I had seen these sleeves before and not really considered them. Used in break open shotguns, they are barely longer than the shell. I couldn't imagine them being of much real use. But today a friend had a set of them in his O/U Beretta 12 ga. He was shooting 28 ga. shells. We patterned it and it looked great! I figured that it wouldn't have any velocity since it was going into such an oversize bore. I was wrong. He was powdering clay birds with authority. Naturally no recoil to speak of. We recovered some of the wads and found that as soon as the wad entered the 12 gauge bore it expanded to seal against the barrel. You could see it clearly on the wads. I made fun of them before but now..............hmmmm.......... I've got a 12 ga. SxS and a lot of .20 gauge shells. Maybe I'll try a set.