20mm (or .80 caliber - to compare it to .50 cal.12.7x99mm bullets found in most large-bore sniper rifles, or the 14.5x114mm cartridge found in Russian heavy stuff) is, as far as I know, the largest cartridge used in any smallarm. The caliber itself was originally used as an anti-aircraft cannon round. The two types I know of are the 20x110mm Hispano and the 20x83.5mm MG151.
The relative size(diameter) of the bullets is about like this:
___ - 5.56mm common to assault rifles
______ - .50 cal (12.7mm) common to military sniper rifles and machine guns
___________ - 20mm (.80 cal) (un)common to the following badass guns:
There are three man-portable guns I know of that fire this calibre:
The Mechem NTW-20 from South Africa
This military anti-everything sniper rifle fires either 14.5x114mm Soviet heavy machinegun ammunition, or 20x88mm MG151 anti-aircraft ammunition from Nazi Germany. The weapon can be carried in the South African plains by two men. The MG151 ammunition can deliver a highly explosive round accurately to about 1500 meters, while the 14.5mm Soviet round can penetrate as much armor as needed to about 2300 meters. It is possible to switch between the two in under a minute by the two-man fire team. It is basically the gun where bad meets ass.
The Croatian RT-20
This weapon fires 20x110mm Hispano (also a WWII anti-aircraft round) to about 1800 meters. I've never seen it in person, but i'm sure this gun would scare the crap out of anyone. It has a big recoil tube along the top which jets out exhaust gases from the firing of the heavy-ass round being fired from the one-shot chamber, and a scope mounted along the side, so the firer doesn't break his shoulder and several ribs firing it. The weapon was used effectively in the Croatian war for independence in the 90's.
The last gun is made in Finland, but I don't know anything about it and I can't find any information on it. I think it fires proprietary 20mm ammunition, rather than all of the WWII vintage shit those others fire. No matter, no big game hunting cartridge can stand up to these bad asses on any day of the week (unless they made 20mm magnum ammo.. ouch!)
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