I currently have 2 SBR’s. One in 556 and one in 300 blk.

I also have a 556 lower with a 9mm adaptor and a 16” 9mm upper.

What I would like to do is to SBR another lower and out that 9mm adaptor into it.

The question is. On the form do I list the bee caliber as 9mm? The adaptor is just that, an adaptor, it can easily be removed.

Or do I list the caliber as 556 since without the adaptor that’s what it will be used for ( I have a spare 10.5 upper laying around )

Are there any state laws regarding pistol caliber SBR's?
If not, either way as long as you can convert back to the other.
But, what I would do is register it as whatever caliber the stripped lower is, if any.
I would not worry about it.
This is my attorney, best in TX regarding NFA laws, very helpful, you could send them an email

Here is NFA Question Forum

Thanks for the help.