Your right you can use a rifle buffer tube. This has been cussed, discussed and debated on AR15.com. Personally I wouldn't as it could get iffy with an ATF agent who doesn't really know the regs. If I did use one I would do a permanent mod to it so it couldn't accept a stock.tRidiot, I don't think that is exactly correct. I think you are allowed to use a rifle receiver extension, but cannot have a stock available that is not attached to another rifle and can easily be attached. The laws around this are wonky, but I do believe that is how it is written. Man people will just put a piece of foam over the extension or just wrap it with paracord.