@GGordonWoody , at 22 ft LOA my mast is around 24 ft, its a roughly 2.75 x 4 inch flat-sided mast, .180 wall (if I recall correctly) and weighs roughly 80 pounds with the spreaders (again, best guess). My mast only secures to the step with a single 5/16 thru-bolt in a slot allowing the backward pivot. That set ups allows for lateral movement so the mast is stepped with the shrouds and aft lowers attached which is what keeps everything on center plane in the arc (somewhat). Your mast is probably a few feet taller and may be a beefier section, just judging by the pics of the foot. If I had to attach my mast section to a hinged step like yours I would have no problem doing it with the 3/16 rivets @ 700 lbs sheer strength, plus I wouldn't have to buy the bigger gun In your case I would always be on the side of caution and would probably prefer the 1/4" rivets having 1300 lb sheer strength. One way or the other, those rivet guns come with 4 sizes of tips for the most common rivets and they have multiple applications, I think it's a tool worth having.
Also, there should be no forces on the rivets while sailing as long as the mast section is FULLY seated on the mast step when it is riveted in place. The shrouds and stays put downward force on the mast which transfers directly to the step if seated... if the mast was sitting up or floating off the step even by a fraction of an inch all the forces would be in sheer on the rivets... but with say 6 rivets having a sheer strength of 1300 lbs each it would probably hold, but don't quote me on that!
Also, there should be no forces on the rivets while sailing as long as the mast section is FULLY seated on the mast step when it is riveted in place. The shrouds and stays put downward force on the mast which transfers directly to the step if seated... if the mast was sitting up or floating off the step even by a fraction of an inch all the forces would be in sheer on the rivets... but with say 6 rivets having a sheer strength of 1300 lbs each it would probably hold, but don't quote me on that!