Only use the caliber designed for that particular gun unless you want the gun to blow back into your face. It is never safe to interchange ammunition not specific to the caliber of weapon you use.
20 mm cannons.
No. The AK-47 is only chambered in 7.62x39mm. A 5.56x45mm round would not fit in its chamber. However, the modernized AK-101 is chambered in 5.56mm.
It is the same cartridge
The F22 can mount a 20 mm Gatling Gun, with 480 rounds of ammo. Primary weapons are not the gun, but missles.
6.35 mm is also known as .25ACP, or .25 Auto for short. Most gun shops will have this in stock.
Some revolvers will take .357, .38 and 9 mm. Whether or not a particular revolver will or not is a different issue. One police department went to them and went to .45 the next year. They may not have worked very well.
11.5 mm bullet diameter
Some folks rather have a 357 revolver then a 9 MM semi auto as a revolver if it misfires you just pull the trigger again If a semi auto misfires you lose the battle as you have to manually reject the ammo and hope the next round fires. Of all the semi autos I did fire I happen to like the sig 226 9 MM the best
"Best" is subjective. A really good definitition of best is, the ammo that functions the most reliably in your particular gun.
Most gun shops, sporting goods stores. Ask for 8mm Mauser- that is the common name, even if it really is 7.9
357 millimeters is 1 foot 2 inches.
No such thing as a 2 caliber gun. That would be a bullet 1/50th of an inch in diameter. Basically, .22 LR ammo may be fired from a gun marked .22LR. It MAY be possible to fire it from a gun marked .223 or 5.56 mm WITH THE USE OF AN ADAPTER.