A familiar feud in American politics has reared its head once more. After the massacre that struck Christchurch, New Zealand in mid-March, the question of whether civilians should be able to own “assault weapons” again became en vogue.

New Zealand responded to the shooting by banning an entire class of weapons just days later, to the thunderous applause of gun control activists and others on the mainstream political left. This response was lauded as reasonable and decisive, with proponents insistent that nobody needs “weapons of war.” In turn, gun rights activists deflected per their usual means, poking holes in gun control logic, and attacking their opponents’ vague and uninformed definitions. But no one is explaining the reason that people truly do need these weapons.

Generally, Americans know what people are talking about when they hear the term “assault rifles.” But when someone tries to write a rigid definition, things get ridiculous. Past definitions have included things like 22-caliber “squirrel guns,” handguns, shotguns, and so on.

This naturally leads to policy failure. It’s worth noting, for example, the Christchurch killer’s rifles didn’t even fit the definition of the “Military Style Semi-Automatic” rifles New Zealand banned after the fact. But let’s accept “assault weapons” as short, semi-automatic rifles which accept a detachable magazine, as this is what most “assault weapon” bans are really getting at.

The argument against these weapons relies on the belief that they’re uniquely deadly. For example, the pro-gun control Giffords Center refers to assault weapons as “a class of [high-powered] semi-automatic firearm specifically designed to kill humans quickly and efficiently.” This conception of assault weapons is inextricably linked to the “high capacity magazines,” usually holding 20 or more rounds, that these weapons are designed to use. So, the idea is that a weapon holding 30 rounds must be designed to kill 30 people. Of course, reality is significantly more nuanced.

Defining “assault” weapons like the AR-15 as “high-powered” obscures one of their defining aspects: their use of an intermediate ammunition cartridge. Intermediate cartridges, as the name suggests, are more powerful than a handgun, and less powerful than a rifle. A weapon’s power, then, isn’t a matter of magic or opinion, it’s one of science.

The 5.56x45mm round — the world’s most common assault rifle cartridge — makes about 1,200 ft-lbs of energy. Compare this to 500 and 2,600 ft-lbs for 9x19mm and 7.62x51mm ammunition — the most common handgun and rifle cartridges, respectively. So, the average assault rifle is about twice as powerful per shot as a handgun, and half as powerful as a typical rifle.

This difference in power translates directly into the magazine size distinction. A full-size semi-automatic rifle with a 10-round magazine can put out about 10 percent more energy per magazine than an AR with a 20-round magazine. But no “assault weapon” ban takes this into consideration. That’s because magazine size has nothing to do with lethality. So why would a person want an AR with a 20- or 30-round magazine, when a full power rifle is more lethal? The answer—which should surprise no one—is that people have different skill levels, abilities, and needs.

Choosing a firearm is a lot like choosing a car. There’s the GMC Sierra, the Chevrolet Corvette, the Ford Windstar — all very different vehicles for different needs. But all, in the end, do the same thing.

Guns work similarly, but a capable shooter might want a more powerful gun that holds fewer rounds. A novice shooter might want something with less kick, and higher odds of hitting the target. The military and law enforcement chose to use the AR-15 platform — not because it’s particularly deadly, but because it suits a wide range of situations. It’s a proven and reliable model. The Honda Civic of guns.

It’s important to consider the realities of defensive gun use, which occur hundreds of thousands, or even millions of times per year. Someone who finds their home being invaded didn’t plan the encounter. Even law enforcement officers have a 39 percent probability to hit a target in a high-stress situation. This, combined with the fact that each hit with a handgun or intermediate cartridge has only an approximate 22 percent chance to stop the target, explains the need for larger magazines. From this, we can extrapolate that a novice law enforcement officer (likely more experienced than the average American) would need about 12 shots to reliably stop a single attacker.

Average people have no opportunity to prepare when there’s a bump in the night or a riot breaks out. On the other hand, mass shooters plan ahead and bring spare magazines. Americans who find themselves needing to defend themselves are unlikely to have spares on hand trained for the occasion. It’s these situations where they need a proven, reliable, and multi-purpose firearm—and yes, that includes “assault weapons.” When innocent Americans can face multiple attackers, and it can take more than 10 shots to reliably stop a single one, is a 30-round magazine and a reliable firearm unreasonable? Definitely not.