Newb questions here guys...

Does a AR15 receiver have to be marked "Multi" to build ANY type of gun or caliber on the action?   i.e.. an AR pistol, or a variant caliber...

Does a lower marked 223/5.56, such as Anderson, have to be built as a .223 or 5.56 rifle?  Or can it be used to make an AR pistol such as a .300 BO, 6.5 Grendel...?

I know, I know...paranoia will destroy ya...

Thanks guys

Caliber markings on the receiver are meaningless. You can build it into whatever caliber you want that's compatible with the AR15 lower receiver.

FPNI.

What he said

Well, they're not totally meaningless, but they're only meaningful for licensed manufacturers, who are required by law to mark calibers or bores on their completed firearms. When they sell unassembled receivers, the receiver doesn't have a bore, so doesn't have to be marked with anything, or can be marked "multi" which many manufacturers choose to do.

As a private person who is not a licensed manufacturer, though, none of the federal marking regulations apply to you, unless it's an NFA firearm you're making. Build whatever you want, regardless of receiver markings, and if you're actually making an NFA firearm like an SBR, you can mark the caliber on the barrel instead of the receiver.

Thanks guys!
I appreciate it!

The ATF requires that a caliber designation be on all firearms sold, however, there is no requirement the markings have to match after purchase.

Not correct.  Prior to 1968 no serial number or caliber was required on non-FA firearms.  Millions of these guns exist today and are bought and sold regularly.

There are no Federal marking requirements for a self-built non-NFA firearm.  Ever.  State law may be different.

They SUGGEST a serial number but that is not a requirement.

Okay.

The ATF now requires a caliber designation be on all firearms manufactured by 07 FFLs, however, there is no requirement the markings have to match after purchase.

What does "prior to 1968" have to do with what the OP is asking? This is 2017.