30-30, just looking for a little more speed than the current 2000 fps. Not quite sold on most of the .311 bullets (except the Barnes) for deer-sized game yet.

GunLoony88,

Yep, all spherical powders are double-based.

I have a Davidson's limited run Ruger 77 MK II stainless 7.62x39 that I bought new in about 1992. I use it now as a cast bullet rifle, but over the years I've tried many jacketed bullets in it. The rifle has a 22" barrel. While I haven't loaded any jacketed bullets recently, my notes indicate a muzzle velocity of just under 2300 fps with a Hornady 150 grain round nose .30-30 bullet using a maximum load of H322. Some of the newer powders might safely beat that velocity, but probably not by much.

When I loaded for the guys I hunted with I was very leery of pushing velocity and pressure in those rifles. I packaged the loads with the exact data on the box. I carefully explained that these loads were for that rifle and that rifle alone and that not only should there be no expectation of any accuracy whatsoever in another rifle, but that the loads could very well be unsafe in any other rifle. I did stay inside SAAMI specs for OAL and velocity (pressure). My own ammo in my own rifles I have on occasion pushed a little on OAL and pressure searching for best accuracy. It makes me twitchy that someone could get hold of some of my reloads and put some of them in the wrong rifle even though all my ammo is labeled for the specific rifle it was developed for. Better for rifles with good reason not to push loads in that we don't do that for the reason that it may wind up in a weak rifle. In most of the rifles I own they have plenty of power for anything I might choose to do with them and there is zero NEED to push them. The couple of rifles that prefer monos to be up tight against the lands remain a problem though.

I find that as I get older, anything that pushes the limits becomes much less attractive to me. There's always a good way that is safer to get there it seems.

^^^^

Amen

I was doing consulting work in the 1990s designing power supplies. My father was chief engineer over 150 engineers and draftsmen in a military gun and vehicle company that did research and some production for Detroit arsenal and Rock Island arsenal.

My father and I were arguing about design since I was a little kid. I made a lot of money blowing up my power supply designs to learn the weak spot, and improving that. I started handloading in 1999 and within days I was trying to blow up guns. I tried every bullet and powder combination to the point of brass failure in 9mm.

My father was disgusted with me at first for blowing up guns. When I showed him how I documented the process he calmed down. He then showed me how to calculate the strength of my 45/70 handi rifle: hoop stress, Lame's thick walled formula, section modulus, pins in double shear, etc.

Knowing the Rockwell hardness of a steel and it's thickness, you would think I could predict the threshold of failure in the gun. But tables of steel strength are for static loads that grow. The one millisecond pressure spike in a chamber never gets it done. Guns typically take 50% more peak pressure to wreck than where they pencil out.

Thousands of overloads later [5 years later of buying carloads of guns at every gun show], I have seen a lot of patterns and can predict what will happen. I had a mechanical engineering professor tell me in college, "if the rocket blows up on the launch pad, you don't learn much." In blowing up gun, a slow gradual work up shows the threshold of failure. It also makes the primary failure easier to find. Blowing a gun up with some random overload will not reveal as much information. It is dangerous too.

With the 7.62x39mm I hoarded RP brass with the small primer thinking it would go 80k psi and the Win large brass would give up at 60k psi.
I was never able to reach pressures where the RP had an advantage.
The SKS rifles would blow the firing pin back out of the bolt assembly.

I gave up on overloading sks rifles... weaker than the brass.

I gave up on the CZ527 blowing pieces of the bolt face off. Tired of paying for bolts.

The Rem 700, Sav 110, Win 70, Mauser 98 are good rifles are far as the bolt and reciever strength go..... but if the case fails they blow extractor pieces at me. Still plenty good for work ups until extractor groove expansion threshold is reached.

The good rifles to overload are Ruger #1, Sav 219, Handi rifle, ext. A rifle way stronger than the brass, and when the brass blows, no rifle parts or brass pieces comes flying at me.

What did I accomplish between 2000 and 2005 blowing up guns? I wrote to all the load book publishers that said the CZ52 was stronger than the Tokarev. I could not blow up a tokarev. CZ52s blow up with factory ammo. So the load books changed. They reduced ALL loads. CZ52 and Tokarev.

How many scars do you have?

Clarkm - what blew out of the Rem 700 when that failed?

Also do you have data/photos that you can publish?

This is the only pic of my guns published.

I have lots of pics I can post.

I have not blown up a Rem700, just compare the bolt lug cross section to 98 Mausers, which I have wrecked.
I do have a friend who had to go to the doctor when a 223 case failed in his Rem700. That Blue Dot ammo was less dangerous in my Ruger #1 223.

I'd really be keen to see your pics and info on what loads did the destruction, thanks

I think overloading a cartridge is like playing Russian Roulette.

Clark,

That's one of the best posts on the fire in a long time.

Thank you.

I think understanding what you're doing and knowing when a cartridge can be safely loaded hotter is worth a lot more than just blindly following a manual.

The OP isn't asking about overloading a 308 or some other high pressure cartridge. He's asking about loading a low pressure cartridge hotter, in a rifle that is designed around high pressure rounds. It's a perfectly reasonable thing to ask, and is commonly done with a number of other cartridges. The 6.5 Swede and the modern 38 Super +P are good examples.

Seems like we've got a number of anecdotal responses here that have nothing to do with the OP's question.