Well, my local gun store guy stated that you can fill out this section with say 7.5" and still be OK using a 10.5", 14.5", etc (as long as it's greater than or equal to 7.5"). I couldn't find anything on the ATF sight about this. Anyone ever heard such a thing? Is this guy talking out of his ass?

If you have an SBR'ed AR-15 lower receiver, then you can put whatever barrel length upper you want on it. You can also change calibers if you like. My original AR-15 SBR was a 11.5" barrel in 5.56mm, however I have a 5" barreled 9mm upper and a 8" barreled 300BLK upper for it.

He is incorrect.  You can use any barrel length you want, whether it's longer or shorter than 7.5 inches--the configuration you specify on a Form 1 is only the configuration in which you intend to originally build the gun, and in the case of a Form 4 it's how the gun is configured at the time of the transfer.  In neither case is the specified length a minimum.  ATF requests, but has no legal or regulatory basis to require, that you notify them of any "permanent" change in configuration of the NFA items.  In the case of an AR, it's up to you whether you consider that any change of configuration is permanent.

He would be wrong.  On a form 4 you must use the length of the gun as it exist.

So, if I put 7.5" on the form, there is no way I can legally put a 10.5 or 14.5 on it?

1. You need to put the specs of how you will initially make the firearm
2. After that, you can run any upper (regardless of caliber or barrel length)...there is no "you can only run longer" rule as you mentioned the dealer stated, which is why he is "wrong"

ETA: sorry...missed the "Form 4" in the title. The rules still apply even though you're not making the firearm. #1 would be the specs if how the firearm is currently configured.

MEGR, are you buying (or considering buying) an already-existing SBR, or are you making one yourself?  If you're buying one already built, there's no question about what to put on the form--you (or, more likely, your dealer) fill it out describing how the rifle is already configured.  End of story.

If you're making one, you fill out a form 1, not a form 4, and your dealer isn't involved.  You fill out the form 1 with the configuration you're going to initially build (caliber, barrel length, OAL).

In either case, you can later change the upper to any caliber, any length (longer or shorter than specified on the form).

If you're undecided about whether to build or buy, electronic Form 1s are coming back approved in under a month, while Form 4 must be filed on paper and seem to be taking at least a few months.  OTOH, to do eForms, you have the cost of setting up a trust (if you haven't already) and engraving your lower.

Thanks. You guys rock. ARFCOM always comes through with the good advice!

Form 4 = supressor, correct?

True.   I haven't ever bought an SBR.  Only made my own.  Only form 4s I've done were for suppressors.  My bad