[gyvel]

"Question: If it starts out as a bare, stripped receiver and is made into a pistol (i.e. purpose built), is it still considered an SBR?"

No, but ONLY if it had NEVER been assembled as a long-arm to begin with; since that's not likely to happen in this case, that leaves the OP with the option of cutting down an existing rifle. However, you are right that if you start with a brand-new receiver, you can "deem" it to be whatever you want, so if he made an entire new receiver, he would simply be making a new handgun, the same way the "Mare's Leg" lever-action "pistols" are built on Italian-produced Winchester receivers that have never been assembled as rifles

OK. Playing devil's advocate here, where would the burden of proof lie if you had a bare receiver and insisted that it had never been a "rifle." ATF would have a hard time proving otherwise, no?