The balance between the weapons seems to well-thought, although the Marksman Rifle could suffer from the bad case of not-having-2-aimed-shots like the assault rifle has, while also missing the massive accuracy/power advantage the Sniper Rifle has.
However, the general balance is more disputable. The options seems to be heavily weighed towards automatic fire; snapshot is basically only useful in defense (except for the Marksman rifle, noted - very high Snap range + triple fire rate compared to Aimed), while in attack, the aimed shot generally takes over only at ranges 30+, while the majority of gunfights seem to take place at 20-25 range (and the range can be easily shortened by walking a few tiles - one automatic burst takes at worst 35% TU, which leaves the soldier with 30+ TU to maneuver). The high autofire rates mean than even with 1/3 accuracy, it grants much higher kill probability than an aimed shot (unless at extreme ranges, ofc).
Another one is the shotgun. It seems pretty useless except at point-blank ranges (1-2 tiles), where again, autofire (even from an assault rifle) would be able to pump equal or higher damage into the enemy, as essentially every bullet hits. Have you taken into consideration that every consecutive shotgun pellet suffers a stacking 5% accuracy penalty? Having heavily used shotguns (7 pellets, 18 damage, accuracy profile essentially very similar to yours), I've found them very unreliable, as very rarely more than 1-2 pellets hit, even at intended range of up to 10 tiles; with worse stats, and very hard competition from automatic weapons, your shotgun is bound to fare even worse.