Are you suggesting that had the Virginia Tech shooter opted for an AR-15 instead of handguns, the resulting casualties would have been fewer?
Not quite. I was saying that handguns are perfectly adequate weapons to get a high body count in a mass shooting.
Whether an AR15 would have changed the number of casualties/fatalities substantially is uncertain. It could well be that it would have led to fewer though. AR15s have a greater firepower, but utility of that in close quarters is limited - dead is dead whether hit by a 9x19mm Parabellum vs a .223R/5.56x39mm NATO. Greater accurate range is also not a benefit in close quarters. On the negative side, rifles are heavier and less wieldy than handguns, which is a plus for the latter when indoors. Handguns can also be easily concealed unlike rifles. It is possible that he would have been spotted with a rifle by a cop before he reached his destination. Or at least somebody who would have called 911 ensuring cops would get there sooner.
So both handguns and rifles have pros and cons, but I think in a situation like VT, handguns have the advantage.