From my studies about high powered assault weaponry, I knew that it was referring to the NATO caliber 5.56, which is also the caliber of choice of many American spree killers, including the shooter at Sandy Hook and most recently at Roseberg.
You should study harder next time. Maybe actually read something on the subject.
You're only telling a half truth there Broke Back. Actually maybe a quarter truth.
Adam Lanza did indeed have a Bushmaster XM 15 on him when he commited his act of terror...but he also had a Glock 20SF and a Sig Sauer P226.
Christopher Sean Harper-Mercer had a rifle.... a .556-caliber Del-Ton. But that was in addition to a 9mm Glock pistol and .40-caliber Smith & Wesson and a .40-caliber Taurus pistol.
With the exception of the Smith all are relatively cheap to buy (you can get the Taurus for less than $250 the rifle for around $750)
So in both instances you cite there were only two rifles...yet there were 6 hand guns ranging in caliber from 9mm to .40 cal
Handguns outnumber rifles 3-1 in both cases meaning your claim that the 5.56 round is "the caliber of choice of many American spree killers" .
In the last three mass murders in the U.S. the preferred weapon has been the handgun...not the "assault" rifle or the 5.56 round.
Oh and just to put a fine point on your ignorance...the shooter in South Carolina used a single Glock .45