1. A lot of sound reduction.2. Ask a lawyer, I am not one nor are you paying for the services of a lawyer right now.If you want you could buy such a thing and send it to the ATF for their review.
This is a slippery slope that effects all airgunners. It's difficult in that different states have different laws.I.E. the national minimum length for a shotgun barrel is 18 inches, but many states have 20 inch minimum. So you can go on line and buy a slug barrel that is legal on the national level, but not in your state. A while back, in the firearm industry, SIG introduced a arm brace for pistol length Ar-15's.Your point People started asking about the legality of shouldering it like a conventional stock.Enough people got on the bandwagon until the ATF was forced to make a ruling that it was not legal, and constituted a short barrel rifle if I remember correctly. Then later they reversed their decision, that if it was used as an arm brace, as intended, it was legal, but if you shouldered it, you were in violation of the law. If I remember correctly. Their unofficial statement after the rulings were "if so many of you wouldn't have asked, we wouldn't have been forced to make the ruling." The ATF at this point in time doesn't control airguns, but with the power factors and calibers coming out daily, it's only a matter of time before enough people are asking the questions, and advancements have taken us to where airguns will be brought under their area of influence .Your point "it's only a matter of time before enough people are asking the questions, and advancements have taken us to where airguns will be brought under their area of influence " makes me think, imagine if we where limited to the amount of pellets we could purchase?