For overall length I read that it should be w/o the muzzle device attached and with the stock fully collapsed. Is that right? What if I change stocks at a later date to one with a rubber butt pad, do I need to change my paper work or just have the original stock around to switch back too?

Stock fully extended, to the end of the muzzle threads (but not the muzzle device).

As for how precise you need to be, you'll get a bunch of different answers. Mine? Don't worry about fractions of an inch...I don't think ATF has the time or the resources (or the give-a-damn) to come measure all the registered SBRs for accuracy.

Barrel length is just that - barrel length. Do not include any extensions, flash hiders, suppressor adaptors, etc...
overall length is calculated with stock extended.

OK so fully extended... glad I asked

I was under the impression that the OAL should be with stock fully collapsed so that you report the shortest OAL of the rifle.

As you can tell, you'll get a variety of answers here....

My understanding is that the stock should be fully extended. If you have a folding stock, it should be unfolded when you measure. This is the method that I've used for my SBRs.

I understand the point that a folding stock should be unfolded so that the rifle is in the form in which it can be used as a rifle (shouldered) but what about collapsing stocks? They can still be shouldered with the stock at their shortest length. Does anyone have a definitive source on this? ATF BS is confusing as all get out.

Nope. My best explanation was here:
viewtopic.php?p=442839#p442839

I've looked and I can't find anything "definitive" but I'm very comfortable with using the extended stock. My suggestion is that folks just go that way and don't spend a bunch of time stressing about it.

Ok, my logic relating to it was that the shortest shoulderable length would make for the most NFA measurement. If that makes any sense. Never seen anything definitive either way though, just heard lots of people saying lots of different things.

I totally get the logic, but the known standard is measure with the stock extended.

Good to know, thanks for the clarification guys.

Submitted

cool. for posterities sake the only F1 I ever submitted not factoring in extended or folded open stock was on an UZI where I intended to use the quick-detach stock. The internet generally believes, though I have never found a letter either way, that quick-detach stocks on UZI carbines can not be included into the OAL due to their 'QD' nature. On and only on that submission I omitted any stock measurements and only measured from the tip of the barrel to the rear of the receiver. That Form was approved (not today) and I re-use that logic for future submissions only in this situation.

You used logic in trying to relate to a government entity/regulation!!!!!