The gun is a Browning XT that had the browning hardware (which I didn't have a problem with)I wanted to be able to use spacers so I had a well known gunsmith change it. I don't know why it won't tighten up and I don't want to strip the threads with to much torque. E-mailed the gunsmith but haven't heard back from him yet.
Beretta had equally crappy factory stuff on my 682. I replaced mine myself(very easy) with the Graco replacement parts. I couldn't be happier. I have friends who have done theirs with the Stock Positioning System parts. It is also high quality replacement comb hardware in a DIY form.
Order one of the above mentioned systems and change it out. They come with the washers/spacers as well. Sounds like you probably have a stripped set screw hole. I guess you could drill and retap it for the next size up set screw. The only problem with that, is you have different size screws.
My brother happen on to the problem that the base plate screws were loose. No thanks to the nationally known gunsmith who didn't answer my e-mail. I will go elsewhere in the future and not recommend him to people.