AR-15's and AK-47's can be easily converted to perform like they are fully automatic. Buy a $100 market available part and you can have an automatic weapon in minutes. Just look at all the
sites detailing this exercise for an AR-15. I have no idea whether this most recent terrorist did this but the San Bernardino terrorists had illegally gotten (but legally purchased) AK-47's that had been converted to automatic.
These military designed weapons
are different than regular guns. Frontline had an interview recently with the lead designer of the original M16, the military grade version of the AR-15. He stated that this weapon was designed for maximum damage. Pointing to the .22 caliber of the weapon, a relatively low caliber, this was intentional. The bullet leaves the barrel more rapidly in the AR-15 than other .22 caliber rifles. The low caliber ensures the bullet doesn't exit the victims body but bounces around maximizing internal damage. The casualty rate for the M16 was off the charts in Vietnam compared to previous military standards, and higher than the AK-47. The designer claimed they had verified reports of deaths from victims with entrance wounds in their ankles. He claimed that the firing mechanisms in the AR-15 is virtually identical to those his team designed for the M16.
For all intents and purposes, the NRA has pushed to allow the AR-15 to be the most common rifle sold and it is a military grade weapon. Combine these weapons with high capacity magazines and the difference between a terrorist wielding this weapon and a group of terrorists in China wielding knives is dozens more deaths and injuries.