Hmmm, people here seem to know a little bit of this and a little bit of that.. I can't say I have the definite answers, but I can clear up a few things.
The Barrett .50 sniper rifle was made to counter lightly armored vehicles (and jeeps, trucks etc). Mainly APC kind of vehicles. And it WILL penetrate that armor. It will NEVER penetrate the front armor of a MBT but with a lucky shot you CAN disable a MBT by shooting off the threads, hitting right between the turret and main body etc.
The .50 caliber of a Desert Eagle handgun cannot really be compared the the .50 rifle rounds. Yes, they are the same thickness, but the rifly round is WAY bigger than what can fit in a handgun :-)
It's also true that you cannot use heavy rounds .50+ caliber against infantry in wartime, and it has nothing to do the size of the bullit or anything like that. It is a warcrime to shoot to kill, it is only allowed to shoot to disable! I'm not kidding here! The reason the heavy rounds are therefore illigal is because if such a bullit just passes by you it'll rip your arm off, and if you are actually hit you'll pretty much explode! So every shot with a heavy round against infantry is a shot with intend to kill, and that is a warcrime.
/Blaznee
