Hi brudive, fantastic to here you are keeping up your diving. I use air guns but I cant see how you could load the models I use because you need to brace the handle on your foot while ramming the spear in the barrel. A short air gun with low pressure would be fairly easy to load but obviously would not have much power. One ansaw might be a roller gun, this type of gun is shorter for the same power than a conventional gun. You may have to get it custom built though because I dont think there are any in current production?
I built one last summer here is a pic to help you understand the principle.
