The AR-15 was first built by ArmaLite as an assault rifle for the United States armed forces.

AR-15, Wikipedia

The Bushmaster XM15-E2S is a variant of the AR-15 first built by ArmaLite... The official manual lists the maximum effective rate of fire of the Bushmaster XM15 at 45 rounds per minute.

Michael Long, Was The Weapon Used At Sandy Hook An AR-15?, GUNFAQ.ORG

The magazine capacity is 30 rounds.

M4 Type Carbines.pdf on bushmaster.com

At about 9:35 am, using his mother's Bushmaster XM15-E2S rifle, Lanza shot his way through a locked glass door at the front of the school... Lanza stopped shooting between 9:46 am and 9:49 am after firing 154 rounds with the rifle.

Sandy Hook Elementary School Shooting, Wikipedia

The distance that Lanza traversed within the school was approximately sixty meters.

Calculated by the artist using plans of the school