If you buy a stripped lower it will be marked "receiver" on the 4473. You can either build it into a pistol or a rifle, but if you ever build it into a rifle it can't be a pistol. MAS Gunworks has several lowers you could start with. They would even get my vote to build you the whole gun. Probably save a little cash over doing it yourself. I bought the parts and had Josh-L build my 9mm upper and he did a very nice job. I'd recommend him as well.