pokerdaddy, Welcome to the forum...to answer your question..a mountain gun is what ever you want it to be...
now for a little clearification:
S&W has put out a line of mountain guns in the following calibers:.357mag, 41mag,44mag 45acp and 45lc...
Now what would a person use a mountain gun for? protection from wild life and other things...I personally want a mountain gun that is capable to handle any situation...most people here would say the 44 mag is the perfect mountain gun..This also depends on what part of the country or the world you live in. I live in NC the largest threat I have here is probably a mountain lion, black bear or wild pig...so for me my mountain gun of choice would be a S&W model 25-5 in 45LC...If I was in Alaska I would probably be toting a 44 mag and a .308 rifle...
As you can see your answers are unlimited and will vary...