Do u need to register a M203 40mm receiver if it has a 37mm barrel on it?

And if u already have a Registered M203 w 40mm barrel and spare 40mm barrels can you keep that in the same location as an unregistered M203 with 37mm barrel?

These laws are confusing

If it has a smoothbore 37mm barrel it does not have to be registered as a DD unless you have antipersonnel rounds.

The test for having a 40mm barrel is the same as having a short AR barrel.  Ask yourself do I have a legal use for this item?

If you have a registered m203, it's perfectly fine to have spare barrels for it.

This.

But those spare barrels can be easily attached to an unregistered receiver.
What if the unregistered lower has 40mm written on it but has a 37mm barrel?

I guess it's like having one SBR AR and multiple SB uppers.  Even w AR lowers around I still have a legal use for the uppers.

It's exactly like that.
What if you put a 9mm upper on your 5.56 marked AR lower? Doesn't matter.
Embrace the freedom! 

The key determining factor is "has a legal use"

If the only thing you can use it for is the creation of an illegal weapon - then you essentially are in possession of said illegal weapon

Having all the parts isn't enough by itself.

Thank you all for the input!

Does anybody have a citation/statute in regards to this?

Thanks.

ETA:NVM. I think I found it:
https://www.atf.gov/file/55446/download

Looks like the official BATFE exemption to having a 37mm, smooth bore barrel, on a M203 40mm Title 1 Receiver is covered under:

18 U.S.C. § 921(a)(4):DESTRUCTIVE DEVICE
26 U.S.C. § 5845(f)(2):DESTRUCTIVE DEVICE:

ATF Rul. 95-3:

Section 5845(f)(3) excludes from the term “destructive device” any device which is neither designed or redesigned
for use as a weapon and any device, although originally designed for use as a weapon, which is redesigned for use as a signaling,
pyrotechnic, line throwing, safety, or similar device.


Please, correct me if I missed something, and/or if there additional documentation backing up this statute.