For "real SHTF/emergency ammo", I prefer to pack them just as they come from the military.  I generally use the 4-pocket bandos.  You need 7 of them per 50-cal ammo can.  4 x 30 x 7 = 840 rounds per can.

The old bandos, meant for use with 20-round mags, had 7 pockets.  7 x 20 x 6 = 840 per can.

You can also use the 7-pocket bandos and put 30-rounds per pocket (withOUT the cardboards) and fit 4 of them per can.  This works fine when full, but I find it isn't as fast or as quiet when you start unloading the pockets.

For range ammo, you can just stack the stripper clips in the cans, and you can fit 1100 or so rounds on strippers per can with optimal arranging.

-Troy