What to do about the barrel I can not say. But that screw your talking about does not hold in the nipple. It is a drum clean out screw. Over time a lot of them kind of weld themselves in place. Not a big deal. Just leave it alone as all you will do is damage the head on it. In fact some rifle makers stopped putting that screw in all together. When they do come out, they are helpful in flushing a barrel, bolster or drum, but really, just removal of the nipple will do a lot of the same work.
"Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, a total wreck, screaming Yahoo, with a big smile on your face."