Check out the Lewis Drake website-

I recently got a very nice cartridge carriage system from them. It's essentially a leather pouch that'll fit a wide belt with a flap that covers the insides, it has a silent button type catch.
Inside is a removable stitched leather 'card' with loops that hold ten rounds of ammo, it can be used on it's own in a coat or trouser cargo pocket.
Ten 450#2 NE rounds fit nicely inside, I'm sure that 500 nitro would work out great in there, Mr Drake notes on the site that it's good for 600 nitro.

I think I paid something in the neighborhood of sixty dollars for it. I'm thinking of getting another and two of the smaller size just to get in on the opportunity to purchace them before they somehow end up dissappearing from his stock list. It's really a very nice piece of outfit.

--Tinker