[ QUOTE ]
So, you can use a rifle action to build a pistol?
No, but you can use a "stripped" or "registered pistol" action to build a pistol. Just make sure that when you do the transfer paperwork, it is NOT registered as a rifle receiver.
What are the rules regarding weight, length, etc., before you start getting into Class III weapons?
There are none. Length and weight rules were part of the AWB which has sunsetted. You may NOT put a vertical foregrip on it, and you may NOT affix a stock to it. Otherwise, have fun. All is legal as shown.
And, technically, it wouldn't be a Class III firearm, it would be an "Any Other Weapon" firearm if it had a short barrel and a stock, or was built from a rifle receiver. Then it needs to be registered as an SBR, which requires the paperwork plus $200.
[/ QUOTE ]
Go to www.AR15.COM
and look under pistol builds to get a link to the actual ATF letter which acknowledges the legality of this.