I also heard that the killer fired something like around 3 shots at each victim.
So it would have been similar to 10 rounds of a more powerful gun.
Before the AR-15 became so popular in America the average gun was more powerful. AR popularity has actually reduced how powerful the cartridge of the average rifle is.
30-06 for example used to be very common.
Today the .223 is more of a standard.
An 8 round Garand for example is not that far from 10 rounds, and was quite popular goin back to ww2.
You can even own them in Canada!
Ironically the military only went to the 5.56x45 round because it was expected to be used in selective fire roles where the reduced recoil was a bigger benefit , not because it was a better semi-auto round.
If the military had stuck with semi-auto rifles they would have never gone to 5.56 and stayed at .30 caliber or larger rifles.
Even more ironic is that today most soldiers are limited to using thier 5.56x45 rifles in semi-auto when the whole original reason for its adoption was being more suited to select fire use. Of course pinning the enemy down while artillery or air power, and support weapons like grenade launchers and mounted machineguns, actually does most of the killing is a bigger use of rifles in the military today, so a lot of little rounds can fill that role better than bigger rounds that actually would do a better job at killing the enemy.
However all of that is really missing the point as to why politicians and leaders want to ban rifles they feel are effective. The truth is a mass shooter with a common shotgun with 1/3 of the rounds could do just as much damage to civilians without body armor, with each round being more lethal. But would be much less effective against those in armor or protected vehicles etc
So such restrictions would do a lot less to change the lethality or number killed in mass shootings, but a lot to insure those working with the government come out on top easier. So when they see the opportunity to go after rifles they feel are effective they use mass shootings to do so. (Keep in mind the whole purpose of the 2nd Amendment being to provide a risk to government forces and act as a deterent to foreign and domestic governments, and that governments in the last 100 years have killed far far more of thier own people than all criminals combined.)