I’m building an AR 9mm pistol with a PSA “Colt” magazine Lower receiver and a MA parts 9mm upper receiver, with a S&B arm brace.

I’m still waiting for my 8.5” 9mm barrel to come to complete this build.

My question, how do you check the head-spacing on a 9mm AR pistol and when installing the barrel into the upper receiver, do I need to “coat” the inner barrel sleeve/shoulder(??) with a product?  If so, what type of product should I use?

I want to ensure that “I” install this barrel properly to prevent any possible damage to either the barrel or the upper receiver.

This build will basically be a “range toy,” but I still want to ensure that I do everything correctly for this build.

Thanks for any assistance with this!!

Typically AR uppers headspace perfect with any reputable barrel installed. To check it you would just assemble and use go, no-go gauges and the bolt to be sure. Or did I misunderstand the question?

Install the barrel like any other barrel, Aeroshell on the threads, season 3 times and send it.  Then go shoot it.  If it is a blowback 9 there isn't anything to headspace, the bolt will stop when the case mouth hits the end of the chamber.  Then press the bang switch.  You will need a proper buffer for a blowback 9, that is about it.

Thanks for the replies and info!!!!

This.

The only way the headspace is dangerous is if the barrel manufacturer screwed up and cut the chamber short.

I’d repost your issue in the Rimfire And Pistol Calibers forum.  There aren’t a lot of issues you can run into with building a 9mm AR upper, but understanding the technical issues related to blowback pistol calibers and headspace is useful.

In short, just install the barrel as you would for a gas-operated AR.  If you can chamber and extract a round with the bolt you’re going to use with it, it’s fine.  Issues come from situations where the combination won’t feed, won’t properly chamber, and/or won’t extract.

In for results…