Breech face gas cutting has occured (consistent with the primer cup), and has for some time in this Glock 17, and it was the same in the G19. I've also run these loads in other guns with similar results (e.g. slight breech face marks), though the ring has not formed due to lower volume shooting. I've even run them in once-fired, sorted .45acp brass (with a hammer-fired 1911: Unique 3.9-4.0gr w/ 200gr LSWC @ 1.135", which is a tame load) to rule out over-pressure, flash hole difference, primer impact deformation, stretched primer pocker (e.g. brass issues), etc. (same result). It does not seem to matter if I use a faster or slower powder, with mixed brass or not, nor does it seem to matter using once-fired brass or more-fired. I have not observed damage to primers, fired primers remain seated (i.e. are not loose), and new primers seat fine in fired brass (i.e. smooth, with slight resistance and seat fully) using two different Dillon SD progressives.
So, becoming more curious only recently, I've narrowed my theory of late, and do not think brass is the culprit. I believe the harder primer cup is not expanding to allow a tight primer pocket seal at the lower handgun pressures, 9mm or .45acp. Meaning, not over-pressure or a streched primer pocket, but under-pressure (weak) loads with these harder older red box CCI SPPM primers. I do think higher 9mm pressure (vs. .45acp) exacerbates it, but the 9mm pressure, at least the weak 1000-1050fps loads I've run, is still below the pressures needed to seal these SPPM in the primer pocket, meaning more gas escapes. And, since most of the cutting in my 9mm is from (older red box CCI) SPPM, the hardness of the cup must be more like rifle primers. I've heard/read, and have experienced myself via light primer strikes, that these CCI SPPM are harder than current production and much more like rifle primers. So, none of this may matter if you don't have those, but could be applicable if you plan to run SRP/SRPM (i.e. harder cup primers) in 9mm.
If I push the 9mm faster, to +p or +p+, it may go away or lessen, and I'm testing that now running all loads at ~1150fps. I have already noticed differences in primer flow patterns (see image below). But, I can't push .45acp fast enough if low pressure is the cause. Regarding primer hardness this picture of primers shows differences in indent depth and primer flow into Glock striker hole. As is often the case, the picture is not as clear as real life. These examples are representative of the lot:
Top (blue marker) = Older Red Box CCI SPPM w/ 4.7gr Unique (some primer flow, shallow indent)
Second Row (red marker) = Older Red Box CCI SPPM w/ 4.2gr Unique (little/no primer flow, shallow indent)
Third Row (cleaned/tumbled) = Range Brass (clear primer flow, deeper indent)
Bottom (cleaned/tumbled) = SPP w/ 4.2gr Unique (clear primer flow, deeper indent)
(Note: A Berry's 124gr FP w/ 4.9gr Unique @1.030" ran ~1150fps for me and 5.2gr ~1210fps, so the 4.7gr load pictured is below 1150fps.)
Update 2/19/12: I ran the 4.9gr Unique load and primer flow and indent depth appears identical to the 4.7gr load. So, >4.7gr provides enough pressure for primer flow that should seal the pocket. Also, I just swapped 4k SPPM with SPP at the local gun show. The guy will trade my remaining 5k SPPM next show. So, that will be the end of it. If I'd have known SPPM and low pressure loads was the cause sooner I'd had swapped them already.
I've pretty much always run weak to mid-level loads. All along, and for many years, I thought the issue was using mixed brass and I did not want to sort. That, and I did not really care since I knew the loads were safe. But, I'm beginning to think conclusively that too weak loads using harder primers is the issue. More rounds downrange with hotter loads will prove or disprove that theory.
I view Glocks as tools, and I consider Glocks "throwaway" guns somewhat anyway. I consider this cosmentic for now, and will monitor to see if breech integrity could become compromised if ring gets deeper. Obviously, this does not matter to me in this Glock 17 (and did not matter in my former Glock 19) as I keep running SPPM, even SRP and SRPM. But, I don't use SPPM, SRP or SRPM in any volume in my other 9mms due to this, nor would I run them in any 9mm I really cared about, and stick with using SPP instead for now. Even if the issue goes away, it may be just these older red box CCI primers, but I won't know that for sure as I've not run other SPPM in high volume. Regardless, I often share SPPM, even SRP and SRPM run fine in 9mm. And, they do. I figured I'd share this as a PSA to let folks know your breech may show accelated wear using SPPM, SRP or SRPM if you run weak loads. To me, though I'll bump up the load to see if it goes away, it's worth it given what I paid for these primers and how cheap Glocks are since I've saved more than enough to buy the gun again.
Edited by contactcole, 20 February 2012 - 12:02 AM.