An update!
I went to the range yesterday, fully prepared:
* cleaned the gun and used a thinner oil, as I mentioned
* all mags were fresh clean, I also used DuPont teflon spray on them (not for lubrication per se, but preservation)
* done a lil' polish to the slide stop tip (as in
here) - nothing invasive, I took almost no metal
* took the factory recoil spring
* took a little ammo box to collect the malfunctioning rounds
* I even took a caliper
So, I went and shot 290 rounds, I counted (shot 4 boxes of 124gr, 1 box of 140gr and 1 of 150gr, all S&B).
Not one malfunction! I was happy like a child. And before that I could be skeptical because, well, 300 rounds may not be enough to jam the gun up. However, this time, I really felt the snappiness. What I mean by that is when I was closing the slide via the slide stop or by slingshotting it, I sometimes (especially later during the range visit) could feel it dragging. Like it wasn't snappy, it was sluggish + I sometimes could feel the round hitting the feed ramp and only then feel the second "click" of the action closing.
This time it was different. No matter if I pushed the slide or used the slingshot method, each time it was really snappy and fast. Same thing during shooting. Sometimes, during the recoil, I could feel a strange movement of the slide. Like it wasn't just back->forward movement, a quick one, it felt like the slide was "sliding" or "gliding" on its own timing. Hard to explain, I'd attribute it to a thicker oil. But to sum up, each time the action closed it was snappy. I had issues with it before when slingshotting (probably because I didn't take my fingers off the slide soon enough and I was riding it back for a tiny moment), but now no matter what I did - snappy (I'm repeating myself so much).
Anyway, aside from the mags and the different oil, nothing changed since my previous visit which had serious hangups after like 70-80 rounds.
I'm telling myself that this has to be the mags, but deep in my heart I feel that this is due to the oil. I really don't wanna entertain that thought, since if it is/was the oil, then I wasted time, energy and money just to fix something that could be done by going to the other room and grabbing a different can of oil. I think it could be just a combination of things that was necessary for my specific gun:
polishing the feed ramp, deep cleaning the mags, changing the oil I use. And probably each one of these helped on its own, as the gun got dirty during shooting, it wasn't enough. Now I did them all and even at the end of the trip, the gun pretty much felt the same as during the first shots.
Soo, I don't know. I think I could say that the issue is resolved. I don't think I can provide an answer like "it was X" or "it was Y". I think that it was "X, Y and Z", but at least for now I attribute the most of it to the oil (really thin one, spray form).
One question: I shot 300 rounds, okay. As it was mentioned previously in this thread, the gun should be able to shoot more without cleaning. Not that I endorse this (I clean my guns after every range trip), but just to check if it's really "fixed" - should I maybe not clean the gun, leave it be and go to the range next week? I would just probably oil it before going out. This could prove that it wasn't just luck and the issue is gone. I thought about that because if I clean it, then the next range trip I'll also be tempted to shoot 300 rounds (my wallet burns) but if the result is the same, it's not that meaningful. But without cleaning it, each shot is a meaningful test, piling up on those 300 I've shot yesterday. And I could then come back and say: "hey, today I shot another 300, it's 600 without cleaning the gun and not one hangup!". What do you think?
