Lower is the SBR / NFA item and will pretty much forevermore be an NFA item. You need to be able to go back to the configuration as described on the form 4 but 99% of the time that is a non issue. Just build an additional upper, keep original handy so you can make it match form 4 description.
You can put different uppers on the SBR lower, perfectly legal. You just have to be able to make the NFA item match the paperwork if/when inspected. Probability of being inspected, having to show NFA item matches description on form 4 happening, that is a different discussion.
As FCS has pointed out, technically yes. Even though it would be in a "rifle" configuration it is still (lower) a NFA item/SBR registered as such. Of course, as also mentioned the chances of being discovered/challenged/enforced would be almost nill. Then, there is "Murphy's" law and also the obligations of a "law abiding citizen." I suggest just picking up another lower and leave the SBR in the safe when traveling to other states, YMMV.
Interesting letter, haven't seen that before. I am aware that the BATF has always held in reference [18 U.S.C. 922(a)(4); 27 CFR 478.28] which requires the 5320.20 for any registered NFA item before transporting across a state line. Changing the configuration doesn't remove a registered SBR lower from the registry, buy hey what do I know.............................I don't have a letter as referenced?
Second article deals with the legality of changing barrel length, not interstate transport of the NFA item. Of course, very familiar with that knowing that although you submit caliber and barrel length on your Form 1 you can use different caliber and barrel length uppers on the approved SBR lower and/or making it back into a non NFA configuration legally, but again it doesn't remove it from the registry.
It is a common misconception that the lower receiver is the NFA firearm. To answer your question; once you put on a >16" barrel, it's just a plain ol' rifle and does not need the 5320.20 to travel across state lines.
if i do choose to take my sbr out of state, do i put the barrel length that is on the form 4, or the barrel length that i currently have on it? i just ordered a 13.7 mid length yesterday. if i don't like it for whatever reason, then i'm going with 16".
i should also ad, if it even matters, that the lower is not engraved as it is a factory DD MK18 sbr, so anyone looking at it wont know the difference.
edit to add:
also, do i have to do one of these for each freaking match? it asks for the out of state destination/address. also, we will be stopping over night in one state to get to the next state. so, do i do one for each stop as well?
IF you transport a rifle with a 16" barrel or longer (and not greater than .50 caliber) it doesn't matter what any receiver engravings are present, as long as you personally don't have any legal liabilities prohibiting you to possess a 16" barrel rifle, or it is illegal to for any individual to possess a 16" barrel rifle in the states you are transiting. IF you are going to multiple matches in the same out-out-of state location with a BATFE defined SBR, you can get a firearm specific BATFE permit for one year; the as-the-crow-flies route will generally apply; can't go from Florida to Kansas via California. IF you have multiple matches in multiple different states that you intend to be in posession of a BATFE SBR defined firearm, then YES it would be legally most efficient for you to apply for multiple one year permits to each out-of-state SBR use destination. It will only take you a few templated letters / forms to the BATFE to put your mind at ease with your travel firearms.
I'm in PA, so would i need one for the stay in Ohio on our way there (wife wants to stay half way there and on the way back instead of making the 8hr trip in one day again), and another for the stay and match in KY; 2 for KY since I technically have 2 stops in KY, the over night at the hotel and the match that is 45 minutes away. then would i need them again for on the way home if we stay at a different hotel in ohio, or we might even go thru WV on our way home and stay in WV?