Hi Barbara!! Russell here to answer your request for a little critique on your photo. I can share some of your vision here, the cannon has some great color and texture, a great subject overall, unfortunately I think a better angle would have worked alot better.
Anytime your subject is placed in the middle it needs to fill as much of the frame as possible giving the viewer the chance to "see" it too, with no major distractions. It truely BECOMES the subject (and this cannon could fit the bill with the color and texture).
If you want to include the background (the arches and lettering ARE pretty cool and gives an air of the "old days") maybe an angle that includes the front and part of the side of the cannon with the lettering and arches in the background and taken from just below the barrel so it gives foreground and background intrest.
Since I have not been to the Persidio since my days in the Marine Corps, I don't know if the rules have changed about getting close to the guns, if they haven't changed then get close, if they have, shoot portrait style from a low angle and use the telephoto.
After looking at your photo in PS CS2 I see that the arches are a bit "blown" hitting 255 on two channels (especially the red channel) kind of makes things a bit distracting. Watch your ISO and shutter speed, seems a bit high on the ISO for an outside shot.
You definately have an eye for photography, as seen by your portfolio (loved your "leading Lines" entry!!) if you get another shot at this cannon, or anything similar, take the time to shoot it from alot of different angles (hard to do when time is short - believe me I know) then check them when you have had some time to relax.