I haven't used one. But on a previous double rifle thread one of our Aussie BGRC shooters mentioning making a two cartridge wrist carrier for rapid double rifle reloading. Lots of shoots and experience make a lot of difference.

I think it makes a lot of sense. Should be able to reload at effectively the shoulder.

A butt stock cartridge pouch would probably be reloaded from at the waist? Don't see the point compared to a pouch on the belt. Only advantage is having the extra ammo on the rifle.

The leather maker makes them all anyway.