The answer you simply go to your FFL Dealer and purchase a Lower. He will do the check on you with the powers to be in Montana. For example here in Tennessee it is the State. But when he fills out the 4473 Form he will put "OTHER" in the Weapon Description Block. He will not list the Receiver as a Rifle or a Pistol. Be sure he does that. I am sure he will do that without any assistance! No need to mention one or the other to him! Since you are building it yourself for now you can build what you want. So nothing else for now.
Yeah, lots of confusion concerning AR lowers. The key question to ask is "is it a commercially produced AR-15 rifle, or AR-15 pistol"? (as in, a complete firearm). If the answer is "yes", then it will be so annotated as such on the 4473 when purchased. So, if you buy say, a Colt AR-15 rifle, Colt will have record that it produced this "rifle" and it will be annotated on the 4473 as a "rifle." You cannot, by law, put a barrel on it shorter than 16", unless you file a form 1 first and get permission to make it into an SBR (Short Barrel Rifle), but a commercially produced rifle can never become a "pistol". On the other hand, if it was manufactured as a pistol, you can legally put a rifle length barrel on it and switch it back to pistol at will.
A commercially produced pistol can also be legally made into an SBR, as long as you file a form 1. Basically, take your commercial pistol and swap put the pistol brace for a buttstock...voila, SBR. Wierd, huh? Yeah, no one ever said the ATF makes any frickin sense.
If you are buying just a receiver, a barrel, an entire upper, a lower, etc, then it is neither a rifle nor a pistol, it is only a portion or part, so it will be marked as "other" on the 4473. What you build on your new lower is completely up to you. If you want it as a pistol, build it as a pistol, if you want it as a rifle, build a rifle, buy several uppers and swap them around whenever you feel like it. Now, if you put a barrel less than 16" but leave a "buttstock" on it, now you're felon as that is an SBR, so you better make sure you filed a form 1 first and paid your $200 tax! Lol, sheer lunacy isn't it?
Sure, by ATF "standards" (now there's a joke) the AR-15 lower is treated as a "firearm" for background check purposes since it is the part that is serialized, but a lower in and of itself is not a rifle or a pistol.
Now, to make it more fun, it is legal to take a commercially produced (or self built from parts) rifle and turn it into an NFA item (i.e. Short Barrel Rifle) with a form 1. But, when I buy an NFA item from a dealer, I have to do a form 4, why is there a difference? (yes indeed, why is there even NFA at all, but I digress)...
A form 4 is for purchase of NFA items from a dealer, or more accurately a SOT (Special Occupational Tax), i.e. a business who has paid, and pays to keep his ultra special government permission to sell such items. A form 1 is the form a "maker" fills out. That's right, put that 12" barrel on your former 20" rifle and you are now "making" NFA items and you better get uncle sam's permission to do it (and pay your money!). The government makes a distinction between "manufacturer" and "maker." Colt manufactured the rifle, YOU "made" it into an NFA item. You are "free" to make all the NFA items you want, as long as you file your form 1 and pay your money and get it approved first. A lot of folks "make" suppressors instead of buying them from SOT/Dealers. Remember the hoopla around the "fuel filters"? Basically, suppressors sold as "fuel filters", the shady area is that they come without a hole drilled in the end cap. You then file a form 1, including whatever "serial number" you want for it (yep, you get to pick it), then when approved, you have it engraved with the serial and then drill the hole in it. A few folks have gone to jail over this shady product, but I still see them being sold.
Anyway, yes, form 1, get approved "make" your NFA item. If you make, say an SBR, once approved, you have to have it engraved with your name, city, and state. It has the "manufacturers" serial number already from the factory, then your info on it as the "maker." Letters/numbers have to be no smaller than 1/16" and the engraving has to be at least 0.003" deep.
Now you can see why we get our panties in a wad any time the ATF or the government starts jacking around with definitions and word play on its "rulings." It's already nuts, makes little to no sense.
No, as mentioned, it should be marked as "other." It doesn't matter what you intend to build (and you can build it however you want). When you're standing their at the sales counter and purchase a lower, be it a stripped lower, or one with all the pieces parts installed (trigger group, pins, tube, buffer, buttstock, etc) it is not a rifle or a pistol at that point, so "other."