I sent a barrel and a barreled receiver out for some work recently.

This 22/45 got cut to 4.5" and threaded.
20180122_072424(1) by Slick_Rick77, on Flickr

The barrel of my ban era AR finally got threaded and re-profiled from a heavy to a medium contour. It took 1/2 pound off the front of the gun.
20180209_103454 by Slick_Rick77, on Flickr

It turned out okay.
20180209_170420 by Slick_Rick77, on Flickr

Both were back well within two weeks.

I've only had one gun stop working and it was a simple fix. My 10/22 built up a glaze in the chamber after decades of use. It required a lot of lube, a bronze brush, and hand drill to clean it up. I ordered a replacement recoil spring, but it was the exact same length as the original. Crazy how that works.

A few mags have needed springs.

Semi auto pistols should have recoil springs replaced periodically if you shoot them a lot.