How about, rather than going down the same path as Arma2 - you utilize the new inventory system, and feature; rather than 'Clips' of Shotgun Ammo, "piles" or 'stacks'?

And rather than Shotgun ammo being a Clip that is attached to the shotgun when you press Reload or via dragging in the inventory, one by one a shell is loaded to the maximum of the capacity of the shotgun upon initiation of the shotgun reload.

If any spare shotgun ammo is in the inventory - it can be fed into the shotgun to the maximum of its capacity.

This would be a real-time, one shell fed at a time with the counter on the GUI going up one at a time also; and with a shell coming from Any in the inventory handy, with a preference for any that are 'lower' than full stack.

And perhaps, akin to many modern shooters, one could press LeftMouseButton to interrupt the reloading process and take a shot from however many had been fed into the shotgun magazine, during the process, say, 6/8 shots reloaded, taking a shot, leaving you to press R once more to refill another 3 shots, 5/8 > 8/8.

If you press R with a shotgun, if you have a half-filled capacity, you shouldnt replace the entire magazine. It should just take from any stacks available in the inventory to fill, one shell at a time. new shells > to the max of the shotgun.

Not only would this distinction between clip style shotguns and 'reload shell at a time' style prove invaluable to realism, but would solve many problems for DayZ style mods or perhaps even the modding in of revolvers or something.