When you say the Gun Doc broke it, do you mean he broke it when he installed it, or do you mean it broke during test-firing?
If you wanted him to test it, he tested it and the part broke, I don't see why he should be on the hook for the part price. You told him to install the TM guide right? If it was the wrong part and broke, that would be the part suppliers fault, or the person who requested the installation.
Now if he broke it while installing, then he should be lible.
Maybe you'll find someone else to help you. Maybe black mesa... THAT WAS A JOKE, ha ha, fat chance.
My Buy/Sell 1337ness rating