The best way I have found to reload a double rifle, or any rifle for that matter, is to carry extra ammo in a buttstock shell carrier. Then as you need to reload just "pluck" a round from the shell carrier with your "trigger" hand and stick it in the chamber[or magazine as the case may be].
On hard kickers just be sure and use a buttstock carrier that has leather around the wood of the stock so that your face is not cut. Been there and done that.
Check out www.murrayleather.com Good stuff.