Everyone here seems to be ignoring the fact that that "minigunner" pic was only made for the C&C Gold Win95 rerelease of the game. They just stuffed whatever prop they had lying around in the guy's hands for that manual pic. The original DOS version doesn't contain that pic anywhere; it uses this image for the minigunner:
Yup... an actual minigun
From the way the 'minigunner' actually shoots ingame, and the fact the cover art of the game has an M16 at the side, it's fairly clear that the actual ingame weapon is an M16A2 on 3-round burst mode.
On a related note... in Renegade they kind of got around this by actually making "officers" who more or less did carry a mobile heavy machine gun, quite comparable to a minigun,
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As for the commando... yeah, no excuse there. That was made purely to look cool. That is in no way a sniper rifle