Dunno if it's different in singleplayer, but I think all silenced weapons start losing damage after about 15 meters, but 15m is already beyond effective range of Marshall so it's not affected.
I also heard a rumor that the under barrel shotgun deals damage of Marshall and Jackal combined. It might be true as it does great damage to Ceph and has very low range in mp but can take down armored opponent in 1 precise shot.
If u carry a rifle with this shotgun attached and another shotgun as secondary weapon u get like 24-28 shots for it.
PS: Real men use pump-action shotguns.
That rumor about the underbarrel shotgun cannot be true. Crytek has lied to us about many things in Crysis 2 (like that adding attachments to weapons make them heavier) but making the UB shotgun so powerful would be against the laws of physics. If you see it, it's way smaller that the jackal, so it's 1% possible that the rumor is true.
PS: Real men cant be killed with one shot, so I guess I'm a woman.