Judy's New G19...
...had several instances of jamming, and since she was a new shooter, I ascribed some of the problem to her.
At any rate, I'd ordered three additional Glock OEM magazines for the gun, and when we marked them with tape, intending to put a mark on the tape for each malfunction, low and behold, all the marks ended up on one magazine. Who'd of thunk it! We exchanged the bad mag, and have since added a 33-rounder. 8500 rounds later--flawless!
My G17 has just passed the 9500-round mark without one hiccup!
But ammo matters! I got two shipments of .45 ACP Blazer Brass from Natchez, and some of the cartridges were 1/8" shorter than average!
These short ones would not feed in my G21, but would hang up on the feed ramp. This might've been lucky, as the short ones may have been over pressure from the bullet set-back!
Those were the only problems I've had with my G21, in .45 ACP mode. This gun has over 12,500 rounds on it--the latter 1200 being 10 mm. In converting the gun to 10 mm, I did have to experiment with ammos, springs, etc., to tweak the conversion to 90%+ reliability. I believe I can yet get it to 100%.
So far, all three Glocks have been fabulous firearms!
--Ray