I have a lower that I plan on doing a Form 1 for as an SBR. At this time I'm planning a .300 BLK upper.
If I wanted to keep options open for other calibers (let's say a .223 upper), would I need to list those on the form?
I'll defer to others who have done it themselves, but it's my understanding that you have to specify a setup style on the form. But from there, you can put other uppers on the SBR lower. If you consider the change "permanent", that you let ATF know the new configuraion.
But since the AR system can be changed with two push pins, unless you sell you the upper you listed, you haven't done anything permanet. So no need to update things.
One issue may be having more sub-16 inch uppers than you have OK lowers for. Now if this is the only AR you have, there's no issue. But who here is so stunted that they only own one AR? So if you have two sub-16 inch uppers, and you only have the SBR, then you could be accused of not having a proper home for that upper.
A pistol lower, which you can get for 300 bucks or so, or make yourself from parts for 250, would be a fully legal home for another sub 16 upper, and that's my plan - have a SBR lower for one, and a pistol lower for another.
List the one configuration that you plan on building as soon as you get the approved stamp.
Own as many other caliber uppers as you want for your SBR.
Even if you sell off the original upper there is no law requiring you to notify the ATF. With their current backlog letters regarding changes are lowest priority. I wouldn't even worry about it.
So as long as I keep the original upper and can restore the gun to the registered configuration, I'm good?
WIthout a doubt. And apparently it's fine even if you do sell off the upper. But with ought to be in prison Holder as the AG, I'd hold onto the listed upper or at least drop ATF an update if I sold if.