Who will ask to see the Form 1 or Form 4?

[lazyengineer]

If they are offering a "no-cost" eform, does it not make sense for me to register all of my lowers as SBRs in case I happen to attach a short upper? That's the beauty of the AR, mutiple calibers, multiple configurations, all on a common lower. Bases covered. What do you think?

Lots of passion on that one. There are two main basis for those who advise against:
1) It's a trap. Personally, I don't think it is, as the ATF has done this before without issue. I think motivation is they simply weren't enforcing NFA as written, and they are doing this to get back to doing so.
2) Solidarity. Sort of a Scabs at the Strike take, on doing anything the ATF wants. Which I kind of get, but not everybody is at a place in life where this is the hill they will die upon, nor in a setting so understanding. To OP's question, if you are going to public ranges - then yes, it's not unusual at all for some busybody to feel important and harrass or a range officer to over-fixate on percieved range liablities, and ask to see your license. If you refuse, you don't get to shoot.

If you do register, then sure, no reason not to just register as much as you can, since it can go into and out of NFA classification at your discretion. So even if it's registered as an SBR, if you put a 16" upper on it, it no longer is, and you can cross state lines, sell it, etc, just like a regular AR (unlike machine guns). You can even ask it to be officically stricken from NFA list by a letter, at any time as well.

HOWEVER, research and have care before piling your AR rifles in on this. The Amnesty is for Arm Braces applied to "handgun" pistols with BBL<16". And only that. If your lower started life as a RIFLE, rather than as a Pistol or Other, then when ATF runs your application serial number - you will now have a signed legal document basically saying you converted a Rifle into an SBR, which is not protected by this amnesty. They might grant it, they might just deny and forget about, or they might chose to give you grief; I really don't know. My guess is they'lll just deny it with instruction to turn that one back into a >16" rifle, but I don't know - not worth it to save $200 for a pile-on SBR stamp.