Thanks for all the replies. Answers, in no particular order:
- The gun probably has about 500-600 rounds total with all original parts except for the Pierce +1's.
- The issue happens almost 90% of the time with the first round in the mag using either the slide release or slingshot. It also happens mid-mag while shooting but, say, only 10-20% of the time once loaded. No where near reliable enough for self-defence use as-is.
- I don't reload or use reman ammo. Only Blazer/Winchester/Hornady/Federal ammo.
- I can't say if this is new or pre-existing as most of the ammo I've run through it has been FMJ. My shooting went idle for a long time and the gun sat in its place, fully loaded, with 7 rounds in each mag for all that time so I could see that the mag springs may have relaxed. I put about 50 self defence rounds through it when new and don't recall having any trouble. So, good when new, several hundred through it, then sat idle for a year or more with fully loaded mags, now 50-60 more rounds with constant FTF. 100% reliable with FMJ's.
- I only have the two original mags. The FTF happens with both of them with, or without, the Pierce base plates installed.
- I've run out of JHP so I can't test anything at the moment until my new order comes in.
- I have two new OEM mags coming in today.
- Didn't think to tear the slide down to check for sludge in the extractor. I'll check that tonight when I get home.
I called Glock today and it sounds like I might just return it and let them check it out if this keeps happening with the new mags. He did ask if I had the -02 mags but I don't have it here so I couldn't answer. I know internet diagnosis is difficult (or impossible in some cases). I posted this to query the hive-mind and document the fix once i have it.
I just bought a 43X and will be swapping slides when my new ammo gets here just to see if that makes any difference before I send it back.
PS - The 43X is
sweet! I traded in my G19 for it.