I understand the reluctance to have someone machine on a gun that's not broken even if it doesn't have sentimental value.
I'm an aerospace engineer. I cannot count the number of times I've seen parts come in that were machined incorrectly no matter how simple the part. This goes for parts made from the ground up either to drawings or models or both as well as modifications to existing parts that are as simple as 'drill a hole this size in this location.'
Even in the best of shops, things can still happen that can make a mess of the job. Cutters can break. The wrong program can get loaded into the mill. The mill actuators take a dump in the middle of a job and crash the head into the item being machined. The machinist for some reason decided to eyeball something instead of a measured adjustment. An adjustment is made in the wrong direction.
Those are only some of the examples and in a controlled machine shop. Don't even get me started on installation errors when something is being installed on an airplane.
(It isn't machining, but I once was told by a mechanic that when making a bent sheet metal clip (just a bent piece of metal usually rivited on both ends to other parts as a gusset or brace) to replace a broken one, they typically made at least three and used the one that came closest to fitting where it needed to go.)
Things that shouldn't happen sometimes do.