If you pull the trigger of a rifle and it shoots, and then you have to add a new bullet to shoot again, you might be firing an old fashioned weapon (that may not even be a rifle, technically) and you are probably a hobbyist.
If you pull the trigger of a rifle and it shoots, and then you have to wiggle a metal object around to move a bullet from a storage area within the firearm, into the chamber from which it can be shot, you are using a non-automatic rifle, and you might be living in the old days (as a soldier) or perhaps you are a hunter, because many excellent hunting rifles work this way. It is also possible that you are a sniper of some kind, depending.
If you pull the trigger of a rifle and it shoots a bullet, and then you pull it again and it shoots one more bullet, and so on, with no additional wiggling of metal parts, you are firing a semi-automatic rifle, or at least, a rifle in semi-automatic mode. This is the ideal rifle for accurately hitting several targets, and if the rifle has a couple of additional design features, it may be the ideal rifle for killing the maximum number of people in a given killing bout.
Like for instance, a classroom full of students or a church full of worshippers or a grocery store full of shoppers.
If you choose, instead, to fire a fully automatic rifle in that school room, church, or grocery store, then you are being a firearms idiot. A rifle on a fully automatic setting fires a lot of bullets all in a short time when you pull the trigger. Most of the bullets will miss their targets, and you will run out of bullets really quickly. The students, churchgoers, or grocery shoppers will duck out of the way, and then when you are out of bullets they will (hopefully) swarm you and rip out your liver.
In fact, an automatic rifle is not really designed, while in full-auto mode, to fire at targets, so much as it is designed to fill the air over and near a target.
Does this seem wrong to you? If so, that could be because you, as a non-gun nut, have fallen into a trap frequently set by gun-nut trolls.
You may ask, if a fully automatic rifle is not an effective means of killing school children, then what kind of rifle should I get for that job? (If you were actually thinking that, call 911 and turn yourself in.)
The misconception arises because non experts tend to put rifles on a spectrum, where on one end is the musket like firearm, in the middle are non-automatic and semi-automatic rifles, and at the far end is a fully automatic rifle such as the MG34, one of the earlier fully automatic war machines that could fire over a thousand bullets a minute. Clearly, the rifles on that high-yield end of the spectrum are best for a mass murder, right?
No, actually. Fully automatic fire is not for killing. It is for suppressing. Consider this scenario. Eight or so soldiers are advancing into a village they intend to occupy. A couple of them are carrying radios, or are medics, or in charge, or whatever. Most of them are carrying fully automatic rifles but set on semi-automatic, so they will be shooting one bullet at time. Two of them are carrying the same rifle, but set on full auto.
They are all hiding behind trees and rocks. The enemy is in sight, so if they advance, they will be shot at. So one of the full auto soldiers pulls out from cover an fires a burst of automatic fire in the general direction of the enemy, and ducks back down, while the other full auto soldier then does the same. They take turns doing this, and the enemy keeps their heads down because they are being suppressed. And maybe repressed too, depending.
Meanwhile, while these two are blasting thin air with lead, the other soldiers with the semi-auto settings turned on, don’t fire their rifles, but they run ahead to a better, closer location with a view of the enemy. The enemy did not see this because they were busy ducking. Then, a bit later, the semi-auto soldiers start picking off the enemy, one carefully fired bullet at a time.
The enemy backs off a ways, maybe one or more are wounded or killed, by they are hit by the soldiers firing one bullet at a time, and NOT by the “bam bam bam” fully automatic soldier.
So, when gun nuts* try to tell you that everything is OK because automatic weapons are not legal, only semi-automatic, you may want to tell them that you already know that the ideal killing machine in a school classroom, temple, grocery store, or nightclub is a semi-automatic assault style rifle, not an automatic weapon, and if you are using an automatic rifle, better set it on semi-auto mode to maximize the number of people you are going to tear apart with bullets.
*I chose the term “gun nut” to single out people who are not merely pro-gun, but rabidly so. Pro-gun people are not necessarily anti-gun regulation.
Cheap: 
