The physical forces acting upon bullets travelling through water are very different from those acting upon bullets travelling through air due to the much higher friction of the liquid medium. This fact allowed Norwegian physicist Andreas Wahl to shoot himself with a rifle in a pool and survive as shown in the video below.
Firing guns underwater produces interesting effects that are explored in the video below by Destin from the YouTube channel SmarterEveryday. He shoots an AK-47 rifle underwater and explains some of the science involved in the effects.
Several approaches have been used to increase the distance that bullets can travel underwater from the APS underwater riffle to supercavitating ammo.