I've been fed up with the pouches on my cartridge belt shrinking after a wet day out so came up with this simple cure...
Buy a length of 22mm electrical conduit and cut into about 3" lengths.
Treat the inside of each pouch with some leather dressing (Ko-Cho-Line for example).
Insert each piece of conduit in each pouch and put the belt somewhere warm and dry like the airing cupboard and leave for as long as you like (without forgetting where it is...oops).
The 22mm counduit is just slightly larger than a 12g cartridge and when it's done the trick you will find that the cartridge goes in/out without any bother.
3" is a good length as it gives a bit more to be able to shove against when putting the counduit piece in and a bit to grip with long nose pliers to take it out.
Note.. you are best to chamfer the outside edge of each conduit piece after cutting it, makes it a lot easier to it put into the pouch and doesn't damage the pouch.
ps... can't help with the "shrinkage" in length, that's probably down to age and beer