The US never ratified the APERS ban, however, the inventory of most US mines is old and there are no "current" plans to replace them with new inventory. Right now, the Army is trying to replace the fuzes for the AT mines (like the ones we use in-game) because they are all 1960s vintage and many have expired initiators.
"Silencer" on a fifty? I can't see that being all that effective. You'd still want to wear earplugs as the sonic boom alone would still be over 150 db. And that can isn't anywhere near big enough to be effective at reducing the report of the bullet ignition. Try the muffler off a Caterpiller bulldozer if you want that to work. My guess is that it serves mainly to dampen the huge muzzle flash associated with that rifle.
"A true warrior fights not because he hates what is in front of him; A true warrior fights because he loves what is behind him" -Saga