Since the formation of UDT in WW2, and US Navy Seals during the Cuban Crisis, frogmen's thoughts have turned to effective underwater guns, something above water guns are not very good at.  The best a Frogman had at his disposal was a spear gun and a knife, the spear gun could reach out 20 feet, and the knife, not so far.  Some effective underwater guns were finally developed, and this led Russian engineers to counter US weapons, and they came up with some interesting guns.

The Russians came up with their first underwater weapon in the late 60's and was very effective for it's size.  The SPP-1 underwater pistol had 4 unrifled barrels and it's cartridge's payload was a long steel dart.  Bullets do terrible underwater, and darts were stabilized hydrodynamically.  The cartridge itself was 4.5mm X 145mm, and as a comparison, the standard Russian rifle cartridge is 5.45 X 39mm.  It's effective range was 50ft at 16ft underwater, much better than a spear gun.  However the deeper you go the worse it gets, which seems to be a problem for all underwater guns.

picture of SPP-1

SPP-1, picture by Vitaly V. Kuzmin

The need for longer range resulted in the APS underwater assault rifle.  It also had an unrifled barrel with similar ammunition, a long steel dart.  The size of the cartridge was 5.6 X 120mm, which resulted in a huge magazine that did somewhat hinder it's use underwater.  The magazine held 26 rounds, could be fired at full auto and the range increased to 100ft at 15ft depth. A rocket powered cartridge was tried, but wasn't as accurate as the standard cartridge. The APS's limitations were soon evident.  It could be fired around 2000 times underwater before being worn out, but if it had to be fired on land, it's designed life was only 200 rounds, and incredibly less accurate.  The Spetnatz took to carrying an APS with an AK-74, not the best of outcomes.

picture of APS

APS rifle, picture by Remigiusz Wilk

The ASM-DT takes a stab at solving the above water combat capability by building a hybrid design.  The receiver can accept two different magazines with a sliding magazine catch.  With it all the way back and a APS magazine installed, the rifle can fire 26 APS cartridges in underwater mode.  If the frogman has to fight on land, the APS magazine is removed, the sliding catch run all the way forward, and the AK-74 magazine installed.  The barrel is a compromise as well.  It has shallow rifling in order to spin stabilize above water bullets, and also has special grooves in the base of the rifling in order to vent water if the barrel is full.  The rifle also sports a flash hider, and can be equipped with a grenade launcher although there are no underwater grenades. The performance is less but close to an APS underwater and an AK-74 on land. The gun was adopted in 2000, but complaints about the long receiver (due to the long underwater ammunition) and a complicated magazine arrangement led to more research.

piture of ASM-DT

ASM-DT, picture by Tomasz Steifer

Adopted in 2013 the newest version of an underwater amphibious gun, the ADS, entered Russian service. Using the same box magazine for the underwater as well as the above water projectiles, the receiver is short, and the overall length is made shorter by being a bullpup design.  The new underwater ammunition has the steel dart fully inserted in the cartridge case all the way to the base, shortening the overall length. The effective range is a bit shorter at 75ft at 15ft depth. All the modern accessories are available for this weapon, including silencers, rail mounted devices and grenade launchers.

picture of ADS rifle

ADS rifle, picture by Vitaly V. Kuzmin

Most of these weapons are still used by Russian military and other countries that use Russian hardware.  China has their own version of the APS, the QBS-06, which they say (the Chinese) is much better than the Russian version.  In any case, underwater warriors are much better armed than ever.

The first Hotchkiss gas operated machine gun was used by several militaries at the beginning of the 20th Century, most notably the Americans, British and the French.  The American gun was used to chase Pancho Villa around in 1916-17, and in WW1 for a short time.  The British and the French were more attached to the gun and it was used up till the end of WW2.

picture of Hotchkiss light machine gun

The Hotchkiss Mk.I, image from Imperial War Museum Collection

The Mk.1 was built in Enfield in .303 caliber.  The Americans and the French used their own rifle calibers as well.  The gun could be fired from 20 or 30 round strips, or from a belt made of shorter strips if used in vehicles.  The odd stock was to accommodate the charging handle, a bit disconcerting due to the fact that when opened it occupies the space your face normally would.  

picture of Royal Navy seaman

Royal Navy convoy duty 1942

The Brits didn't use these guns with infantry as such, but were used in many other ways. The Royal Navy used them in escort ships, and Q ships (armed trawlers meant to suck U-Boats into a trap) in both World Wars.  They were used in early tanks mainly in the Destroyer role, which were meant to smash barbed wire and keep German heads down till the infantry arrived.  They were used by the Aussies and the Imperial Camel Corps in the Sinai in 1917 as they were easy to transport by horse, camel back,and early armored cars.

picture of Hotchkiss dual gun mount

Picture of Royal Navy dual mount, picture by Imperial War Museum

Following video from SitsinShadow's channel