"Beta C-Mags"

Beta C-Mag
These little beauties should be first on the list for any gun-fondlers using AR-15 style weapons, in effect they area 100-round magazine for your M-16!
The magazines are described as being extremely reliable, and not prone to jamming or feeding problems at all, they are temperature and drop resistant and have passed a variety of NATO Reliability tests.
Beta-Mags come in two pieces, the twin drums, and the feed-clip tower which is weapon specific, and can fit AR-15 style weapons, M-249 SAW's (FN Minimi's), HK G-36's, Steyr USR's, and HK 33 and 53's. A compact HK-53 or bullpup Steyr with one of these attached would be an awkward weapon.

Disadvantages
Now, obviously, these things are not light, but they do sit in the centre of the weapon, so balance shouldn't be affected to badly. They will slow your aiming speed down, and they will be hard to carry on the march.
Obviously they are over the ten-round limit for civilian weapons according to Uncle Bill, so the ATF doesn't like them.
Size, these things protrude from the gun, but apparently not enough to make it difficult to use bipods.
Note, Beta-mags do not have any transparent sections to inform you how much ammunition you have left, so round-counting is a necessity, after all, you don't want it to run out just as the tcho-tcho's have climbed over the barb-wire and are about to enter your fox-hole!

Extra's

A new 9mm C-Mag is on the way, imagine an uzi with a 100 round capacity!

LINKS

SIMULATION STATISTICS

M16 + C-MAG
Price: $200-250 for LEO or Military only, refurbished magazines available to public for ~$350-500.
Weight: 1 Kg (Unloaded)
Dimensions: 25.0 x 11.7 x 7.95 cm
Feed Rate: AdjustS to the rate of fire of the weapon, but will do 1300 RPM!
NB. GM's may wish to reduce the use of these items by calling for luck rolls to avoid snags, jams and other bad things when the agents get overly energetic with these things.