Saw this with a couple of .308 "F" guns where they put a can on , and shot there normal target load . Stretched the cases , and the primer fell out of the case , when extracted . So I'm curious about the shell casing . If you got it out intact , and you have it available , check all the dimensions of the case and it should tell you a lot . I'm guessing you have a case that very closely "mirrors" your chamber , dimensionally . Most "cans" are only designed to suppress noise , and do very little with regard to dissipating pressure build-up , causing a pressure spike when the round is discharged .
Most people don't realize that mil-surp is on the high end of the scale , both pressure , and velocity-wise , so I'm glad you weren't hurt , or the gun damaged . Some mil-surp has a listing of over 2,900 fps and that could have made for a very bad day .
Let me get into my data books and see what I can come up with and I'll try to post it today . Tomorrow , latest .