I believe that the 3 ton proof is about an equivalent to 10,600 PSI so would be lower than SAAMI allows for any US 12 gauge shell of 2 3/4" or longer. A 4 ton pressure would be approximately equivalent to 12,300 PSI which does put it in range of US 12 gauge shells in either 2 3/4" or 3".

Miller/TN
I Didn't Say Everything I Said, Yogi Berra