After getting very good answer concerning which kind of weapon would be reasonable on a planet with air density of 3 atmospheres
and doing some googling, I started to analyse this concept a bit deeper.
So effectively I end with people armed with flechette sabot stabilized guns, which ammo has comparable size (but some specially designed ammo seemed a bit longer than one designed for conventional bullets), is smootborne, has extraordinary aerodynamics, extraordinary armor piercing quantities and... awful terminal ballistics (which means that pierces enemy but tend not to do much actual damage).
As such it would be an acceptable trade off. But in close combat situation long range properties are not an issue while stopping power matters. And over penetration is not always advantage... How should it be solved? Especially that not rifled weapons tend to have low accuracy.
(Normal bullets for short range and accept this low accuracy? Should there be some fattened sabot stabilized projectiles? Shotgun like slug bullets with their own "rifling"? Or maybe such different kind of ammo would be too problematic, and just using a short burst would solve any problem of inferior terminal ballistics? Or just submachine guns if someone really plans and needs some house to house combat or hostages saving and otherwise just normal guns?)
Should there be proper short distance ammo? If yes, of which type?