Actions

Machine Gun. Ranged Weapon Attack: +4 to hit, range 80/240ft, one target. Hit: 7 (1d10+2) piercing damage.
Rapid Fire. The gunner rapid fires their weapon, consuming 10 rounds of ammunition. All creatures in a 15ft cube within 80ft must succeed a DC 14 Dex save or take 7 (1d10+2) piercing damage.
Additionally, all creatures in the cube must succeed a DC 14 Wis save or become suppressed until the end of their next turn. A suppressed creature has its movement speed halved, can not take reactions and can not concentrate on spells. A creature that is immune to the frightened condition can not be suppressed.
Shortsword. Melee Weapon Attack: +4 to hit, reach 5ft, one target. Hit: 5 (1d6+2) piercing damage.
Smoke Grenade. The gunner throws a smoke grenade up to 60ft. One round after it lands, the grenade emits a cloud of smoke that creates a heavily obscured area in a 20ft radius. A moderate wind (10-20 mph) disperses the cloud in four rounds, a strong wind (>20 mph) disperses it in one round. The guard can use this ability twice before needing to restock.

Bonus Actions

Mount Bipod. The gunner uses the machine gun's bipod to mount the weapon on a surface. The weapon can not be rapid fired while it is not mounted. The gunner is immobile while the weapon is mounted. Dismounting uses 5ft of movement.