doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com The Value of Proof

Interesting discussion. We probably need to start with the obvious, which is that there is only ONE peak pressure, and that occurs in the chamber. Pressure doesn't start high, drop, and then go back up. It drops all the way from the chamber to the muzzle. The only difference is in how quickly it drops. Different pressure curves . . . but they're all headed in the same direction.

And it's quite possible to obtain "decent performance" from shells that are loaded well below max service pressure. We don't care what the pressure is at the muzzle. We only care about sufficient velocity to give us sufficient energy (and penetration) to do whatever we want the load to do. For several years, I've used a reload for pheasants in a Federal hull with Fed wad and primer, 22 grains Unique, 7,200 psi. That's well below both SAAMI and CIP max service pressure. Yet it yields 1200 fps velocity at the muzzle . . . and the pheasants don't like it at all. That load may well retain a higher pressure as it moves down the barrel, relative to peak pressure, than does a load that generates 11,000 psi peak pressure. But performance, based on my field tests, is definitely "decent".

That peak pressure is maintained for only a very short distance within the chamber. Obviously for a 7,200 psi & a 11,000 psi to end u giving the same velocity to the same weight charge that difference has to be made up. Generally speaking this "Makeup" space will occupy the entire rest of the barrel, which may contain on the order of 100 times the length of that occupied by the peak. Thus it doesn't take a large increase in the pressure ""Down the Barrel" to compensate for a major difference in the peak.
One of the main concerns here is to be sure the chosen powder is compatible with the reduced pressures to insure it will burn consistently & reliably in ant temperature the load is apt to be used in, otherwise squibs & failures can occur.
Unique should burn well down to that 7200 psi figure which your results would tend to bear out. but personally I would likely not trust it much lower than that. As I recall the Federal primer is a comparatively Hot primer. This is good when working to minimum pressures. Even though "Mild" primers can lower pressures even more they can also be a major factor in incomplete ignition & combustion at the lower pressures.

So Larry... your pet load seems to be loaded closer to CIP maximum than SAAMI maximum. That was my point in the context of the discussion, not that it's terribly relevant.

Neither CIP nor SAAMI member companies 'load below their rating' for liability reasons. That's one of the purposes of the standards in the first place. They are quite free to load right up to maximum peak pressures, and certainly sometimes do.

We were trying to figure out what the pressure might be at the apparently arbitrary 162mm point which is about 6 3/8" from the breech face and not the 9" quoted in this and other discussions as the mythical 'peak pressure point of a second proof load'.

The intent is clearly to test not only the chamber but the forcing cone to a reasonable distance.

Another interesting thing to consider that was mentioned earlier in this discussion is that since this was not a catastrophic failure, the shooter could just have stopped shooting and all would be well.

He could have, IF he had noticed the problem and heeded the warning of impending major failure.

Well, who would be that unobservant you ask...

I know of three instances where a shooter had ample warning that something wasn't right, yet persisted and had a catastrophic failure.

In fact, I'm of the opinion that a sudden unexpected failure is rare , and that the gun will usually give warning of it's distress before the catastrophic event.

It's up to the shooter to be in tune enough to recognize it, and STOP to take stock of the situation should anything unusual occur.

The only exception might be in combat.

Jones, I think most of us understand that the major ammo manufacturers work for consistent velocity and, as you point out, only worry about staying below the max average service pressure. But I think the Brits may, on average, provide more of a cushion below the allowable service pressure than we do. And I think that's because they have a whole lot more old guns (as a percentage) still in service--including, of course, Damascus--than we do. That probably has something to do with the fact that our guns were designed for rugged use, more on the order of tools, and that they were more likely to be the victims of neglect (not seeing a gunsmith until something broke) than British guns. Vic Venters, in one of his "Gun Craft" columns for Shooting Sportsman (my copy unfortunately "ate" the date of the issue in question!) wrote:

"Although the maximum mean pressures for service loads for standard proof guns are 740 bars, CIP-regulated cartridge manufacturers typically work to far lower pressures--usually between 450 and 650 bars as measured by CIP piezo transducers."

Re my 7200 psi pheasant load: I'm not one who believes in ultra-low pressures for vintage guns. As pointed out above, they can sometimes suffer from inferior performance in cold weather. (I can remember old reloading manuals carrying notations that such and such a load was not a good choice for cold weather.) I've field tested that one down to at least the teens above 0 and it works quite well. Any more, much colder than that and I may very well stay home!

While we don't seem to know what pressure test point is being targeted at 162mm's from the breech, there does seem to be some agreement on what a shotgun shell pressure curve looks like. It might be that 'testing' downstream of the forcing cone is intended to exceed stated proof pressure at 17 and 25mm's.

Say, for discussion, that the second proof load firing is the 162mm test. I wonder what percentage of failures are racked up on that second shot. Rivelling, bulges and a cracked frame? Maybe, there's a possible reason why, 'tightening proof tests in recent years has made it a rather risky business'. Note, no anecdotal or otherwise reports of specific failures at 162mm's. I think other options could be reasonably guessed at.

"It might be that 'testing' downstream of the forcing cone is intended to exceed stated proof pressure at 17 and 25mm's."

It depends what you mean by that.

If you assert that proof chamber pressure is being applied to the 162mm point, no. Each barrel would blow up.

If you mean they are using overall hotter proof loads to assure the 162mm test spec is being met, perhaps.

The rules of proof are fond of the term 'at least' so you really don't know what The Masters intend to do to your gun, do you?

There is certainly nothing wrong with establishing your own lower service pressure, as Larry and many of us do, in lieu of risking some sort of reproof official or otherwise.

Like Miller, I'm glad we have that option.

But then I'm not in the business of selling guns.

The Birmingham Proof House was established by an act of British Parliament. Don't their policies and actions speak to their intent?

I had Toby restore an Edward Lang hammer gun for me. I knew the left barrel was honed beyond 8 thou (out of proof), when we started. It needs reproof prior to sale.
Here is the thing. I wasn't going to sell it. I didn't want to risk my gun at the proof house. My barrels are plenty thick (+32 thou). It is still out of proof in the left barrel. I bought it with a big dent in the left barrel. I raised the dent. I then used it to practice browning. It came out great. So, I shot it. It shoots great. It needed the left firing pin sorted out as the nipple was prone to fall out. (puns expected)
I now have a 30" E. Lang hammer gun with IC/Mod chokes. It is a a great gun.
At my request, Toby did not submit the gun for reproof. This was my choice as the owner.
I would never offer this gun for sale in the US without the disclaimer that it is out of proof. It's a keeper.

Joe