Geez....wonder why?

There's something about this Bushmaster AR-15: Both Adam Lanza and William Spangler, the two gunmen in the Newtown and firefighter shootings, respectively, got their hands on the same make of semi-automatic, the .223 caliber rifle, pictured right. This popular sporting gun seems to be the weapon of choice for many a mass murderer. It's also the same gun used by James Holmes, of the Aurora theater shootings last summer, as well as Jacob Tyler Roberts, the mall shooter from a few weeks back. What is it about this Bushmaster that makes it so available and desirable for these gunmen?