The ATF decided that barrels could not be imported with kits a little while back, so the original barrels where left overseas. No legal requirements different that I know of except for receivers.
The barrels are not restricted except for importation and of course any sbr rules about length.

As long as you follow the rules for sbr in your state, and find good receiver, you may build what ever you want.

Not legal advice, and subject to change and local or state and federal regulations.
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