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Okay, lets say theoretically HArry had a Desert Eagle handgun each year, and even had it magically upgraded to not jam or break and the proffesors never confisccated it(I added this part to thwart troll answers) Given that any human within 500 feet wouldnt hear a gunshot before a bullet would hit them or have the ability to react quickly enough to dodge one if they saw him shoot at them, on top of the fact they wouldnt expect him to use an unforgiveable curse and so they definitely wouldnt expect HArry Potter to be packing heat wouldnt he have been able to resolve the problems/antagonist in most books much faster, easier, and would have even been able to avoid being captured a couple of the times he was captured in the book? If he used a silencer with a his invisibillity cloak couldnt he have assassinated people like Bellatrix Lestrange and other death eaters without being caught?by them or the ministry of magic? Technically couldnt Dumbledore have even done this before Harry even started school? Seems like it would have saved a lot of innocent lives, potential danger, time, etc.
Heres the information about how fast a bullet travels and human reaction time etc.
The average bullet travels at 2,500 feet per second (around 1,700 mph). If you reacted to the sound of the gun going off and required 0.20 seconds (twice that of the fastest Olympic sprinters) to react, then you would need to be at least 500 feet away to successfully dodge a bullet. The problem is that sound travels at 768 miles per hour (1,126 feet per second)—about half the speed of the bullet. In this case, the bullet would hit you before you even heard the gun fire off. http://www.mythbusterstheexhibition.com/science-content/dodge-a-bullet/