[KLRANGL]

Theoretically, if you had a .177 pellet rifle with an integral suppressor you would be fine, but the problem is in actually building the suppressor. The ATF seems to think (or so I hear) that a single suppressor baffle would constitute a firearm suppressor. So unless you have a Form 1 laying around, it might not be a good idea to start fabricating baffles even if the end result is in the use of an integral suppressor for an air rifle.

I had this problem a while back while trying to legally customize an airsoft bolt action rifle to one with an integral suppressor. I couldn't find a satisfactory answer, so I dropped the project.