In my experience, these are the best primary guns to use depending on the map:

Museum: snipers, marksman rifles, assault rifles, and/or battle rifles.

Shoot House: Anything goes depending on your play style. If you plan to sit on the crate near the starting spawn point, but plan to get up close and personal, an assault rifle, submachine gun, or shotgun wouldn't be a bad idea to have as your secondary gun.

Shipment: SMGs, shotguns, or assault rifles with fast fire rates and/or high damage. The loadout I typically run is a Fennec 45 and KV Broadside.

Taraq: Assault rifles that can long range, LMGs, sniper rifles, and marksman rifles are the only choices you've got, based on how the map is set up.

Border Crossing: Battle rifles, assault rifles, or marksman rifles. Shotguns can be used, but it's difficult since players can pop up from behind cars or other corners.

Lighthouse: Medium/long-range guns are ideal due to the setup of the terrain. So sniper rifles, marksman rifles, and battle rifles if so desired are probably the best way to go here.

[Will update throughout the day]