20 Round Mags????

Hey Family, new to the group.... Im trying to find a way to have a 20rd mag for my custom built .458. I know you can fit 10 in a standard 30rd AR mag, and Ive been doing some research on 60rd AR mags and drums. Has anyone tried it, or is my best bet just to use a 40rd 556 mag and just max it out at 13+1? Thanks!!!!