French Double,
Artisanal Spanish shotguns, such as your Martin Ugarteburu (makers mark MU), are purpose made shotguns intended for some fairly narrow kind of use. This kind of use is generally described as falling somewhere on a continuum that runs from light game gun (carried much, shot little), through medium game gun (carried little, shot much), to heavy completion gun (not carried, shot continually).
Your MU, judging from your description, is most likely (but not certainly) built as a medium game gun. If you are running shells through it fast enough to get it really hot, you are using it as if it were a heavy completion gun. Thats going to cause problems, and wear the gun out pretty quickly. Think of it as using your putter at the driving range; thats a fair analogy.
Hope this helps get a handle on what is happening, and likely why.
Kyrie
