Form 1 lower for 300 blackout upper?

[sonofmt]

I am thinking about getting a 300 blackout upper and form 1 sbr'ing a lower for it.

Regarding the ATF--
Would I need to have a specific 300 blackout lower on the form 1?
Could I use a form 1 556 lower, provided the lengths match, etc?
Alternatively, could I run a 556 upper on a form 1 300 blackout lower?

My suspicion is the ATF would want the upper and lower calibers to match. I realize I could contact the ATF, but from what I have read that is sometimes more confusing than helpful.

[prepare]

In the caliber section just put Multi.

[sonofmt]

Ah, yes. Thanks very much.

[Bret]

What is cartridge designation on the lower receiver?

[sonofmt]

Caliber on lower is 5.56.
I guess I could notify the ATF of a change of caliber. I have read conflicting things—ATF apparently ‘requests’ but does not require a change of caliber notification?

[the_1iviper]

your overthinking it

if you have a registered lower you can swap uppers on it , even different calibers

the caliber engraving on the lower is irrelevant to what upper your running on it

you can notify the nfa of the change but it is not a requirement

[the_1iviper]

In the caliber section just put Multi.

i don't think this flies with the batfe , when filing out the form they want what caliber the gun is being built as

[prepare]

i don't think this flies with the batfe , when filing out the form they want what caliber the gun is being built as

Mine was approved. Plus my lower says multi.

[Alex V]

i don't think this flies with the batfe , when filing out the form they want what caliber the gun is being built as

My BCM lower for approves as “MULTI”

[JimmyB62]

Unless things have changed (yet again), it doesn’t matter what the current marking is on your rifle. The caliber that you designate must be engraved conspicuously on the rifle. So if your rifle is engraved 5.56 (or multi) and you are going 300blk then just add 300 to the makers engraving. Multi used to be accepted when filling out the F1, then in the recent past F1’s were getting denied for Multi. ATF will not know what the markings are on your rifle so that really has nothing to do with approval but on the form 1, its required. For anyone claiming their form 1 was approved in the last 3-ish years with “Multi”, please take a screen shot of the caliber box on the form 1.

Joe Smith (or trust name)
Bumblebee, WY 300

That’s all that’s required and it doesn’t need to read 300 blk, just 300. Once your form 1 one is approved you can change it to any length or caliber without notifying ATF. THey *request* that you notify them of a permanent change but its not required. And how does anyone define permanent anyway?

The safest way to do a form 1 is to indicate a caliber on the form 1. There’s no reason to use multi on the form.