If you've ever used the things to charge your rifle, you know what a pain they can be. It's not simply that they're for a rimmed cartridge; the .303 British slicks right through theirs. It seems to be two things:
Different rifles will or will not work well with them, and
Most of those available are crap.
As to the rimmed part, you have to load them in so that the rim of one is under the rim of the one before, and then charge the rifle with them so aligned. No big deal, the .303 has to be loaded into the chargers in a proper pattern, too. But even with that done, a lot of the chargers/rifles just don't work well. A while back I saw a picture somewhere of a charger with a slot cut in it, don't know if it was original or modified. So I got the jewelers saw and cut a 1/4" slot in the ends of a couple of chargers that worked so-so and gave them a try. Big improvement. Then, at the gun show a couple of weeks ago a guy had some actual Russian-made chargers, Izhevsk marked and all. On the left is one I modified, on the right the Izzy:
I just tried the new ones out, and they work well.
So if you've got some of the reproductions that don't work well, you might try cutting a slot in one and see if it helps.