I used to load shotshells for my revolvers. Thin cardboard wad over powder, followed by shot size of your choice and anther thin cardboard wad on top. Used a heavy crimp to hold it all together. If I knew the weather might be damp I would seal the top wad with candle wax. Upon firing, the pattern of shot left a lot to be desired and in my case I didn't want to be dependent on them to do the job. A normally loaded cartridge aimed at the snake would do the job. Even if the bullet missed him the debris flying around him would send him high tailing.