My understanding is that you can mail firearms state to state as long as it is not for a  sale. I think pistols need to be next day air and rifles can go however. Is that about right?

I live in one state and will be working in another for 6 months. I was thinking I might want to build up an AR. Can I just mail one of my lowers out to myself?

I'd say you're GTG.

FAQ


6. May I lawfully ship a firearm to myself in a different State?

Yes.  The package “should be addressed to the owner “in the care of” the out-of-State resident. Upon reaching its destination, persons other than the owner must not open the package or take possession of the firearm.”

Why don't you just take the lower with you?

I second the "just take it with you" comment.  If you're flying it's a pretty easy affair.  If you're driving it's an even easier affair.

And yes you can mail firearms to yourself HOWEVER you may NOT mail handguns via USPS unless you're a licensed individual (I.E. FFL), UPS and FEDEX only and they'll only mail to FFLs.  You WILL want to declare the contents because if your package gets lost and you try to claim it after the fact you're proper fucked.  

Rifles and Shotguns you can mail to yourself via UPS, FEDEX, or even USPS.  If by USPS they must go priority ground with tracking, insurance, adult signature.  If you'll not be there on the other end by the time of delivery you can ship them "Care Of/C.O. XYZ name" and that's kosher too.