Gotta agree with this. It is big, it is impressive, and it fires a “mofo” round. I don’t know about the .50, but the .44 Mag will definitely get the job done. And, important to** StarvingButStrong’s** story, this is something that his killer might reasonably carry, and find ammo for pretty easily as well.
BTW, kudo to you, StarvingButStrong for trying to get the scene right and not insulting the reader’s intelligence! So little of that nowadays. 
You’re taking into account expansion and fragmentation, but not velocity and cavitation. A low velocity round will mostly cause damage surrounding the permanent cavity, the track of the bullet, and leave an exit wound comparable to the bullet’s expansion, if it can penetrate. However, at higher velocities, the bullet can damage tissue a good deal away from the path of the bullet, and send a shockwave through the entire organ.
For most areas of the body, the temporary cavitation (the stretched tissue surrounding the permanent cavity of the bullet’s track) of a high velocity pistol round won’t make a huge difference difference due to the elasticity of the tissue, but the tissue of the brain is extremely delicate and inelastic. A high velocity gunshot wound to the head can cause a bursting fracture of the skull due to temporary cavity formation. This is obviously much more likely from, say, a Mannlicher-Carcano rifle, but a .357 Magnum with hollowpoint centerfire high velocity ammunition (1500 fps) is theoretically capable of bursting a skull.
http://karws.gso.uri.edu/JFK/scientific_topics/wound_ballistics/How_a_high-speed.html
Dr. Martin Fackler, formerly of the Wound Ballistics Laboratory, Letterman Army Institute of Research, has done the most extensive body of experiementation and research with regard to wound ballistics in the US. It was his conclusion that hydrostatic shock was of minimal effect with regard to damage; while a temporary wound channel is created by the shock wave of a bullet moving through tissue, the tissue is flexible and compressible enough that it isn’t significantly traumatized, and that the biggest factor in wound ballistics is tissue that is lacerated or torn by the bullet or fragments thereof. He came to this conclusion even with regard to military rifle (large bore) calibers which move at 2000+ fps. Certainly with the much slower pistol rounds–even the vaunted .357 Magnum or a hot loaded 10mm Auto doesn’t get much past 1500fps–hydrostatic trauma is minimal, espeically in brain matter which is less dense and more compressible than muscle tissue or organs like the liver or kidneys.
While Fackler’s work isn’t the last word in wound ballistics it is the best available and is generally considered definitive. Here is a collection of articles, some by Fackler, provided by B&T Ammo Labs, a forensic ballistics laboratory (see “Wounding Mechanism of Projectiles Striking at More than 1.5km/sec” and “Effects of Small Arms on the Human Body”), and here’s another collection including the famed FBI study by Urey on effectiveness of handgun stopping power (“Handgun Wounding Factors and Effectiveness”) and several articles by Fackler from Wound Ballisics Review. Coates and Beyer’s Wound Ballistics (out of print I think, but long a standard reference for forensic pathologists) put the lower end for permanent trauma from hydrostatic shock at about 4800fps; well above even a hot-loaded 22-250.
A rifle bullet can cause considerable damage by keyholing–tumbling after entry–and can make larger wound channels and exit wounds, but handgun bullets are stubby, typically no more than half again in length that diameter. No handgun round is going to create a fist-sized exit wound or cause someone’s head to explode. That’s pure Hollywood.
Stranger
The FBI study specifically deals with law enforcement uses of handguns, i.e. targeting center mass in the torso, and states that headshots are not the focus of the article since they are impractical for training. The article does cite Fackler extensively for the proposition that fragmentation causes much more extensive damage than temporary cavitation even in a high velocity rifle, but is careful to point out that that is due to the elasticity of the body tissues that will be most commonly targeted. If you’ll read page 7, there’s a discussion of why temporary cavitation from a high velocity round does not cause extensive damage to most of the human body in comparison with fragmentation:
This means that the relative inelasticity and density of the brain compred to other tissues renders it more vulnerable to the kinetic energy of the pressue of the temporary cavity: