For a rifle it would be the 2 Bore.
for a rifle it is thought to be a .50 calibre but a gun company made there own for there own sniper rifle the .408 calibre
The 9mm Parabellum(9mmLuger) is .355 caliber.
Smaller than 20mm.
about 600 metres
Depends on your ability to accurately place a round. 25M or so
The Paris gun of WWI. It weighed 256 tons. it was 28m long and fired a 240mm round. It was made by Krupp of Germany. The German army used it on Paris France.
The best sniper rifle has to be the barrat 50 calibre it has an awesome firepower, and an awesome zoom. Following up to the barrat 50 cal is the walther 2000. it has a 45 calibre round, but makes up for it in speed.
Depends what kinda of round (bullet) it was, if it's from a small calibre weapon (i.e. 5.56 mil ammunition) it would probably take your head off with no effect to your body however if it is something like a 50. calibre round (often used in sniper rifles) it would probably take your shoulders/chest along with your head.
95 is the highest calibre for a bullet
Calibre One was created in 1999.
The velocity depends on the precise firearm, not the calibre.
The largest calibre i came across was 30 calibre russian rifle (gas chambers punctured to make it bolt action). owned by a retired army personnel. I guess they allow them to purchase it from the ordnance. Don't know if they can sell it though.