Does your serial number have a USA prefix?
Mossberg made an "over-run" of 5000 M500 MILS guns in 1983, and another limited run in about 1997, with the USA prefix. Genuine government contract guns have a US prefix, not USA, and the US serial numbered receivers cannot be released to the public.
So says Canfield's Complete Guide To US Military Combat Shotguns anyway...
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.