"I mean it is close to 0.50 calliber bullets..."
Okay, this line tells me you are have been swayed by the media hate campaign directed at the 50BMG.
For starters, the 'caliber' is the diameter of a bullet or bore. Unfortunately the media often simply states '50 caliber rifle' when talking about a very specific and powerful chambering, the 50 BMG, as used in the Barrett rifle.
But in actuality there are many other totally unrelated guns that fire a bullet that is .50 inches or larger. A 12 guage shotgun is much larger in diameter. Many old muzzleloading muskets used balls that were at or bigger than .5 inches in diameter.
What is more relevant is how FAST the bullet is going plus how heavy the bullet is. Most guns don't fire perfectly sphereical bullets any more, they fire pointed cylinders. A long pointed cylinder is of course heavier than a short one.
Obviously our technology has improved, we have better gunpowders nowadays and can have much more powerful guns than the old muzzleloaders. Still, some .50 cal chamberings like the 50BMG are very very powerful firing it's 660grain bullet at 3,000 feet per second producing 12,500 foot-pounds of energy. The current most powerful handgun is the 500 Smith and Wesson, fires a 400 grain bullet at 1,600 feet per second producing 2,400 foot-pounds of energy.
Now, the lowly 45ACP fires a 200 grain bullet at 900 feet per second producing 400 foot-pounds of energy.
So no, it is not too powerful.
Regarding metric vs inches for caliber. It is up to the individual who invents the round what he wants to call it. In general, "American" rounds are given inch designation based names, and "European" rounds are given mm designation based names.