A cartridge which is more powerfull than a normal round of the same caliber, or a gun made to fire such a round. As there is no precice
definition of what is a magnum round and what is not, the name is up to the designer.
Magnums are most commonly used in revolvers, but are also used in semi-automatics and rifles.
Magnums, while powerful, will not "blow somebody's head off" or kill anybody with a
single shot. As with all rounds, they will not kill except by central nervous system damage or by blood loss. While a magnum round will most likely cause more damage than a non-magnum the location the round strikes has more effect than
anything else, and even a round to the chest should not be assumed to be a automatic stop. Look at Richard Blackburn, who was shot five times in the chest with a .357 magnum, but survived and returned fire, killing a
South Carolina state trooper, Mark Coates, with a .22LR handgun.
There are many magnum rounds, but some of the most popular include the .357 magnum and .44 magnum. Also worthy of note is the .460 Wetherby Magnum, a _very_ large, powerful rifle round for large game hunting.
A .460 Wetherby Magnum will stop anything it can hit properly. It is what you use
if you want to hunt an elephant.
At the other end of
the scale is the .17 HMR, which is only very
effective against fairly small animals.