If memory serves, CA SR-25's (AR-10's) use the custom long type mechbox that is next to impossible to find replacement parts for. The G&P SR-25 usually uses a standard short mechbox with a custom super long nozzle and a metal spacer between the mechbox and hopup unit.
Either way, its a ver.2 mechbox which can break down after time. I would definitely recommend a sorbo pad and a MASK piston head (both available from airsoftparts.ca.) Another option would be to get the often hard to find angel silent piston head set. The SystemA silent head set is another option but doesn't work as well as the angel for reducing the impact on the mechbox.
But as said, being a designated marksman is not the easiest thing to do and most new people who try it end up getting really frustrated.