I've never shot….or even seen a roller gun. But i don't think reloading speed is really a big issue anyway.
I can't recall even one time where being able to reload even 20 seconds faster would have made a difference in getting another fish.
If a school of fish is around and I get one….dog snapper?, mutton?…whatever we hunt here, and I get one, it takes minutes to get a decent sized fish off the shaft and, in my case, usually on the boat. But even if I used a stringer, it would take a bit of time to load a fish onto there.
Given all that time, what's a few more seconds needed to load a gun?
I have had a jack or fish on where a few other snappers came up to look. But before I even got the fish off the shaft, they were gone.
Maybe with other species in other parts of the ocean it's different, but that's what I've experienced.