Could he have an airsoft Thompson? Airsoft pellets are 6.5 mm.
This one's actually listed as "6mm cal"
Thompson M1A1 AEG Airsoft Submachine Gun by Cybergun
... maybe he is just an old airsofter? Or maybe he traded his old bringback 1911, Garand, or carbine or something to someone for a 6mm Thompson SMG.