Just go for an AWSM with a Bipod, a 10 round magazine, tracer/hollow-point ammunition, a silencer, and a rag cover to make it look badass.
(Silencer because the AWSM is LOUD, and tracer/hollow-point ammunition because I assume that explosive rounds would have some sort of noise when they exploded. If you don't miss, tracer/hollow-point rounds should be pretty darn quiet when they're burrowing through your enemies' empty skulls.)