Re: air rifle supressor
I think this says it best:
Federal law defines a firearm as a device capable of projecting a bullet, by the use of chemical reaction, ie, combustion, and using fixed ammunition( primer, powder, bullet in one unit), and speaks nothing of pneumatics, ie air rifles, pistols, etc.
Only if and when your lawnmower starts shooting projectiles from its muffler might you have problems... :-)
That said, why not get a suppressor for a .22lr then thread your air rifle for the same and you've covered your bases and your @$$!