Originally Posted by WiskyT
I load plenty low enough for my G17 and never had this problem. I have seen it in other peoples' guns with overloaded rounds though. Kind of makes you wonder, doesn't it?
A "magnum" primer is just a regular primer with a lrch more priming compound. Some "standard" primers are as hot as other magnum primers, and Winchester uses magnum primers for all LP loads.
Heck, many pistol loads are under 10kpsi and they don't pit the breach on guns. 45 Colt for example is well below the OP's pressure and you never see those guys with problems like this.
By "makes you wonder" I suspect you mean "some error on my part". However, it certainly does make me wonder as these SPPM loads have been well under load data max and run well under 1100fps (w/ 124gr FMJ and 125gr plate FP) with the recoil impulse to match a weaker load. That's the mystery to me.
As for load data examples, many of these SPPM loads have been a 3.7-3.9 TG load (some less) or a 4.2-4.4 Unique load, both with typical OAL. With Unique, a 4.9gr 125gr plated FP load I ran came in at about 1150fps and 5.2gr ran about 1210fps. I'm not aware you can have over-pressure without correlated velocity, felt recoil, flattened primer, primer pocket/cup scoring, brass flow/bulge, etc. - i.e. signs of too much pressure - so I welcome that insight.
I can say from experience these older red box CCI SPPM are harder. Actually, thinking about it, they do act much like SRP and SRPM in my Glock in terms of cup hardness (light hit indent, lack of primer flow into striker hole and intermittent light primer strikes). I've run about 20k of 'em and have maybe 10k left. I've had many light primer strikes with these particular SPPM in my former Glock 19 (had the shorter cruciform) and a few in my current Gen3 G26 and about as many (as the first) in my current Gen2 G19, whereas this Gen3 Glock 17 (with longer cruciform) will pop most of them the first time and most that don't won't pop the second time either (I have to use them in a hammer fired gun). But, all the Glock hits are much lighter on these SPPM than SPP (see pic added to OP). Meaning, they are in fact harder. I've not had much experience with other SPPM than these though. So, I presume your direct experience and applicable "problem free" comparison is running the older red box CCI SPPM at ~1000-1050 fps (w/ 124gr 9mm) in high volume?
I'll be running all 9mm loads at 1150fps (~4.9-5.0gr Unique). I do know these weaker SPPM loads have not showed primer flow into the Glock breech face primer hole as SPP do. So, that may be a clue and if I see that in hotter loads it will be telling. Though I'm curious now, I've never really thought so much about it before dismissing it as a mixed brass issue since 1) I did not care (and still don't in this gun), 2) I never heard/read harder primers could be the issue, 3) had >30k of these CCI SPPM and 4) I knew loads were weak/safe.
Regardless, I'll report back after after more rounds downrange. And, pic of primers added to OP that shows pretty clearly the weaker 4.2gr Unique SPPM load lacks the primer flow of the other loads.
Originally Posted by Colorado4Wheel
It could be that his load is so low in pressure that it just doesn't seal the primer at all. I don't know for sure. Perhaps dirty pockets and a low load. To me he has wondered of the reservation with out a guide and anyone is just guessing at this point. I would go back to using the right primer and standard loads and stop fiddling around with non-standard things until he figures it out.
I think you are spot on. My recent testing and observation has ruled out brass issues while velocity, recoil impulse, inspection and load data rules out over-pressure. Plus, I confirmed the same issue exists in much lower pressure .45acp with SRPM (and presumably SRP and SPPM given enough time). Furthermore, I have other experience that supports these older red box CCI SPPM are harder, and we all know SPR and SPRM are harder than SPP. That pretty much, and I think fairly conclusively, leaves low pressure and primer hardness. So, my next effort is to bump up all loads to ~1150fps.
I posted a pic in the OP showing differing primer indent depth and primer flow patterns. The higher velocity SPPM load, the SPP load, and the factory loads all show the characteristic Glock primer flow into the striker hole. The weaker SPPM load does not. So, that's something I'll be looking for in the ~1150fps loads.
It will be news to me that running weak SPPM in 9mm can result in gas cutting due to lack of primer flow/expansion to seal the pocket. It may be just these older red box CCI SPPM (that are harder), or not. That I won't know since I've just loaded these red box CCI SPPM in any volume.