Yeah, the reason there's only one type of assault rifle is that I don't think there'd be enough in-game difference between an M16 and an AK-47 to warrant including both. Their stats would be too similar to be worth the added micromanagement, in my view. So for the sake of accessability, I won't include any additional weapons in vanilla Xenonauts.
EDIT - Also, you're aware there is already a pistol, shotgun, assault rifle, sniper rifle, machinegun and rocket launcher in the game already? Just to check - your post kinda implies you only think the M16 is available to soldiers.
However, I know lots of people are interested in this sorta thing and we've made it EXTREMELY easy to mod in new weapons to Xenonauts. To the point we're deliberately trying to have a few spare weapon tiles for people who want to add more weapons. Like, for instance...
(That's basically an example piece we've done for a Kickstarter video, specifically for the Soviet Weapon pack)