Does anyone check their headspace after attaching a 9mm barrel to the upper receiver?

What locations and measurements would be checked for headspace on a blow-back 9mm AR ??

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There is only a couple of us here that understand  real values for an AR-15 in 9mm

Have the barrel on table top, and measure away

SCREW the SAAMI .754" gauge

Most commercial 9mm on open market sport a 0.743 - 0.748" case length..

I use the  shortest case I have as gauge, and set chamber depth so tail end of case stick out end of chamber 0.135" with the shortest case

With the std pocket depth of 0.125" for Colt pattern bolt,, we will have case firmly held by case mouth in barrel, and by bolt face at rear, with a wee bit of clearance between face of bolt and face of barrel,,  only time bolt should spank barrel is on empty chamber

Remove barrel, drop a sized case or cartridge in chamber, use depth mic and measure how far case stick out end of barrel

So yes, but....

Until you have a standard bolt face dimension, there isn’t a way to standardize the relationship between the parts.

We do have standards for the ammo, so with a given chamber design, you could get started as above. The bolt face differences will drive the discussion.

So are you two the "only a couple of us here that understand  real values for an AR-15 in 9mm"?

Can you offer any insight on why SAAMI doesn't understand this?  Or, why most other large manufacturers don't understand this either and keep doing it "wrong"?

Just asking for a friend...

I never have.  Never have had any issues with them either.

I didn't think it was important for blowback guns

Clarify ??

Its blow back. Lots of room before a big kaboom !
You would most likely notice bulging or splitting on near every case if it gets way out.
Then decide if it is really that far out or reloaded cases used way to many times ?
If you are not having issues or noticing cases deformed then it is most likely GTG.

The only thing I worry about is bullet seating depth. Had some ammo that was a little long and when the bolt slammed home it was very hard th pull the charging handle back