This one is iffy. Way back when several years ago the ATF would approve Form 1's with things like "under 16 inches" or "various" in the place where the length goes. I have a couple like this.
ATF has said that you have to meet whatever is on the Form 1, so in the case of "various" it appears you are good no matter what.
Now, that said, ATF at some point stopped approving those forms without a specific length so the best way to deal with it now is to put ALL barrel lengths you might want to use on the form, as well as all the possible calibers if you intend to use different uppers. They won't approve "various" in the caliber portion of the form either.
So for length, I would put "10, 11, 14.5," for mine, since that's what I want to be able to have the options to use. Maybe even shorter if you wanted to put a pistol upper on it.
Then put ".22, .223, 6.8SPC," etc in the caliber section.
Not legal advice of course, I'm not even sure a lawyer would know this one since ATF seems to make it up as they go to some extent.
