I would wait till it's approved and then send in a letter stating that you've made a permanent change to the measurements. Not a big deal.
The more I thought about this last night the more I agree with you. My only rationale was that my eForm 1 was filled out for a 10.5" barrel and has an OAL of 29.5" so at OAL of 20" my barrel would be 1.0", which is highly improbable. The easiest and cheapest solution is to let it process and then send the change letter.
I would wait till it's approved and then send in a letter stating that you've made a permanent change to the measurements. Not a big deal.
You can also switch and swap uppers and buttstocks as long as it's not a permanent change and the rifle can be returned to the configuration listed on the form.
I've never seen or heard of a BATFE agent knocking on anyone's door with a tape measure in their hand.
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