Twenty millimeter rounds have traditionally been the ammunition fired from mounted weapons, like aircraft cannons and anti-aircraft guns. Occasionally, they can be found in semi-automatic shoulder fired weapons in the anit-tank role. Those weapons are particulalrly heavy and punishing to the firer.
Imagine a 20mm shoulder fired weapon on fully automatic.
That Japanese Army fielded one such beast during World War II as the Type 97 anti-tank gun.
At 150 pounds, the Type 97 weighed as much as the operator who wielded it. So heavy was the weapon that while it was said it could be carried by a two man crew, a cradle and handle assembly that could be attached to the gun meant it was more likely born by four men. Looking like bicycle handles, the bars could be mounted beneath the barrel or at the stock. More often than not the heavy metal carrying handles were discarded to save weight.
The overall weight came from the beefy gas operating system of the rifle. Dual gas pistons operated the action, cycling spent cases out (ejecting downwards) and stripping fresh rounds on return.
Heavy, the Type 97 was also not very small. With butt extended (slight retractable stock was built into the rear of the gun) the Type 97 was nearly seven feet long from muzzle back. Half of that length was the barrel.
Fed from a top mounted, seven round magazine, the Type 97 was a punishing beast. High Explosive or Armor Piercing 20mm rounds exited the muzzle at near 2,600 feet per second with an effective range of 1,000 yards.The small, seven round magazine also meant the Type 97 had a limited full-auto capacity.
Placed and loaded, the Type 97 had adjustable bipod under the gas-assembly and monopod below the stock comb. Early use of the Type 97 proved effective. However, as the war moved on, the 20mm round found limited effectiveness against American armor.
At 250 meters the armor piercing rounds could penetrate 30mm of armor, which meant classes of tanks and armored vehicles easily deflected the 20mm round. However, the Type 97 could be used against soft skinned vehicles, light armored vehicles and even slow moving/landing aircraft.
The beast of a gun lasted through the length of the war, proving reliable but never a force to be reckoned with.