Is it necessary to check headspace on a 9mm build? Running the following:
Qc10 small glock frame
VLTOR mur 1st
7" Adco bbl
JP ent. Bolt

Thanks guys

Headspace in a blowback gun isn't nearly as critical as in a locked, gas-operated gun.  I've never checked this in the several 9mm builds I've done.

Basically there are three potential conditions.  First, it's too short a chamber, and you'll bulge a couple of cases firing it before you figure this out.

Second is a too-long chamber, and you won't get any to fire, because the primer won't be where the firing pin needs it to be.

Finally, there's the almost certain condition where ADCO made sure the chamber was well within spec, and your gun will just run.  I'm betting on this condition.

Measure the protrusion of a cartridge from the barrel....it should be around .135 .......then measure the bolt pocket, it should be about .125.......you want a little clearance between the bolt face and the breech face when a round is chambered.

That how I been setting my 9mm stuff for years..  Makes sure that there case is sandwiched between bolt face and case mouth, and allows for the case length variations of makers brass too,, also makes for slightly quieter suppressed firing as the bolt face not clunking on the barrel face.

I would only add this (from personal experience): On item one, if the chamber is short.....you may get a Kaboom! rather than a bulged case.  Damn it gets loud when those cases blow out!

I bet it is!  That would have to be a pretty short chamber, though, wouldn't it?  I hope nothing got hurt - you included! - when you found this out!

MadMachinist and SBR, where is your reference point on the barrel for the protrusion measurement?  Does it matter?  I have a depth gauge on my dial caliper, but I'd like to know if there's a preferred spot to check.  And do you check with a case that you've trimmed to SAAMI standard length, or any specific length at all?  I ask because there are 9mm (and other pistol caliber) headspace gauges out there, but with rounds like this headspacing on the case mouth, I'm not sure a precision gauge actually brings anything useful to the equation if you can do pretty much the same with a sized (and maybe trimmed) pistol case...

I get bulged cases with my 9mm AR Is this something I should be worried about?

Do they look like they're pregnant, or just overweight?  A small bulge is common and not a problem (the whole case head just can't be fully supported), but if it's a LOT of bulge, that is a problem.

It is an indication of either too deep a feed cone in the barrel....or too light a buffer......more mass in buffer.....means a heavier combined bolt weight.....meand the breech opens slower ....less chance of bulging cases.

I measure protrusion form the flat of the breech face on a colt pattern barrel. I fresh factor round of what you are going to shoot the most is a great place to start......winchester bulk pack tends to run close to Sammi Max so it is a good round to work with. You are correct....headspace gages are not required....I have them, but they aren't a requirement when dealing with blowback actions.

Good evening from Ohio my friend Mr Porter..  You going to keep me busy here..  AR-15 measure is with case in chamber to barrel face,, I prefer to use a surface depth micrometer, as it has large wide flat footing to sit on case, and thimble the rod down to the barrel face (flat)

To answer your question about case for chamber/headspace measure.. SAAMI says 0.754 for chamber.. PHOOIE !!!  There is no commercial ammo that comes with a 0.754 long case, so setting chamber with that long of gage is beyond stupid and waste of time... all the yrs I been playing with 9mm, most all the cases you'll come across run from 0.743 to 0.748" inch long..

I use the shortest sized case when setting headspace here on 9mm AR-15 and my M11-9 SMG barrel cut from blank.. then with the 2 sheet of paper longer 9mm cases, I am still good for capturing case between bolt face and case mouth..

In a past argument here, fella maintianed that 0.754 was the rule.. ok,,,,, use a 0.754 SAAMI GO gage to set chamber,, now load a off shelf store bought cartridge in the chamber,,,, remember the 0.743 - 0.748 cases that we all have.. they sit 0.006 to 0.011 deeper into the chamber,, now we have front of bolt clunking on barrel face, short dowel in long hole, fire pin lost that much hit to primer, and case does not get snapped under extractor proper..

Only use I have for a 0.754 gage is "Field" test a pistol with tilting barrel, I would prefer my 1911 or GLOCK not close on a SAAMI 0.754 GO gage.

I try to keep it simple for you, maybe Rudy ?? jump in and help out too

SBR7_11 ...........Great minds think alike......your .135 protrusion number is EXACTLY what I set mine at when I make them....and with blowback I too use a factory round.............once we start running blowback barrels on the CNC I'll come up with a set of gages to use as my programmer isn't a gun guy........hell of a machinist and programmer but cringes when I do a chamber on a blowback manually with a cartridge.

Look overweight. They will resize.

Thanks, guys.  I remember SBR's "exchange" with the SAAMI purist, but I couldn't find the thread with our "wonderful" search engine.  I can use my caliper/depth gauge to check for case protrusion without any real issues - it's not a surface depth mike, but it's good enough for this application, as long as I pay attention.  With a factory case (or a sized, once-fired factory case), look for 0.135" of case sticking out, and then check bolt face depth similarly.  Not that I have any issues with my 2 different 9mm barrels (a RRA 16" and a Spike's 10.5") and 3 different bolts (RRA, CMMG and one Rudy just Glock-milled for me), but sometimes it's "good to know."