Hmmmm Granted, it is a comic book and anyting goes, so long as consistency is held from one issue to the next.
However, I think the young lady asks some legitimate questions and deserves a legitimate answer.
1. No, the .50 DE will not take off an arm on contact. Depending on where the person was struck and with what kind of bullet, there could be massive damage resulting in instant death, to a clean hole through and through, causing pain and leakage, but not even incapcitating the person hit.
The gun is difficult to handle and aim for someone untrained, when compared to some others. However, to an experienced shooter, who has practiced with a DE, the tool is not that hard to use effectively.
Many revolvers can use different types of ammunition interchangably. The .22 revolvers can use .22 short, .22 long, .22 long-rifle, and/or .22 CB, to mention a few. These are merely variations of the basic .22 rimfire cartridge, differing in length and power. The .357 revolver can use 38 S&W, 38 Special and .357 Magnum rounds. These are all variations on the basic .38 S&W cartridge, differing in length and power. (However, note that a 38 Special revolver cannot use .357 Magnum ammunition.) There are others as well.
I recommend the Lyman books highly. They appear, at first glance to be for handloaders and reloaders, but there is a wealth of basic background information in each one, also. The Shooter's Bible is very good for a beginner's look into the fascinating obssesion....er... hobby of guns.
(Click on "publications.")
Have fun and learn. It is the only way to grow.