I think the individual snipers should make that call.
Individual snipers picking their own weapon makes zero sense. Despite today’s hit movies, a sniper is not some loner out in his own private war. He is part of a machine. The commander needs to know that when he sends a sniper somewhere, exactly what that capability means. You don’t get that if everyone picks their own.
Then there are logistics. A sniper can’t just go to the Benghazi Cabelas for resupply.
If they are a Marine, they fight with the weapon they are issued and the Corps trains them on.