Try carrying 338Allen Magnum ammo, now THAT is a challenge. I finally found something to use, a choke tube pouch RedHead Shooting Bags - Choke Tube Case
that I made a styrofoam bottom for (to protect the lower half of the shell).
Talk about funny, carrying 6 rounds of ammo on my belt in a carrier made to hold long 12ga choke tubes.
I use MTM shellholders and cartridge pouches like this RedHead Cartridge Pouch
for my 'normal' size cartridges.