Does anyone happen to know why some platforms can be registered in one caliber yet swap pieces to shoot other calibers (M16, SBR AR-15) while others are registered for multiple calibers (HK sear, UZI)? Are there features of certain models that require multiple caliber registration on the Form 4?

Thank you!

It's my understanding you can swap at will, same as changing the barrel length after initial registration.

I do know putting "Multi" on the form gets them kicked back.

At least in the case of the sear, I'm guessing because it may be because the piece being registered doesn't have an inherient caliber. When registering a AR lower for instance, you have to give certain characteristics of the rifle (length being one of them along with caliber). While it's arguable that it's just a lower, and as such doesn't have a length/caliber independent of an upper, that's what the form says.

The sear might be seen (in the eyes of the law) as something that will knowingly be swapped around. You and I know that the lower will do much the same, but the law doesn't always keep pace with current capabilities.

Anyhow, that's my uneducated guess. Would be interested to hear from someone that's more in-the-know on the matter.

BATFE used to allow you to register your NFA item as "multi."

Then we got told that "Multi" is not a caliber. So you couldn't use "multi" on your form 1/form 4.

[This confused folks for a while because some manufacturers started putting "multi" as the caliber marking on their lowers.]

Unless it's changed, you should register your item in its primary caliber. That is, if you intend to shoot your SBR mostly as a 5.56, then register it as a 5.56.

That does not keep you from shooting .300 BLK or 9mm, etc. As long as you can put it back in the configuration it was originally registered in (bbl length, caliber, etc) you are fine (i.e. you have the parts for the original configuration). If you permanently change the configuration (e.g. you sell off the original upper and don't have another one that fits that configuration) then you should update your registration to reflect the new configuration.

Going from memory. But I think as long as it can be easily converted back to original configuration. It's fine.

If permanently changed I believe the ATF recommends you notify them.