It's not a "machine gun" unless it has a full-auto mode. You either have an SBR (short barrelled rifle) or an AOW (any other weapon - i.e. a short barrelled shotgun or something like that). AOW's, by the way, only take a $5 stamp . All of these are considered "Class 3" weapons.
You'd have to check either your dealer, but I think an SBR with a supressor (one that's not permanently mounted and the overall barrel length is still under 16") needs two stamps - one for the rifle, and one for the suppressor.
22LR isn't so loud anyhow, but suppressors are handy for them.
What is the point of rules that are not enforced?