Go back and read the link on post #4. Straight from the BATF. That's what I go by for my rear grip SP builds. And I keep a printed copy in my range bag just in case the question ever comes up. What is it about a 16"+ barrel and an OAL that you don't understand? It's still a rifle, no matter what it looks like. And shorter in the OAL or barrel length and you'll be running into problems. Or, build on a Striker action or go undesignated. Just more $$.

And as far as a left hand action? That may have been the only one I've ever seen listed for sale. Be easier to buy a complete rifle and brake it up than wait on a left hand action. Like I said, few and far between.