What happens with them?...
Basically, this:
...I had brand new G19 with 10 round mags, and it would almost always hang-up on the l last round in the mag, even with fmj's
... but not only on the last round.
There have been a couple of large tests done by police departments that I know about. Roughly 10 years ago, ten OEM 10-round G17 mags were tested by a west coast department. All 10 cripple-mags malfunctioned, but none of the standard capacity mags in the test had any problems. "9mm3" followers were substituted for the funky followers in the cripple mags, and they were still not as reliable as 17-round mags.
Within the last few years, another test was done with Glock OEM 10-round G19 mags. Ten proven-reliable Gen3 G19 pistols were used, testing the 147 grain practice and duty ammo used by many police departments. By about the 500-round mark, all of the pistols had stoppages due to the 10-round cripple mags. As with the previous testing, the good (standard #3) followers were substituted, and all ten pistols still choked within the next 500 rounds.
Based on a tip from someone within the Glock organization, they even tried replacing the springs in the G19 cripple-mags with springs from G17 cripple-mags, but they continued to experience malfunctions... most typically nose-dive failures to feed and jams within the first several rounds in the mag.
Please note: All of this testing seems to have been done with 147-grain ammo, as that's what was/is in use at these police departments. Federal AE9FP 147gr FMJ practice ammo is commonly used because in weight and velocity it mimics Federal P9HST2 147gr JHP, a common duty ammo. AE9FP has a flat nose (like a hollow point), and that "non-round nose" could be part of the problem.
Round-nose 115gr and 124gr ammo may not exhibit the same problems, but I took great interest in these tests and recorded the info because I prefer carrying 147gr ammo. Winchester RA9T 147gr JHP duty ammo was also used for some of the testing.
When you have time, if you have links to this, I'd be interested in reading them. I'd never heard it. (and I'm not a Glock guy, so I've got no dog in that fight. I'm merely curious and want to learn more about something I'd never heard before.)
Sorry, no links, as I copied all of this material some time ago for my own records. I can tell you that the sources are Dr. Gary Roberts and some other people who are similarly in a position to know. They did not divulge official police department documents or anything, but I personally trust their accounts of the test results.
Dr. Roberts also stated, "In each Glock Armorer Course I have taken, the Glock Rep has clearly stated the neutered 10 rd magazines are NOT as reliable as the correct standard capacity magazines designed for the pistol and to NOT rely on the neutered 10 rd magazines for duty or defensive use."
It seems that Glock rushed the 10-round cripple mags to market to comply with changing mag capacity laws and that they did not test the mags to the same degree that they tested the mags that were designed to be used with their pistols. That makes sense to me, and I do not hold it against them.
Everybody should test their defensive ammo with the pistol it will be used in to make sure it functions reliably. (How many hundreds of rounds fired without a malfunction is a personal preference, but I would recommend firing at various tempos and positions in simulated self-defense scenarios.)
Yea it's weird as I have a dozen 10 rounder G19 mags for my sub2k and never had that issue with JHP
That's a good data point with respect to that firearm, but just so everyone understands
it doesn't say anything about cripple-mags in Glock pistols. The internals of a sub2k are not the same as a Glock, and everything (firearm, mag, ammo, person operating the firearm, etc.) all work as a system, so changing just one little thing in that system can make a difference.