Changing Caliber of SBR?

[Ricochet]

This week I will have completed my NFA Trust and will immediately begin to process the ATF forms for my first sbr build.
However, is it possible if I want in the future to change a 5,56mm sbr not a 9mm sbr with a new upper and Hahn Precision magwell kit?

Basically, if a sbr is originally a 5,56mm , does it by law have to always remain in that caliber?

Thanks!

[theunrulychef]

You can change calibers & barrel lengths all day long. Just keep the original upper / configuration so you can put it back. You can put a rifle lenght barrel on if you want, then switch it back. If you make a permanent change (ie sell the old upper), you should send an update to the ATF with the new specs. Basically, the ATF realizes that ARs are like lego kits, so they give you some leeway.

[Ricochet]

I'm not worried about the upper because that's not considered the firearm.

But with the Hahn precision magwell 9mm kit, it changes the caliber of the NFA Registered lower receiver.
That's the info I need.

[theunrulychef]

It's a temporary kit? It shouldn't be an issue.