A couple different things, starting with take a deep breath. Then:
1) Have you actually talked to FX USA? There is a lot of ALL CAPS calling out of people, but so far its all "this guy said." Did FX USA refuse to honor the warranty when you spoke to them?
2) Did they show what the machine marks were? Any specific information on the issues at all?
3) This gentleman who you traded it to for a Wildcat, he waited a month and a half to notice these issues?
4) Why would/should any of this be covered by warranty? Not knowing what exactly the damage is, or what it was caused by, is one issue. But who caused it is also pretty unclear. Several parties along the line could have caused it here, and to my eyes FX seems the least likely of all of them.
Stray tool marks and over-torque are both uncommon on precision large-scale manufactured parts, not so much because of possible sloppiness, but because it is inefficient to do things badly on a large scale. Whoever is installing FX barrels almost certainly has a torque tool to tension the barrels so that they come out the same every single time. This would be done, not just for quality, but because it is more efficient to just have a torque wrench zip each and every one in. They probably also don't make weird tool marks in weird places, because their tooling is probably optimized for exactly the job that needs doing, and so it just fits. Stray tool marks tend to be caused by using the wrong tool for a job, which at some place like the FX factory is unlikely because the right tool would be right there at the assembler's workstation whereas they'd have to go looking for the wrong tool.
So yeah, aside from a recommendation to switch to decaf, my suggestion would be to just go straight to the horse's mouth (FX USA) and ask for documentation and their side of the story. From your description though it sounds like someone borked up the gun, and that someone seems least likely of all possible candidates to be FX.
My 2c.