We got these good-quality Blackhawk ammo pouches now in used surplus, too! Here's an ammo carrier that holds 19 rounds for you in a convenient package. If that ain't enough, buy a few of these.
The first batch you can grab at any time is the six rounds on the lid. Once they are gone (which will be fast if you're having fun) you pull a tab to gain access to seven rounds in a second layer. Pulling a second tab reveals the final six rounds. It's actually easier than it sounds, try it if you don't believe it.
Size: 9 x 18 x 4 cm (3.5" x 7" x 1.5"). The thickness of course depends on how many layers of ammo you squeeze in this thing.
PALS size requirement:
These are used but in serviceable condition. Because people have waged war with these things on or at least trained for it, there might be some smudges or surplus sand on these. If that happens, you get that wonderful "military vintage patina" for free. Made by Blackhawk! for the British armed forces. Manufactured in Vietnam.
At least for this price, these can't be beaten if you want to assemble a high-quality kit for demanding use. Availability in the future is uncertain so act now.