This whole thing about using a suppressor on an airgun has recieved a lot of discussion on the internet over the years.
Appearently suppressors on airguns are common in other countries.
One thing I know for sure is that if the suppressor can come off the airgun, then it COULD be used with a cartridge type gun and is therefore covered by the NFA.
I recently read something on-line (meaning it could be total BS) about airguns with integral suppressors being OK without getting a tax stamp.
If I was you I would look into this suject long and hard before I made any moves unless you simply buy it as a legal suppressor and get the required tax stamp to avoid any possible conflict.
You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.