I asked George G. a question about Fremont's gun the other day, and then I remembered this site. I don't recall if this site has been posted here before, but it contains some interesting information, not the least of which is a high resolution picture of the plans for the "12-pounder bronze mountain howitzer, model of 1835. Source: Alfred Mordecai, Artillery for the United States Land Service."
Lost Fremont Cannon M1835 Mt. Howitzer Drawing