Can projectiles pass through enemies? If yes, what weapons allow this? How does the damage work? Will the impact on the first enemy hit by the projectile, lessen damage to the second enemy hit?
Okay here's a list of all (non unlockable) guns and if they penetrate the enemy.
Just as an extra the recurve bow arrows do not pass trough enemies.
I didnt include the launchers for obvious reasons. Also the special unlockable weapons because they are just upgrades from other guns
Yes, this is possible.
Snipers don't kill two people with one shot. It's ruined my undetected bonus many times. It only kills the first guy. I've tried many times, with a special sniper as well as with regular snipers, and I failed every time.