The gun choices are symbolic. The Alliance officers have two-tone Berettas, the frontier sorts have lever-action carbines, the cowboys have SAAs and other gunfighter-esque revolvers, and Mal, the guy who was on the losing side of a civil war, has something that looks like a Walker. And Jayne has absolutely everything.
Plus, if you were homesteading on a frontier planet far away from parts and armorers, would you, for a ranch gun, want a super-complex laser that needed to be charged and that you couldn't fix, something with tiny, complicated plastic parts like an FN 2, or a sturdy, powerful lever-action carbine, a drum of powder, bullet molds and reloading press?