Wow, this is an old thread.
Anyways, I myself use MOLLE II frag pouches like Greatguns mentioned earler in the thread. This thread was actually where I got the idea
two years ago. 
You can actually carry a lot of ammo this way. I built a rig for my M39 using a USGI pistol belt and suspenders. Running four frag pouches on each side makes for 200 rounds (five clips per pouch, with two upside down in the pouch as described earlier in the thread). For a big guy like me, that still leaves room for two 1QT canteens on the sides and an IFAK on the back. If you want even more ammo, you can buy these clips:

You can get them from
Strapworks.com. Get the 2" model, they're $1.25 a piece. These are just like the clips used to attach the suspenders to the top row of eyelets on the belt.
You clip them on the
bottom row of eyelets, and slip the MOLLE straps of the pouch through the clip (may take some force as they
barely fit through). Then you can have a second row of pouches below the ones on the belt. Criss-cross the straps/snaps on the bottom row to help keep the pouches from flopping around. And presto, you have 400 rounds of 7.62x54mmR readily accessible! You will have to raise up the suspenders though, almost like a chest rig. If you don't like this, you can still carry 300 rounds or so this way either in a "4x2" or a "3x3" configuration on each side, leaving the front more open so you can lower the belt without the front pouches obstructing your legs.
My personal rig has 300 rounds. A bit overkill for a Mosin? Yes. Awesome?
Heck yes!If anyone is interested to see, I'll take a few pics and post them.