I can't provide info on law but:-
- A .22 firing sub-sonic ammo with a silencer will make a "Puut" sound, at about the same level or less as you clapping your hands.
- Some semis won't cycle on semi-auto with subsonic ammo.
- If you silence a center-fire and use super sonic ammo you will still have flight noise and the super-sonic crack but not the muzzle blast. This means that while you won't be silent, if you are "bombing up" a mob of goats they won't know where you are but will know they are being shot at, giving you some more time to get off a follow up shot.
- Get an over-barrel silencer. The main reason for this is that it has two contact points and is harder to knock out of alignment.
- The over-barrel silencer will lengthen the barrel. I tended to recommend cutting 2" of the barrel to improve balance.
- An over-barrel silencer will affect fps. You will generally gain velocity (due to free-bore boost) and accuracy.
- Due to action noise you won't get them silent. If you do use the 10/22 look at something like this to reduce action noise.
Have a look
here. It may be cheaper than getting one in the US.
Here in NZ anyone can buy a silencer and as a consequence we silence almost anything. The shop I have linked to had an 870 with a full over-barrel silencer.(Note: I did used to work for them but now have no affiliation with them. I just referenced them as they are Australasia's biggest firearms retailer.)