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Posted: 5/5/2021 10:52:30 PM EDT
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!!

Link Posted: 5/5/2021 10:57:55 PM EDT
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?
Link Posted: 5/5/2021 11:08:36 PM EDT
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.
Link Posted: 5/6/2021 4:24:09 AM EDT
Thanks for the replies and info!!!!
Link Posted: 5/6/2021 1:08:22 PM EDT
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Originally Posted By s4s4u:
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.
View Quote

This.

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

Link Posted: 5/7/2021 11:48:22 AM EDT
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…
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