Iran recently announced that its weapons developers have designed and built a pistol for use underwater. Foreign weapons experts, or history buffs, pointed out that the Iranian four-barrel underwater pistol looked like a copy of a 1960s Russian design, the SPP-1. Introduced in the 1970s for use by Russian frogmen, the SPP-1 was later upgraded as the SPP-1M and is still in use. Iran often copies older Russian weapons without mentioning that then they reveal their new weapon design. While that no longer fools foreigners, such deceptive announcements have some use as internal propaganda.
Russia and Western nations have been developing these underwater weapons since the 1960s and new models and technologies are regularly introduced. For example, in the last decade Russia introduced a new underwater bullet design and an underwater assault rifle. More recently (2019) a Norwegian firm introduced an even more effective underwater projectile design that can be fired by existing weapons used outside the water at targets in the water and travel farther and with more impact in the water than ordinary bullets. Russia was the first to introduce such a dual-use bullet and the Norwegian design was the first improvement on that Russian design.
In 2013 Russian scientists revealed their more effective underwater bullet design. 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 more than 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 underwater bullets. The ADS underwater bullets are designed to operate underwater. To do that they are longer and use a discarding sabot like anti-tank guns long used to obtain longer range and penetration. Using the new ADS bullets the ADS assault rifle can be fired on full-automatic (like a machine-gun) underwater. 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, the 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, where it looked like a bulked-up AK-47. 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. The ADS allows amphibious commandos to carry one type of weapon (but two types of ammo) for both above water or underwater combat. The new Norwegian SDG bullets enables existing weapons to fire special bullets that can hit and injure targets fourteen or more meters beneath the surface.
It’s only a matter of time before Iran reinvents all this new underwater weapons tech.