There is no reason for any civilian to own a full automatic or semi-automatic firearm, a firearm with an external magazine (often referred to inaccurately as a clip), or military style rounds (a.k.a. bullets). Full and semi-automatic weapons firing military style rounds from external magazines are typically referred to as assault rifles or assault weapons but we will not use that term here. Rather we are going to examine why each of those 3 components by themselves should make a weapon ban worthy.
There are only two legitimate civilian uses for a firearm: Sport shooting (including hunting and target shooting) and self-defense (which includes the killing of vermin and of predators attacking livestock). Both uses put a premium on marksmanship; i.e., hitting what one is aiming at. (This includes shotguns, which are much more forgiving re absolute accuracy, for reasons described below.)
Military firearms have two and only two non-ceremonial uses: For marksmen to shoot an individual enemy with accuracy, and to lay down a withering cover of supressing fire in order to pin an enemy unit down long enough for air or artillery strikes to be called in.
The former may involve standard issue sidearms, rifles, or specialized sniper weapons, but the latter typically involves the tools of an infantry unit: Rifles, machine guns, grenade or rocket launchers, and mortars.
The military marksman, like the civilian sports shooter, seeks to hit and kill the target with the first shot since a miss has several results of increasing undesireability, from the target vanishing to the target attacking and killing the shooter. Military marksmen, like civilian sports shooters, strive to be effective with a single shot.
Supressing fire, however, doesn't rely on accuracy but rather on filling the air so full of lead that an enemy doesn't dare move for fear of being hit by a stray shot. Enemy kills are a by-product of this sort of use, not the primary objective. As cited above, the main purpose of military firearms is to pin an enemy down until heavier weapons can be brought to bear.
This is where the semi-auto function comes in.
A semi-automatic weapon fires a round, then using expanding gas from the round (or, much more rarely, the physical recoil of the round being fired) ejects the spent round and loads another bullet into the chamber, readying the weapon to be fired again (a full automatic weapon keeps firing as long as the trigger is held; some semi-automatic weapons have selector options that allow a shooter to fire a three round burst with every pull of the trigger).
To maintain accuracy, every weapon needs to be re-aimed after every shot. Unless a weapon is firmly anchored to a far more massive object, its recoil alone will throw its aim off.
Many air / ship / vehicle mounted weapons use the vehicle's mass plus gyroscopic mechanical and/or computer aiming systems to maintain steady accurate fire by re-aiming the weapon every split second between shots.
Handheld or shoulder firearms can not maintain steady accurate fire even on a semi-auto setting without being re-aimed. Not only is there recoil and muzzle climb to deal with (i.e., a tendency for the business end of the device to rise as it's fired), but the very operation of the semi-automatic reloading will produce enough vibration to require a shooter to slow their rate of fire to re-aim.
For civilian sporting and defense use, there is no purpose served by a semi-automatic weapon since to fire it accurately the shooter must slow their rate of fire down to the level of a bolt/lever/pump operated weapon.
To compensate for the appaling lack of accuracy in full and semi-automatic rates of fire, modern military weapons use large external magazines to hold several rounds of ammunition thus enabling the shooter to throw out a lot of lethal lead very, very quickly. In modern combat it takes literally tens of thousands of rounds to produce a single gunshot casualty; the real killing is done with high explosives delivered by rocket, howitzer, aircraft, or vest.
The round in the middle is the type used by the M-16 and its civilian counterpart, the AR-15, which was the weapon used to killed 20 unarmed first graders in Sandy Hook. They were all shot multiple times with this type of bullet.
Further, in combat situations military personnel concern themselves only with self-protection. If fired on they will fire back, tough titty if some civilian gets hit with a stray shot. In civilian sport and defense shooting, however, the shooter will be held criminally and financially responsible for any injury, damage, or loss of life caused by a stray round. As a result it is not in a civilian shooter's best interests to fire wildly at a target, so there is no need for a large external magazine.
If it takes more than two shots to bring down a deer or a duck, you are clearly not a good enough shot to be a hunter. (Go practice.)
If you think defending your home will involve a prolonged gunfight against several intruders, you should really analyze your life to see why people would want to engage in a gun battle with you. (Lifestyle changes may be in order.)
