Is a barrel extension legal?

[Bigfoot]

I've posted here before about using long barrels instead of suppressors for pistol caliber subsonic shooting. Since then I put a 28" sporter profile barrel on my 10/22 and it's almost silent with Rem Subsonics. I can shoot it indoors without hearing protection. Now I'm looking at this .40S&W Mech-Tech carbine. I could have someone build a long barrel but it would need some expensive machining because of the mounting system. I'm wondering if I could thread the 16" barrel instead and attach an 10-12" extension on it, probably with no rifling to match up.

I don't remember seeing anything like this. Possibly because pistol caliber carbines aren't common and rifle calibers could cause problems with their much higher pressures.

I wonder about the legality because anything that reduces the sound could be deemed a suppressor.

I live in a very liberal area or I'd just get a suppressor.

Bigfoot