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Double D, thanks for the "heads up..I just assumed 40mm was closer to a cannon than a rifle..
..But now that you mention it, he is a highly qualified gunsmith, here's one of his custom jobs; a special .223 for a survivalist !
THe next photo shows the rifle he built up in iraq, for himself. He was one of two "overwatch" (sniper) Marines for his unit.
Still limited to the .223, he being the armorer, selected parts after much testing, to build his own "Old Betsy"!
I suspect he built a similar rifle for his best friend, the other overwatch and best man at his wedding.
Oops! Sorry, the last photo won't blow up ! Don't know why..