It's a suppressor. It can provide two separate functions, depending on your need.
1) With subsonic ammo, you can rely on it to be practically silent (and not give away your position), as well as keep the round from "cracking" (the supersonic shockwave sound) to the people you're shooting at. Technically you could miss all day and no one will know until you hit them, OR you could get one or two additional people before they realize what's going on. The subsonic ammo limits the range, but not usually the accuracy.
2) Mask your position but the crack can still give you away with regular ammo, which isn't really an issue past 500 yards. So you select the ammo for range, more or less.
The USMC has a dedicated armorer (MOS 2112) that builds and maintains their precision weapons. You can buy weapons that are essentially the same, but they are made by other people/companies. I do believe that they are basically M40A3's outfitted like Patriot Arms.