We know... we know... this is an Army-focused system... but we still get a lot of questions about USMC gear. Here's a handy info sheet that we located recently, which gives a little more detail on USMC MOLLE pouches.
9mm 15 Round Magazine pouch
http://www.armyproperty.com/nsn/8465-01-558-5105 Designed for modularity and accessibility, this pouch holds one 9mm magazine.
M16/M4 Speed Reload Magazine pouch
http://www.armyproperty.com/nsn/8465-01-558-5122 Designed to be sleek and accessible, this specialized pouch holds the Marine rifleman's first choice M16/M4 magazine.
M16/M4 Single/Double Magazine pouch
http://www.armyproperty.com/nsn/8465-01-558-5167 Designed to be the sustainment pouch for the M16/M4 Speed Reload pouch, this pouch holds up to two M16/M4 Magazines and is single magazine wide.
40mm Grenade pouch
http://www.armyproperty.com/nsn/8465-01-558-5113 Designed for accessibility and modularity, this pouch holds one 40mm grenade (all DODICs).
Pop-up Flare pouch
http://www.armyproperty.com/nsn/8465-01-559-3148 Designed with unit leader in mind, this pouch affords the user quick access to one pop-up flare.
M67 Grenade pouch
http://www.armyproperty.com/nsn/8465-01-558-5185 Designed to hold one M67 grenade, this pouch affords the user quick access and modularity.
Squad Automatic Weapon (SAW)/Utility pouch
http://www.armyproperty.com/nsn/8465-01-559-0116 Designed to be a multifunctional pouch, this pouch holds one 200 round SAW drum or six 40 mm grenades or six M16/M4 Magazines securely.
12-Gauge Shotgun Shell pouch
http://www.armyproperty.com/nsn/8465-01-558-4476 Designed to hold ten 3 inch 12 gauge shotgun shells, this pouch affords the user the ability to breech three doors.
Multi-Grenade pouch
http://www.armyproperty.com/nsn/8465-01-558-4687 Designed to be a multifunctional single grenade pouch, this pouch holds one Smoke grenade, Thermite grenade, Star Cluster grenade or two Flashbang grenades.
Dump pouch
http://www.armyproperty.com/nsn/8465-01-558-5130 Designed to hold seven expended magazines, this pouch can be stowed out of the way during administrative operations.