"Actually if you buy a stripped lower it should be on the 4473 as "other" since its not a pistol or rifle."
This statement is correct. Unless some state's law tries to modify the 4473, dealers should not be listing a stripped lower as a pistol.
Re: chambering 300AAC in a .223/5.56
I personally have not tried this with every possible type of ammo but I've also heard/read that it is possible. Some of the very long ogive tapered point bullets may enter the .223 chamber shoulder far enough to allow the bolt to close. We use "sand" colored magazines with big, red markings for the 6x45MM and gray mags with blue numbers for the 300AAC(blue because Granddaughter wanted "her" little AR to have blue accents).
I have several different chamberings on the AR 15 (5.45x39, 5.56, 224 Valkyrie, 6.5 Grendel, 6.8 SPC, 300 AAC, 22Nosler in the works) and AR10 (.308 , 6.5 Creedmore) platform(s) so we MUST be very careful. I've even gone so far as to use colored tape around the pistol grip or handguard with matching tape color on the magazine. If I like the look, I use a permanent color code.
I tried my hand at builying an AR9 pistol using an AR556 stripped lower. To adapt to the Colt style smg magazinea, I used a Sylvan Arms magazine block.
I've had nothing but problems with this block. At first, I needed to file the "ears" of the feed ramp down to clear my bolt. Now that I got it to a point where it the upper locks down and the bolt operates, it jams the rounds up onto the top of the barrel and receiver. I think I need to do some grinding on the feed ramp to cut down the angle.
When I had the first issue, I called down to Sylvan Arms. I spoke to a great CS Rep and he offered to replace it. However, as they are located in Myrtle Beach and less than a mile from the shoreline and it was the day before Florence was to make landfall, I didn't think my problem should be at the forefront of his concerns at that time.
I was toying with the idea of going with the .300 Blackout, as I built a carbine in that a few years back. However, I thought the 9mm would be cool and easier to handle.
So, I'm a bit torn between still fighting with what I've got, sending the block back to Sylvan and trying a different brand adapter (Midway has an AR Stoner one that looks identical to my Sylvan Arms one...maybe it'll work), or swapping out the barrel and changing it into a .300 Blackout. If I finish it as 300, I've got all this 9mm stuff to sell and that'll be a PITA.
So, that's where my project stands...I don't even have photos yet.
"Love hearing 208 grain lead at 950 fps hitting steel at 100yds without needing ear plugs"
With the right light, you can actually see those long, slow bullets in flight. We were zeroing Scooter's little 300 and I got a bit cocky by saying "On the way" as the trigger broke so my spotters could see the golden streak.
I don't have much(any) interest in a pistol caliber AR. I already have the two "best" 9mm carbines made: Ruger PC9 and Marlin Camp 9. I'm not demeaning those who want a PCC, it's just that I 'm not going to the effort to have a 9mm AR(9mm is the only semi-auto pistol round I have on hand).
I have several thousand 300 cases and bullets so it's my "pistol round of choice" for the AR platform.