IF the box of the shells to which claycrusher is referring is marked
These cartridges are suitable for use in:
70mm case length: Guns with a chamber length of 2 3/4 or longer, nitro proofed to a service pressure of 3 1/4 tons per square inch (900 kg per square cm)
We can believe the shell maker
10,800 psi however is not what the British consider to be 3 1/4 Tons, which I assume is the service pressure for which claycrusher's gun is proved
Drew, I think we may have discussed the guidance on British shotshell boxes previously. Unfortunately, that guidance contains a fairly significant disconnect. We know that 3 1/4 tons, as stated on the box, is a service pressure measurement. However, we also know that 900 kg per square cm is NOT a service pressure measurement, but rather a proof pressure measurement. The British, even after converting to metric marks, continued to use proof pressures that were, in fact, lead crusher values. 850 bar, per the Birmingham Proofhouse, equates to a SAAMI proof pressure of 13,920 psi, and a service pressure of 10,730 psi. If we go by the more recent (metric) figure and increase those values based on 900kg or bar, we arrive at a service pressure which is just shy of 11,400 psi--bumping up against the 12ga SAAMI service pressure figure of 11,500 psi.
All of which leaves some doubt in my mind. I own an Alex Martin 20ga, original proof 2 1/2", 7/8 oz. It has since been reproofed and properly marked as 2 3/4", 3 1/4 tons. Should I shoot my Gamebore Pure Gold 1 oz 2 3/4" loads in that gun? Going by the 3 1/4 ton reference on the box and my gun's 3 1/4 ton proof, I would seem to be good to go. IF that gun carried the 900 bar (or kg) mark of a more recent 20ga reproof, I might do so. But given the gun's age and extremely light weight (under 5 1/2 pounds), I think I'd stick with the Gamebore Traditional Game loads made for 2 1/2" guns, service pressure 3 tons (850 kg/sq cm). Or even better, I might opt for RST 7/8 oz factory loads.
The two different service/proof pressures do leave a degree of doubt . . . but I'd sooner err on the side of caution.
