I was comparing reloading recipes recently, and was surprised at the extent to which using different wads can affect load pressures. Which led to this question: Assuming you are shooting a gun with modern steel barrels, and using loads within the gun's proof, is there any benefit to shooting lower pressure loads? If a gun is proved to 13,000 psi, does shooting 8,000 psi loads instead 10,000 psi loads (assuming the fps and shot charge are the same) extend the time before a gun shoots loose?
Put another way, is it worth spending extra money on higher quality wads to reduce the pressure in my reloads 1500-2000 psi?
Thanks in advance, Doverham
DH, I take it from your post--since modern American guns are proofed at a pressure far higher than 13,000 psi (pushing 20,000 psi)--that you're speaking either of vintage American guns, or of British/European guns with the "standard" CIP proof. Per the proofmaster of the Birmingham Proofhouse, standard CIP proof equates to 13,920 psi as measured by SAAMI transducers. Standard CIP service pressure for those guns is 10,730 psi (versus the 11,500 psi SAAMI service pressure standard for an American 12ga).
Let's say you're reloading based on a formula that tells you the pressure of the load in question is 10,000 psi. I don't know that the gun would wear any quicker, assuming that you never slip up and nudge above the service pressure limit for the guns in question. But those of us who've had our reloads pressure tested by someone like Tom Armbrust know that pressures will vary, sometimes by several hundred psi. So if you don't factor in a sufficient safety cushion, say by reducing your powder charge a bit, you might well exceed the service pressure for which the gun was designed.
Also, if--like many of us--you're working up low pressure reloads for a gun with 2 1/2" chambers using 2 3/4" hulls, you need to remember that the longer hull will produce somewhat higher pressures when fired in a shorter chamber. In Sherman Bell's tests, while most of his loads showed an increase of only a few hundred psi when fired in a short chamber test barrel vs one with a 2 3/4" chamber, in some cases the increase exceeded 1,000 psi. So if you're using 2 3/4" hulls, you need to build in even more of a safety cushion--if you want to make sure you don't exceed the service pressure standard for the gun in question.
While I don't work to develop extremely low pressure reloads--talking 5-6,000 psi or less, as some do--I am careful to build in significant "margin for error" when reloading 2 3/4" hulls for 2 1/2" guns. Factoring in the possibility of a 1,000 psi increase due to hull length, and the possibility of maybe a few hundred additional psi if I drop more powder than the recipe calls for, I like to start with a load with a "book" pressure figure of not much higher than 8,000 psi. Working from that point, I feel confident that I'm not going to end up with anything much over 10,000 psi, and probably well under it.
Of course the other solution is to feed the gun a steady diet of something like CIP standard proof loads, or RST's. The problem with the former is that most of them are field loads with a fair amount of recoil due to shot charge and velocity. RST does offer some nice, soft loads at modest velocity, and 3/4 or 7/8 oz of shot. Because I shoot a lot of targets with vintage or CIP-proof guns, I reload a lot of those--thus giving me both low pressure and reduced recoil. Good for both me and the gun in question.