Okay. That's lug setback. The rounded top edge is probably from the flanging as the slide passes over it and ironing it out rather than from a likdown timing or clearance problem.
This is one of those one-in-five Norincos that I started to see later in my examinaton program when I put out the call to any owners to bring'em in.
I went through a lot of'em without seeing any serious barrel issues...and then they started to show up more often. The 1:5 ratio is an average of all that I looked at.
You're due for a new barrel, and a Norinco take-off barrel won't do it. You're gonna need a hard-fit barrel, and you'll wind up with the firing pin strike low on the primer unless you have the rails peened to lower the slide to spec height from the slidestop pin centerline. This isn't a simple rail peen/swage operation as with a gun that's within spec, but just worn. I did it just recently with a Norinco...my second one that required a full frame reworking...and it's labor-intensive. The cheapest route is to go with a full hard-fit barrel and live with the off-center pin strike.
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and I can't quite understand the metallurgical situation of the barrel, I thought I saw a stress crack in an area of not great stress
Soft steel underneath with a hard chrome plate. If the lugs are sufficiently engaged vertically the barrels hold up well. If not... this is the result.
And... Stress in an area of not-too-great stress... The lugs are under a huge amount of stress every time the gun fires. Specifically, on the front faces of the barrel lugs and rear faces of the slide lugs... as evidenced by the severe lug setback and deformation seen here. This is the worst example I've seen yet, no doubt from a lot of use before the former owner noticed the damage and decided to dump it.
