"They have some odd "gotchas" in the US wrt guns -- silencers (and also "sound moderators") are scary, possibly because of their connotations as assassination weapons. It takes a Federal Firearms Licence (FFL, effectively a dealer's licence) to own them and it costs a chunk of money to buy and transfer ownership of such a silencer or a silenced weapon, like the DeLisle carbine. It is classed in the same way as fully-automatic weapons like machine guns."
You're a little off, but close. Anyone in the United States who can legally own a pistol can legally own any weapon covered by the NFA (National Firearms Act). This includes Machine Guns, Sound Suppressors (A.K.A. silencers), Short-Barrelled Rifles, Short-Barrelled Shotguns, and All Other Weapons (A.O.W. is basically anything that doesn't fall under any of those definitions, or Long Gun on Handgun).
One must be a licensed Class III dealer to sell these items. Owning them requires a lengthy background check and BATF approval, and a $200 transfer tax for all but the AOW, which carries a $5 transfer.
Machine Guns are no longer legal for civilians to purchase new, but are legal to possess so long as they were registered before the ban took effect in 1986. This means there is a finite supply of legal machine guns, and the entry-level full-auto costs about $4000, for something like a shitty MAC-10.
When it comes to detachable shoulder stocks and the like, one can legally fit them to a modern pistol, so long as one is willing to jump through some hoops. Typically one has to register the weapon as an SBR, which entails a $200 transfer tax and a pile of paperwork.
And regarding the Anonymous Coward's suggestion of jimsonweed, are you trying to kill someone, or simply make them hallucinate? It's a pretty mild neurotoxin, and takes quite a bit to kill.