Technically, you drive a soft slug through and then mike it, but be careful, you can bend the crane real easy. If it's a top-break it would be easier, but just make sure whatever you are going to shoot passes through the throats, either easily or with very minimum resistance. If it's chambered 38 S&W it's for the stubby cartridge with the slightly fat bullet, so any newer slug should be safe. The gun may be black powder, so load them real light. I assume you just want the satisfaction of firing it a few times, and don't intend to burn up the range with it.