I am going to take it apart and clean the internals... I don't really trust myself with taking a stone to them though. I have no experience with that type of thing, admittedly.
Assuming this isn't just a cleaning issue and it actually is a QC issue with the gun (though I would find it weird 391 rounds caused this, almost all jacketed ammunition and not bare lead), I'm wondering if my best bet to get a reliable out of the box .357 would be to find a new/unfired Model 28 Highway Patrolman or something. But then there's spare parts and what to do when/if there's a breakage. I managed to find a 25-2 unfired, and I do see M28's unfired from time to time. Actually to be honest, I just want an M28 anyway to go with my Rugers and M25-2.
Ruger SP101 Trigger Job Guide (kevinsworkbench.com)
This works for the GP100 too. I have done all of my Rugers to some extent by following this guide, just do what you are comfortable doing. I improved a really bad gritty trigger (I would have sent it back but he insisted) on a coworker's gun by doing steps 21 - 23 on the latch/trigger return spring
Three new things that didn't work would get the grownups all wound-up: cars, lawnmowers and guns. You are right about new guns even then; a lot of them got hauled to the LGS that had both a gunsmith and a reputation. There was indeed more QC on a brand-name firearm before it left the factory back then, but they weren't all perfect. I remember guys breaking in guns as carefully as they broke in their cars, and some of either would take their sweet time.
gn
"On the internet, nobody knows if you are a dog... or even a cat."
I could be wrong, but I get the impression that QC was higher on revolvers back then since they were modern issue service handguns at the time with forged components. These days I get the feeling S&W sees the revolver line as more of a consumer/recreational market thing. Again, I could be wrong.
Looking into what I'm going to replace my 7-shot GP100 with, I've discovered that TK Custom will sell a stainless 6 shot 4" GP100 with the option of hand work done to it with a Basic Action Job and Cylinder Chamfering. There are options for moonclip cuts and a more advanced action job, but after corresponding with the owner, who has assured me the gun will be looked over and hand worked to make sure there's no defects, I think that is the route I'm going to go (basic action job, cylinder honing, cylinder chamfering). It's more for the additional QC than the action job and cylinder chamfering, but those are a bonus.
I had forgotten about this because it was a while back and not my gun, but a new Ruger SP101 4" .357 that was the first one I had seen was not behaving for its owner, and the trouble ended up being a combination of a marginally fitted hand and some tolerance stacking. I installed a new hand and checking via my magnifier made sure there were no burrs on the ratchet or the cylinder stop notches. It still required a little use before I thought it was as good as it could be, but the owner was happy enough right away.
That turned out to be a very nice launcher of .38 Specials and a dandy - if a bit long - compromise between a kit gun and a utility revolver.
gn
"On the internet, nobody knows if you are a dog... or even a cat."