[url]http://www.flashdistributor.com[/url]
Rifles have a similar exception for sporting purposes. It is in there, look for it. I doubt though that the ATF would let you use it for anything other than a factory built Slug Gun.
You can try a 410 as they are below .5". A 12 Gauge is .75". I believe a 20 Gauge is .6" For example, If not for the 1994 AW Ban, we could make USAS, Street Sweepers, etc...in 410 Gauge. I personally consider 410 kinda small, but it isn't relegated to sporting purposes.
I have seen alot of 410 Pistols. All have Rifled Bores. I saw a Specialized 12 Gauge Pistol that somehow does not qualify as a firearm. Flash Distributing is kinda weird in that they sell alot of guns you'd think were either illegal or require an FFL. They somehow alter them so that they are legal. The 12 Gauge Pistol uses special ammo. which I am willing to bet the powder doesn't "yield readily to combustion." Rapid Gas Expansion ?
They also have Full-Automatic M-16s for $99. They also do not qualify as Firearms. Strange, but again special ammo. At close range it is supposed to knock the person senseless, but not kill.