A high capacity magazine for high powered rimfire cartridges comprises a header portion that has a form factor to be received in a rotary magazine receptacle and an arcuate cartridge bank portion that holds a column of cartridges. A pair of clamshell halves define an arcuate cartridge bank and a lower portion of the header potion. A header caps the upper ends of the clamshell haves securing the clamshell halves together. The head and clamshell halves define the cartridge pathway to the magazine exit to be fed into the rifle chamber.