The No. 1 demand from many in the Parkland community after 17 people were gunned down in a high school there last week is to ban the gun used in the attack.

It sounds simple enough. In 1994, Congress passed a 10-year ban on these types of guns after a spate of mass shootings in schools and restaurants in which the attacker used a semiautomatic weapon. Seven states and Washington still have some kind of ban.

And it appears most would agree with a ban; a new Quinnipiac University poll found two-thirds of Americans think semiautomatic assault weapons should be banned.

But, as illustrated by an intense, emotional exchange during Wednesday's CNN town hall on guns between Sen. Marco Rubio (R-Fla.) and Fred Guttenberg, the father of a student who was killed, there is no easy way to ban assault weapons. That's because there's no easy definition of what an assault weapon is, and it may be virtually impossible to ban them. But there are fairly simple ways to heavily regulate them.

Lawmakers have two ways to ban assault weapons: List specific guns they think shouldn't be legal, or list characteristics of guns that they think make them too dangerous because they fire rounds too quickly. But guns can be modified, which is why even supporters of the 1994 federal assault-weapons ban acknowledge gunmakers just tweaked illegal guns to make them legal.

Here's the statement from former congresswoman Gabby Giffords's gun-control group, Giffords Law Center to Prevent Gun Violence: “The inclusion of some purely cosmetic features created a loophole that allowed manufacturers to successfully circumvent the law by making minor modifications to the weapons they already produced.”

The Giffords Center, which was founded after the congresswoman was shot in 2011, says there's a way to tighten the loophole, albeit not close it completely. Instead of requiring that guns have two certain characteristics to be classified as an assault weapon, you could require that guns have just one. That's what New York and California have done. “A one-feature test captures more assault weapons and makes it harder for the gun industry to evade the law by slightly modifying banned weapons,” the Giffords Center says.

To which opponents of the ban say: We can still find a way around that. As long as you classify guns by how they look, we can simply change certain parts.

When pro-gun writer J.D. Tucille visited a New York gun shop in 2014, the owner explained how he sells guns that get around a state ban.

“It's basically still a semiautomatic rifle,” he said, holding up a modified assault rifle that is legal in New York. "The law was basically written by people who don't know anything about guns.”

Rubio has clearly heard that argument before. Here's what he told Guttenberg on Wednesday:

“In New York, they have passed that ban. And you know what they’ve done to get right around it? It took them 15 seconds to do it. They simply take the plastic tip off of it. They just take the plastic grip off of the front or the back … [of] the same gun, and it becomes legal, performs the exact same way.”

Here's where the really heated part of Rubio and Guttenberg's exchange comes in. Guttenberg asked Rubio what's wrong with banning at least some guns. A version of a federal assault-weapons ban introduced by Sen. Dianne Feinstein (D-Calif.) in recent years would ban 150 to 200.

“It’s a place to start,” Guttenberg said. “We can do that.”

David Chipman, a former agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives and an adviser to the Giffords Center, says that's the right way to think about assault weapons: They may not be able to be banned, but they can be heavily regulated. And more regulation could mean fewer of these deadly weapons in the hands of killers. “I think there are people trying to shoot other people all the time,” he said, “but the outcomes are different depending on the weapon used.”

To which opponents say: Would-be killers will find a way. Connecticut has a ban on semiautomatic weapons, but the attacker in the 2012 massacre at a Sandy Hook, Conn., elementary school used a legal version of a semiautomatic weapon to kill 26 at the school.

So the debate about banning assault weapons — as all complex debates often do — comes down to a degree of scale. Is it worth taking some deadly assault weapons off the street if you can't take all of them off? The data is mixed. Studies have shown that assault-weapon bans have meant communities have fewer assault weapons; other studies have shown that banning them hasn't significantly reduced violence because they are used in 1 to 2 percent of attacks.

But perhaps the real question we should be asking is this: Could an assault weapons ban pass Congress right now? The answer to that is another “probably not.”

Both times in recent history Congress considered and/or passed an assault weapons ban, it did it under a Democratic-controlled Congress, and even then, the ban just barely passed.

To sum up, the data is inconclusive about the effectiveness of assault-weapon bans, and there may always be some loopholes for gun manufactures. But a ban might have prevented the carnage in Parkland, Fla. Under the 1994 assault weapons ban, the AR-15 that shooting suspect Nikolas Cruz legally bought would have been illegal.

All of this is why it's easy to see the same Rubio-Guttenberg exchange two vastly different ways: Is the senator giving an excuse for not taking tough action on guns? Or is there genuinely not much change that can come from a new assault weapons ban? And that debate has yet to be settled.