doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2.75" shells in 2.5" chambers

Raimey,
Based on the results stated here by some of the low pressure loaders that use very slow powders, I'd say it's likely the low end pressure/efficiency issue is dependant upon the burn rate of the powder. I've been using a bit faster powder than some others but loading to similar pressures and velocities and have noted relatively good powder burn, albeit not as clean as a higher pressure load using this powder (Hodgden Clays).

ChuckH:

Thanks for the info. Regarding posted pressure, for every cartridge a manufacture makes do they fire the cartridge in a test gun to get the pressure or it is a calculation. group velocity outside the barrel isn't that difficult to measure but I was curious about their pressure values like you would find here:
http://www.tiropratico.com/ricarica/manuali/REX%20Nitrokemia-.pdf
Is all the given data from measurements?

Kind Regards,

Raimey
rse

Here is the procedure for testing ammunition http://www.hpwhite.com/101-00.pdf

Based on ANSI/SAAMI Z299.2-1992 which basically says for 12ga,

Maximum average pressure: 11,500 psi
Maximum probable lot mean: 12,100 psi (a calculated figure)
Maximum probable sample mean: 13,000 psi (a calculated figure)
Proof loads (AKA Blue Pills) are 19,000 to 20,500 PSI

A selected number of shells from each production run are tested to provide a satistically meaningful result. To not do this is inviting litigation. There are many examples of ammunition makers issuing a recall.

The procedure for testing firearms is here http://www.hpwhite.com/100-00.pdf

If you want the complete ANSI/SAAMI specification it is for sale here: http://www.saami.org/Publications.cfm

Pete

The local CIP proof house does actual tests on batches of locally produced and imported shells. They say the local production is more consistent than some imported brands.

JC

Re hull length in shorter chambers and resulting pressure increases . . . In one case, Bell got a pressure increase of almost 1,500 psi. But that resulted from a 3" shell being fired in a 2 1/2" chamber. I'd guess that those instances in which extreme constriction takes place, apparently due to very short forcing cones and hulls that are too long--resulting in ends blown off the shells and wicked recoil--there might be similar pressure increases.

My recollection of his article is that the only shells that blew off part of the crimp end were extreme mismatches. 3-1/4 or 3-1/2" shells in 2-1/2" chambers. I don't think he even mentioned recoil. He maybe did not shoot the shells from his shoulder, either. I don't have that magazine anymore. Assuming that the small increase in pressure that MIGHT occur when firing a 2-3/4" shell in a 2-1/2" chamber, because the extra length ends half-way up a 1/2" long forcing cone MIGHT cause an increase in velocity and would therefore cause an increase in recoil is as silly as the long forcing cone salesmen claiming the longer cones cause NO loss in velocity but magically reduce recoil. Ditto the same claim illogical for backboring. BS.

Jim,
I think you're right about not shooting it from the shoulder. I believe it was a laboratory bench mounted pressure gun.

One unknowable variable in this entire equation that no one brings up is an individual gun's history. I hasten to think that a 100 yr. old gun has likely had any number of various rounds fired through it during its lifetime, from low pressure loads to magnums. If it hasn't failed by now, I doubt that shooting a slightly longer low pressure shell is going to all of a sudden make much of a difference. It may sound like I'm answering my own original question regarding this post, and perhaps I am.

Having said all that, and having read these fifteen pages of posts, it's still probably better to be safe than sorry. Why tempt fate?

Put another way, be reasonable, careful until someone defines what is safe. Everything we do involves risk.

Not quite that much of a mismatch, Jim. "As with the previous magnum load, the 3" hulls that emerged from the 2 1/2" chamber looked terrible. Several had portions of the star crimp area torn off. All had ragged cone-shaped mouths." And no, they were not fired from the shoulder. "I was glad we had the use of the strong test receiver and barrel for this stunt. You would have to be a complete idiot to do this to a real lightweight game gun."

Jim, if you don't believe that an increase of 1500 psi and blown ends on shells might also result in an increase in recoil, why don't you try it for yourself and see? Sort of in Bell's "Finding Out For Myself" mode. Personally, I am more than willing to accept the reports from Thomas and Fergus that long shells fired in short chambers with short cones produced similar visible results to the ends of the shells; so why is it unlikely that they also produced similar increases in pressure? And in both cases, they reported significantly increased recoil. And in Fergus' case, he fired the same long shells in another gun, also with short chambers but with longer forcing cones, with totally different results: no blown ends on the hulls, no noticeable increase in recoil. He also reported that true 2 1/2" hulls worked fine in the gun with the short cones.

Generally speaking, as Burrard, Thomas and Bell all agree, I don't think a 2 3/4" hull loaded to appropriate low pressure will result in anything other than a slight and inconsequential increase in pressure when fired in a gun with 2 1/2" chambers. I do it all the time with my own 2 1/2" guns. However, I have heard enough "exceptions to the rule" to accept that there are indeed exceptions to the rule, and that the exception appears to be the result of guns that not only have short chambers but also very short forcing cones.