Picked up and Armalite AR-10 in a trade. Couple questions on the upper. I cant find any marking on the barrel or upper receiver and when I took the flash hider off to mount my AS#R suppressor mount the threads are 3/4" long vs the more common 5/8". Is this normal for Armalite or is most likely an aftermarket barrel and upper on an Armalite lower? At this point I'm not shimming the suppressor mount so I'd have to have the barrel rethreaded or replace it. Since I'm not having much luck with an Armalite barrel, is their any issues with buying a complete DPMS pattern barrel and bolt carrier group?

Best bet is to get a hold of Armalite. They have their own tech forum with a lot of FAQs. Or just call them and ask.

When you say 3/4" long it confuses me. The thread is usually 5/8" in diameter, not length. So, is the 3/4" the diameter or length?

Sorry, the threading is 5/8x24, but  it is threaded .750 deep vs the more common .625. So with my ASR suppressor mount torqured I have a 1/8" gap.

Gemtech sells thick shims.  Brownells and others carry them.  They are just a couple clicks away.  Don't overthink it. I have had to buy this kit twice. I seem to recall I had one that needed two of the thickest shims. 

Link

Many barrels aren't threaded all the way to the shoulder with a proper relief cut.  In some cases, even the chamfer cut on the can or muzzle device will appear tight, but will be skewed by the interference.  A proper shim fixes this. 

This image shows the relief cut on the smaller thread size: 

Armalite uses pretty thick crush washers. Im at work so cant measure mine, but they would take up that 1/8" for sure.

On a side note, there is nothing wrong with shimming your suppressor mount if you use the proper loctite or rockset. its actually better because it doesnt affect the barrel harmonics as much as a torqued down muzzle device.