If your dad is still in the market, have him stay from all rimless cartridges. The extractors made for the rimless cartridges are specially made and are springloaded for their use. There are a lot problems raised especially when using the higher pressure cartridges. It doesn't happen that often, but often enough for me to never play with rimless cartridges in any double. He needs to always use a rimmed cartridge in any double that he buys. Since I've only been shooting doubles for only a couple three years, I don't know as much as the rest of the heavy users here.