Beware trying to convert the old British "tons" proofmark to pounds per square inch (psi). It doesn't work that way. Those guns are NOT limited to 6,000 psi loads, nor is the proof pressure 6,000 psi. As lagopus pointed out, that was the proofmark for a "standard proof" 2 1/2" 12ga. British proofmarks have changed twice since then, but the maximum service pressure limit for shotshells used in those guns has not. That limit is a bit less than 11,000 psi, which is a little lower than the American (SAAMI) max service pressure of 11,500 psi for the 2 3/4" 12ga.
