Re: seal rifle
Any legal biggame cartridge, ie any cartridge putting out a 139gr or heavier bullet.
Most norwegian hunters use frangible varmint bullets as seals are almost exclusively shot in the head whilst resting on the slopes of naked rock or when popping up in the shoals. Body shots are avoided as it frequently results in lost seals. The hunters use small boats to pop from rock to rock and shoots from land, prone position mostly.
LR for seal? I'm afraid your frequency of lost seals would be very high due to sinking. It is already hard enough to recover them when your pal is sitting in the boat just waiting for your shot before rushing it.
Remember to get your permit from your local fishery manager.