firearms proof testing pressures??

[g.willikers]

What increased pressures are firearms proofed tested at?
Fuel lines, compressed gas bottles and fire extinguishers are proof tested at, (if memory serves) something like 5/3rds working pressure.
Are guns proofed at anywhere like that amount?
Wiki sez it's somewhere between 17% and 30% over.
Doesn't seem like a huge amount, though.
But even so, why are we overly concerned with being so careful with our reloading ammo going over the maximum load data on occasion?
Curious minds want to know.

[RC20]

That's a good question or questions.

To add some into this, when the military does the bolt proof testing on the M16/4 bolts, they magnauflux it to see if its been damaged.

You can add into this that I recently saw the remains of a blown up gun on the range. Blew the top right off the receiver (modern gun, not sure how modern_)

Upshot is you can blow up a gun and it is not necessarily going to blow the bottom out of the magazine.

My take is if you do a proof shot you should also magnalfux and see if there is any developing damage (I doubt they sell the guns after but...)

[wogpotter]

I think you're falling prey to the percentages thing a bit.

Most of the tanks you're discussing run between 100 & 2475 PSI, so a 5/3 test is actually 166 ~ 4125 PSI. That's a 66 ~ 1650 increase (or "proof" overload).

Now I know "PSI" in firearms terms isn't the same as actual "Pounds Per Square Inch" of a fire extinguisher or SCUBA tank, but the pressure & the shock of climbing to that pressure darn near instantly isn't just higher, but orders of magnitude higher.

Lets take a pretty average middle of the road load like the .308 Winchester. It's a 62,000 psi (430 MPa) max safe load according to SAAMI.

Now lets overload a "blue pill proof load" by 30%. That's an 82460 PSI load, an increase of 20460 over max!

Plus the heat & shock of the instant pressure jump (SCUBA is filled slooooooowly immersed in water for cooling) & add a few other test for stress items like a greased case just for giggles & its way more of a test than the 17~30% would indicate till you run the numbers!

[Unclenick]

The proof pressures are 125% in Europe and under SAAMI they have a range of 133%-149% of MAP depending on the cartridge.

Here's the basis. If you look at an N-S curve, which is the number of cycles to fatigue failure vs the peak stress in each cycle, you find a level of stress that lets steel survive, say half a million cycles, when increased by 40% drops to only about 1000 cycles of fatigue life. Loss of cycle life with increasing stress is exponential. Since proofing is supposed to be a non-destructive test, the thinking is that it shouldn't shorten the remaining normal pressure cycle life appreciably, either, which is why the numbers aren't bigger.

In the U.S. a gun may be proofed only once, if at all, by the factory. Maybe a second time by a gunsmith who makes a serious modification. In Europe it used to be (not sure if it is currently so) every gun had to be taken to a proof house every time it changed owners or saw a gunsmith. So a gun that was bought and sold several times, might have half a dozen or more proof loads fired in it over time, and thus the lower CIP level of proof load prevents that from taking away too much cycle life.

It should be understood that the pressure we see listed as SAAMI Maximum is not an absolute maximum. It is called the Maximum Average Pressure (MAP), and as its name implies it is an average, and individual rounds that go into making up that average can, it turns out, be up to a little over 18% higher than the average if the others are uniformly enough lower that the average doesn't exceed the MAP. This limit is governed by a number called the Maximum Extreme Variation (MEV). The CIP only allows 15% above the average for an individual round in a test sample, so it's a slightly tighter spec.

Also, it's important to know the MAP number is only an average of 10 rounds taken under specified conditions. Because 10 is not a large sample, an allowance has to be made that the next sample of 10 might average randomly higher. They have a second average pressure number for this called the Maximum Probable Lot Mean (MPLM), which is two standard errors higher than the MAP, based on a standard deviation of 4%.

Further, it is important to know that the MAP and MPLM only apply to a freshly loaded lot of ammunition. Because of "cold soldering" and corrosion tending to increase start pressure over time, they have to make an allowance for pressure to go up some as a lot ages. Therefore they have another number called the Maximum Probably Sample Mean (MPSM) that limits how high a pressure a sample of ten rounds from the same lot can average at some time years down the road as the lot ages toward the end of its life.

[RC20]

My question is how much of an overcharge would you need to blow the top of a receiver off?

In this case a Model 70 (era unknown but looked 80s on) Weatherby 300 mag (unknown if original Winchester or converted)

[michaelcj]

RC20….. I think the only definitive, short answer to your specific question regarding the Mod. 70 .300 is:

"It depends"

[Jim Watson]

From kaBoom reports, I would think a 75 to 100% overload would be sufficient.

[briandg]

Unkle put some good accurate information there.

One thing that is missing is that proof of a cord, cable, chain, hose, etc, is all based on static numbers. 100 pounds will break that rope, 400 on the chain, etc. A firearm isn't any bit that precise. You have a high spike of pressure at first, the pressure tapers off as the barrel does, approximately. Just look at a standard rifle to see the approximate curve representing needed strength.

The problem with thinking in a straight line for proof pressures and max pressures is that there are too many variables in shooting. Another problem is that there are so many variables in firearms themselves. A proof load must be calibrated to give a clear indication that the firearm is capable of not only holding the peak pressure in, but also maintain pressure integrity throughout the whole trip down the barrel. The proof test isn't just about whether the barrel splits, it is also testing bolt lugs, receiver integrity, etc. Lots of small things can fail during an over charge shot.

as nick said, though, what must be worried about more than anything is the slow degradation of structural integrity. You can run over maximum loads in a gun for years without blowing it up, and actually be slowly tearing it apart. The bolt lugs and the receiver slots can be peening, fatigue cracks can be forming in various places, etc. We can understand and accept that a throat will eventually wear out, that a .458 will eventually loosen in the stock, and many other things, but it's a little harder to understand that your steel parts may eventually loosen and stretch as a constant diet of overpressure rounds are fed through it.

One of my friends when I was young had his grandfather's old military 38 revolver. The thing had splits in the barrel by the cylinder. He had +p ammo in it at the time, but it also turned out that the cylinder was straight bored. I don't know what caused the cracking. Obviously this 60-80 year old pistol was made of weaker steel. The cylinder may have had .357 rounds run through it, there may have been too many +p, the thing may have had a barrel obstruction, etc. The point is that this thing appeared to have very serious material failures, but it has not had a catastrophic failure. Most people look for only a Kaboom, and never consider that there could be other, serious damage occurring in longer term.

Why would a .300 magnum go into suck spectacular catastrophic failure? Did the guy forget to clean out his powder measure and dump a half load of bullseye into his first charge?

[Unclenick]

And once you get a crack, the stress risers at the ends make it take a whole lot less pressure to go on to full failure than an uncracked gun requires.

Actual blowups take very different amounts of pressure depending on the shape and thickness of the steel. Board member Clark blows guns up as part of his professional work. Check with him about the range he's seen.

BTW, if you look at pressure vs. barrel length after the pressure peak, it actually is an inverse exponential curve rather than a straight line because as the pressure drops, the temperature drops, lowering the pressure still further.