The weapon Hasan used in the Fort Hood slaughter was an FN 5.7, according to news reports. It's an unusual gun. Semi-automatic pistol that shoots a center-fire .22 caliber bottle-neck cartridge that looks more like it belongs in a long-distance rifle. It's an unusual round for a handgun -- intended for extreme accuracy for military personnel. And, yes, some ammunition can penetrate Kevlar, at up to 200 meters. Here's a review.
Yes, it is available for sale to the general public, unlike other FN products, which include fully-automatic machine guns.
Hasan's weapons choice is really unusual for someone who was going off the deep end and thinking about going on a shooting spree. He had to have researched this. This is the gun he wanted. Bullets would have traveled through their human targets and likely hit another. The clip holds 20 rounds, unlike a .45 (1911) which has clips that can hold 13.
Additionally, contrary to MSNBC reports, it's an expensive gun, retailing for more than $1,000.
The gun was designed for one purpose – to kill humans. (regular readers will note that I'm not opposed to citizens owning such weapons, in fact, I own two such weapons -- a Kimber 1911 .45 cal. pistol, which I carry, and a 7.65 Argentine Mauser, which was manufactured in the early 1920s, and which has been customized for use in hunting large game.)
What makes the FN Five-seveN interesting is that it really isn't an impulse buy. The bullet, if not precisely placed, would likely travel through the target without doing much damage unless it hit bone or vital organs, which likely explains the number of wounded.
I think the guy planned this attack for some time. And, with this particular weapon, it's a wonder more people did not perish.