When I loaded BP shotshells and w/o crimping the case,,I kept the OS wad in place with cheap white door& window caulk.
A little placed onto the top wad and then with your thumb press down on it and smear it around the case on top of the wad. It will work itself into the joint where the wad meets the case wall and form a small fillet of the stuff.
Let it cure, usually at least overnite (the directions on the container will say how long it takes to cure). It forms a rubbery seal that simply comes apart upon firing and blows out the muzzle with everything else.
Usually sold in the caulking gun type cartridges, but you can buy small tubes of the stuff that just squeeze out small amts thru a slender applicator. Very handy to use.
