Russian scientists have created a more effective bullet for use underwater. There is not a huge market for such a bullet, but there is some demand from special operations troops (like U.S. Navy SEALs) who practice a lot for underwater operations. The problem with firing weapons underwater has always been that water creates a lot more resistance than air and the bullets are not effective for very far (a few meters at most). Moreover, the gas operated recoil systems do not work underwater, which means you have to manually pull back the bolt to eject spent rounds and load another.
The new Russian ADS “amphibious assault rifle” is a Russian A-91M 5.45mm assault rifle modified to fire the special ammo or normal 5.45mm rounds. All the user has to do is flick a selector switch and use the special magazine carrying the longer bullets. The ADS underwater bullets are designed to operate underwater (they are longer and use a discarding sabot like anti-tank guns long have used to obtain longer range and penetration) and can be fired on automatic (like a machine-gun). All this would be useful to amphibious commandos or those defending ships or coastal installations against underwater attackers.
In the air the ADS bullets still have the same range and accuracy of regular bullets (but not as much hitting power). Underwater, effective range of these bullets depends on the water depth. The deeper you go the more water pressure, from all the water above you, which slows down rapidly moving objects like bullets. At a depth of 30 meters (nearly a hundred feet) an ADS bullet is dangerous out to 25 meters (nearly 80 feet). Regular bullets have much less range as the water slows down the bullet very quickly. A regular rifle bullet becomes harmless about 100 cm (1 meter or 3 feet) out and will stop after about 5 meters. A pistol bullet won't hurt anyone after 60-70cm (2 feet). However, if fired from just above the water, say from a dock or boat, bullets will hurt someone underwater to a depth of 2-4 meters. When fired outside the water, the bullet is able to get up some speed before hitting the water and starting to slow down.
Without special bullets there are still some weapons available to SEALs or other underwater combat troops. There's always the knife, but you have to get in close, too close, to use it. Think of the knife as a last ditch weapon. During the Cold War, especially in the 1970s, The Soviet Union and the United States (with a lot of help from West Germany) came up with some more effective underwater weapons. These weapons fired heavier bullets that created an air bubble around them (cavitating) which enabled them to maintain higher speeds for longer distances. There was no spin on these bullets (they were fired from a smooth bore barrel). These projectiles could hurt you out to about 30 meters. Beyond that, most of the time, you can't make out a target.
Until they developed the ADS the Russians still used their underwater "nail gun" (as underwater warriors call this type of Cold War weapon), in both pistol and assault rifle (it looks like a bulked up AK-47) forms. The U.S. still arms underwater troops with a similar pistol. And then there's also the spear gun. It's used for hunting fish, is a one shot affair, and has a range of about 4-8 meters.
The nail gun, unlike the ADS, was really only useful under water. It was not as effective as a regular rifle or pistol when fired in the air. Thus the ADS allows amphibious commandos to carry one type of weapon (but two types of ammo) for both above water or underwater combat.