Whoa, whoa, whoa. There are some stipulations and it's the reason you can't find stripped lowers on Form 4's. Assuming you know the new process for NFA submissions in general:
1. Notice the Bbl. length and OA length questions? Caliber? This assumes a completed firearm. You can change your uppers out, but ATF likes to be notified if you have multiple uppers for multiple lowers and at minimum you are supposed to have the original upper you applied for on the original Form 1 handy somewhere. I suppose if you lose the original upper and can't replace it, then a letter explaining such and the new upper you plan on using would suffice, but I'd talk to them, don't take my word for that.
2. You engrave the lower with "Joe Blow Trust, Anywhere City, Some State", I like to do it on the top of the right side mag well. Letters must be certain height and depth, it's in print somewhere but my laser engraver does 'em all the time to the minimum required unless you want something different and blackens the exposed aluminum after too.
3. If you get another upper, different bbl. length, caliber, etc., then just send a letter saying so and easiest way is enumerating the changes for each upper you plan on using with that lower (box x: 5.56mm, box y: 11.5", box z: 27" --you get the idea) and they like it if you send a copy of that first page of the original Form 1 with it, the one with all the info. Make sure to call ATF to send it to the right people, they make changes there from time to time.
So technically, you can Form 1 an SBR but not a lower, you need to have caliber, barrel length and overall lengths listed. DO NOT use "Multi-caliber" even if that's what it says --they don't like that and will send it back. Put one caliber, one bbl. length, etc., send the addendum letter after you get the stamp back, not before, not with the original submission. And you need to have that original upper/bbl. length handy if you decide to use a different upper. If you change it, they like to be notified in writing but ONLY if you actually have that upper in your possession. They told me they aren't interested in what I PLAN to use, just what I AM using, so if you have even ONE registered SBR, they don't care how many short uppers you have and how many non-registered lowers you have (provided those uppers don't wind up on those lowers prior to registering them). I was afraid of constructive possession but they said that's not a problem if you have a registered weapon that uses those parts already (even if you haven't told them about the extra uppers --that's still considered a courtesy).
Hope that helps, sucks having one returned --only happened to me once and it was because they didn't know if "LMT" was part of the SN or not. But after fixing that, making it more clear, I got the stamp like two weeks later. I put "The" in front of my trust one time and they warned me not to do it anymore but that they let it slide that time, that sometimes they return the submission. Not having the barrel length, caliber and OAL will definitely get it returned. Promise.
They are pretty technical about shit and if you think about it, they register SBR's and other stuff but you never hear of them registering stripped OR complete lowers. It's because they don't. Trust me, tried when we had the fiasco in WA when some asshat started a stink asking if it was legal to manufacture on a Form 1. I was one of the lucky few that actually got two registered Form 1's back complete prior to it happening. It took a year to explain to these fuckers, despite ATF saying we were right, that none of us are "manufacturing" anything, we are assembling rifles from parts no different really than had we taken them apart to clean. During that time, if you wanted to register and build your own SBR a la Form 1, the only way to do it was to buy it as a custom rifle on a Form 4 from, say, Rainier, who of course is gonna take their cut, and you have to wait for them to register it themselves as a production rifle, blah, blah. Never did that but wife did buy one on a Form 4 and it was in part due to that. That was all because you couldn't buy lowers on Form 4's and can only register rifles on Form 1's. Assembly vs. manufacture was a WA specific issue. It's fixed now, but I learned a lot that year.
Good luck registering your SBR, not your lower.
