A revamp of my original (and lackluster) post. Enjoy.
Although standard rounds do in fact work underwater, their performance is extremely hindered. Surprisingly, subsonic ammunition actually works better underwater, with an effective range of around 2.4 meters (8 feet) before slowing to non-lethal speeds compared to the 90 centimeters (3 feet) before supersonic ammunition disintegrates. Taking this into account, normal firearms are nearly useless in aquatic environments.
Surprisingly, weapons such as bows and crossbows actually perform better than firearms when submerged despite the drag factor due to arrows having a more streamlined design.
Special ammunition known as supercavitating ammunition has been developed which function significantly better underwater, however these rounds are limited to a select few cartridges, which are 5.56×45mm, 7.62×51mm, and .50 BMG. Despite their rarity however, these should still be considered.
TL;DR, if you’re underwater just ignore firearms entirely, it’s not worth it and you’re going to waste tons of ammo. Use a specially designed underwater weapon like the ones below, or if those aren’t available use a crossbow or bow.
(Please note that these are all just suggestions for reference and are to be taken with a grain of salt)
Heckler & Koch P11
A rather peculiar five-barreled German-designed underwater firearm which fires 7.62×36mm projectiles, which are electronically actuated from a battery pack in the pistol grip. However, after firing all five cartridges the weapon must be sent back to the factory for reloading, effectively making it a single-use weapon.
A break-action, four-barreled pistol developed by the Soviet Union for use by frogmen and underwater demolition teams, the SPP-1M fires a 4.5×40mmR steel dart that is somewhat innacurate when fired out of water due to it requiring hydrodynamic effects to stabilize the round underwater.
The Soviet-designed Avtomat Podvodny Spetsialnyy, or “special underwater assault rifle,” was designed to supplement the SPP-1 pistol and uses a 5.66×39mm cartridge in 26-round magazines. However, the weapon was extremely innacurate and the service life of the rifle was only 180 rounds when fired out of water.
The replacement for the APS assault rifle, the ADS is a bullpup layout rifle that can accept both standard 5.45×39mm ammunition and underwater ammunition, and also has an integral 40mm grenade launcher (it is not known whether or not this can also fire underwater). Unlike its predecessors, however, it can also accept suppressors and optics due to a Picatinny rail.
A Russian-designed special-purpose double-barreled 45mm grenade launcher designed to defend coastal installations from combat divers and naval special forces. Its grenades behave much like conventional depth charges and explode upon reaching a certain depth in order to maximize its lethal potential against submerged personnel. It can use fragmentation, flare, and concussive grendes.
Mk1 Mod 0 UDG
Developed at the United States Naval Surface Weapons Center, the UDG is a six-shot pepperbox-type revolving handgun developed for use by naval special forces. Unlike other underwater firearms, however, the self-contained design of the cartridge eliminates muzzle flash and reduces sound, making it also an integrally suppressed firearm.
Also known as a “bang stick” or “shark stick,” a powerhead is a specialized contact-type firearm which is designed to be attatched to the end of a spear, and can be modified to use a wide variety of cartridges, from 12 gauge shotshells to .357 Magnum. By firing while in contact of the target, the round does not expend any kinetic energy travelling and thus is nearly unaffected by the usual penalties that a normal firearm suffers from when fired underwater.
An air-powered weapon relying on air pressure in order to fire a projectile at high speeds (note that I decided to suggest a pneumatic speargun instead of a band speargun as the latter would be too similar to a crossbow).
Alright, this doesn’t nearly have enough meat and potatoes as many of many of my other posts and is really just a glorified weapon suggestion post, but the regular badge doesn’t pay for itself, and I have to meet quota. That being said please leave any suggestions and concerns below.