Bags 101
The WELT would be as Bull described it, a piece of leather that is laid flat around the sewn circumference of the bag. It provides protection to the SEAM, especially important when constructing a knife sheath.
Now this has absolutely nothing to do with muzzleloading, but I recall about 50 years ago when I was building my '39 Chevy hot rod, part of the upholstering I had done on it was having new WELTING made to put between the fenders and the body of the car.
The GUSSET is a piece of leather added between the front and the back of the pouch. It can be any width you want, depending on how deep you want the bag. When we had October Country, we generally used an 1 1/4" wide gusset for our smaller bags and a 2" wide gusset for our larger bags. Seemed to work pretty good that way. Of course, there were variations, the 01-401 Freestyle Bag was a large bag but used the smaller gusset. And so it goes.
John