According to the guys on various Curio & Relics lists and forums, including some guys who are really experts with the P-38s, those stories about slave labor sabotague are just that -- stories.
The guns were closely watched during production, all were inspected by Wermacht inspectors, and they were tested.
I don't know, personally, of any factories that used slave labor in gun production -- but I am not that well read in that subject and haven't found a way to chase that story down.
Lots of slave labor in other labor-intensive activities, though. Food production, clothing, many other pits and pieces of the war machine.
I would think that the German Army and industry would have been very wary of letting slave laborers near assembly lines where the workers could, through subtrefuge, get access to weapons a piece at a time -- or fabricate something even more concealable.
And, I would think that the only way to really screw up a slide (to cause cracks, for example) would be to screw up the heat treatment. I'm sure THAT process was closely watched.
That's not to say that some factories weren't better than others, etc., or that whole batches of guns couldn't have been screwed up due to bad heat treatment. (You know the stories about the early 1903 Springfields.)
The P-1s, for there to have been so many in use in the US over the past 4-5 years, seem to be relatively trouble free. I suspect the one mentioned above is an exception.
