Can you put a silencer on any gun?

Answer 1

You can, strictly speaking, attach a silencer to any gun with an attachment device on the end of its barrel.

Shotguns require special silencers because of the wadding. They don't work very well.

You can only put a silencer on a gun legally if you have done the proper NFA paperwork, which is expensive and time consuming.

If you put a silencer on a gun that shoots a super-sonic bullet you will still hear the sonic crack, which is loud enough that you will definitely still need ear protection.

Despite what you see on TV, most revolvers are not really "silenced" by a silencer, due to the gap between the cylinder and barrel.