Check the upper barrel lugs and the mating slide lugs for peening, deformation, and a "Stair-Stepped" appearance.
Please note that these old pistols aren't really suitable for a lot of use. They're dead soft, and because there's no way of knowing how much ammo has gone through'em...you don't know if the next round will fracture the slide or if it will stand up to the next 10,000.
Back in those dear, dead days when you could barely give a USGI pistol away, and the gun show vendors had'em stacked on the tables like so much cordwood...and the price for an average example was about 35 bucks...my father and uncle bought'em up and refurbished them with cheap USGI parts available on the table across the aisle...to use as shooters. GI ball Ammo was about a penny a round if you bought a thousand at a time.
My father, having a fondness for the pre-A1s, bought those over the later models at about 3:1. We averaged between 5 and 6k on a given pistol before the slide broke. No problem, because slides were also cheap, and the two rascals usually picked up a spare at every show.
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