I know it's been said a billion times, but I'll personally vouch for the Mk. 23 too. It's just too great to be passed up as a NBB gun, I can't think of anything else that surpasses it in terms of its quietness. I'd say if you're really gunnin for stealth, dish out the extra money and get a solid, silent performer aka the Mk. 23.
Also, to further answer your question about the usefulness of the silencer, it does make a difference. Not a whole lot, but mostly the difference is that without the silencer, it sounds like a fairly high-pitched popping nose, kind of like when you're popping the top of a metal bottle cap but louder. With the silencer on, it's the same sort of sound but minus the really high pitches, making it harder to hear (lower pitch sounds are harder to perceive over distance than higher pitch sounds).
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