After Christmas I want to begin shopping for a used "shorty", 11-14"upper for a pre-ban (Form 1 is pending).  I understand all the normal issues with a used upper except many of them are sold incomplete, eg no bolt or carrier. I also know that when you use a different bolt/carrier the head spacing must be checked. But, what do you do if the head spacing is out of spec?  I would be throwing "good money after bad" if I bought a used upper and then had to have a lot of work done on it to get the head space in spec, or buy several bolts/carriers until I found one that was OK to use.

Also, if I find one with a long flash suppressor or break to bring it up to 16" can the suppressor or break be easily remove and a regular length (birdcage) suppressor easily added?

As far as the headspacing issue goes for buying a used upper I would specifically discuss this with the seller and have an understanding that you want to be able to return the upper if its out of spec. Then check the upper with your bolt upon its arrival. Definitely reasonable and at most you'll be out shipping.

As soon as you take the long brake off of it, you have a "sawed off rifle". You for dern sure better be on the good side of ATF (registered or whatever).

Yes, I understand it's a SBR.  That's why I stated the Form 1 has been submitted and is "pending." Won't be purchasing or removing brake/supressor until after stamp is received.

gecko
I know it's a little more expensive but your best bet is to but the two together. Upper with carrier/bolt complete. That way there is less of a chance of a headspace problem and the results if there is one. Kinda like Armalite AR-10's... always sold as a complete upper half.  Solves the liability of a rifle blowing up and the litigation afterward.

I have several uppers for various AR and FA rifles/carbines. I too was thinking I could just swap out carriers/bolts and save then I started thinking each upper would have a certain amount of wear and the bolt would have the same amount of wear times however many different combos I used it with.

I'm probably paranoid, but the additional cost is a small price to pay considering the additional performance and safety. Good Luck!

If you have a chrome lined chamber and barrel and the headspace is out of spec then the only thing you can do is swap bolt and carrier assemblies until you find one that is in spec.
if you can't find one that specs out then you have to toss the barrel and buy a new one.

If you have a non-chrome lined barrel and chamber you can have a gunsmith ream it out a little to see if that brings it back into spec.