Military rounds are typically hard tipped steel or copper jacketed slugs designed to penetrate metal, heavy wood, cinderblock, etc. with enough force to kill a human being on the other side. The steel or copper jacket holds the slug together when it hits hard objects, thus facilitating lethal ricochets.
Military rounds were so powerful there are well documented cases of them passing through a human body so swiftly the victim was not even aware they had been shot and continued fighting. As a result modern military rounds are designed to start wobbling when they strike a target, translating the bullet's velocity into a devastating kinetic shock wave that rips through a human body.
There are numerous documented cases of people having limbs and heads blown off by a single military round; there are documented cases of human bodies literally being blasted to bits by a burst from a full automatic weapon.
While "varmint" shooting may not worry about the damage done to the target, hunting generally does not want the meat or trophy damaged by reckless shooting.
Many civilian hunting rounds have soft or hollow tips if jacketed, or no jacket at all. The reason for this is to deform the bullet when it hits a target, flattening it out to spread the kinetic damage around.
The objective is not to send a bullet through the target, but to get it to impose maximum tissue damage on vital organs such as hearts and lungs. Conversely, military rounds need to keep causing damage by ripping through one human and hitting another, possibly going through a brick wall and ricocheting off an embedded steel beam before hitting another human (and if that human is an enemy, so much the better).
Using a civilian weapon such as a shotgun as a last line of home defense makes sense. Fire a shotgun at an intruder and the chances of hitting them with some pellets are greater than hitting them at all with a bullet, and pellets that miss are unlikely to penetrate the walls of one’s home to harm others.
Conversely, shoot an intruder in your bedroom with a military style rifle round, and the bullet is fully capable of going right through them, going through an external load bearing wall, traveling half a mile to smash through the grill of an automobile, ricochet off the engine block of said auto, punch through the fender of the car, penetrate the external load bearing wall of another house, go through two more internal walls, and then blow the head off a sleeping infant.
By definition, military weapons are designed for use that renders them impractical at best and totally worthless at worse for sporting and personal defense. Military small arms are designed for and deployed by governments seeking to use them to kill large numbers of people very quickly, either directly or indirectly by pinning an enemy unit down until it can be destroyed by air or artillery.
The reason governments want to do this is because they are seeking to impose their will by force on others. As a result they do not care how much “collateral damage” occurs to civilians so long as the enemy capitulates.
Military grade weapons (full or semi-automatic firearms with external magazines firing military grade ammunition) are tools of the trade along with saturation bombing, nerve gas, biological warfare, and nuclear weapons, all of which are weapons with no practical civilian use that governments employ against civilians.
Now, it has been argued that citizens need these military quality weapons and ammunition in order to oppose a tyrannical government. Let the record show that the last time there was a large scale uprising against the government of the United States, it was by tyrants seeking to maintain their ownership of slaves. Let the record further show that said uprising, despite being led by some of the most brilliant military minds in history, ultimately failed and at a terrible price to those who supported the tyrants.
And let the record show even further still that it is historically proven armed resistance succeeds only one in four times it’s tried, and when it fails it leaves the victorious establishment with even more power than before, and that never leads to democratization, but when nonviolent passive resistance is employed, there’s a fifty-fifty chance of success and even if the resistance fails, there’s a one in three chance the government will loosen up and democratize within five years.
Nonviolent passive resistance is clearly the best method of resisting a tyrannical government.
Bottom line: If you want to defend your home from criminal intruders, first get a dog, then get a cell phone, then get a burglar alarm, and only then get a shotgun. If you want to defend your community from an oppressive government, lay down your weapons and resist nonviolently and passively.
 For one thing, irresponsible weapon ownership apologists enjoy indulging in meaningless semantics over the term, sometimes correctly pointing out that two weapons with identical operating mechanisms may be identified as either a sports weapon or an assault rifle based on purely cosmetic appearances, but then using that to attempt to negate reasonable attempts to regulate such weapons.
 And that’s just a single round; imagine how popular you’ll be with the neighbors after spraying a 30 round clip at an intruder.