changing caliber and barrel length

[yep i did it]

I have 1 registered lower, paperwork says 11.5" barrel and caliber 5.56. I want to switch to a 8.5" barrel and 300blk. Is this a whole new tax stamp or just send ATF a letter stating the changes?

[BigWaylon]

It's a "nothing" other than put the new upper on it. You made the firearm to meet the F1 specs. How it's currently configured doesn't matter.

No requirement to notify them of anything. They recommend you notify them of any permanent change, but never define what qualifies as permanent.

[yep i did it]

Ok. The reason I'm asking , I'm going to try and hunt some whitetail down here in SC. I am unsure our game wardens, DNR, are up to speed on NFA papaerwork. If he reads the wrong information on the papaerwork,I don't want him to take away my birthday.

[BigWaylon]

Ok. The reason I'm asking , I'm going to try and hunt some whitetail down here in SC. I am unsure our game wardens, DNR, are up to speed on NFA papaerwork. If he reads the wrong information on the papaerwork,I don't want him to take away my birthday.
He serial number will match. That's what matters. Most of them won't have a clue what they're looking at anyway.

I have several 5320.20 forms approved with caliber and/or lengths different from the F1, and I've never notified the ATF of anything. That's the first real world signal to me (in addition to the fact no statute exists that requires it) they don't require it. If you ever tried to sell it, and the F4 didn't match the F1, you'd probably have to explain it...