BLACKSBURG, Va. – Holiday shopping season is here, and tens of thousands of Nerf Gun Blasters will be under Christmas trees this year. More than 25 years after the toy was first released, its constantly growing popularity has led to an active market for knock-offs, especially for the darts.

Stefan Duma, a professor of engineering at Virginia Tech, who spent the summer testing the darts, found that off-brand darts are two to three times more likely to cause eye injuries. A big part of the risk is because of how they're made.

Scroll through Amazon or any online retailer and you can find dozens of "Nerf Compatible Darts" for less than half the price of the name brand darts. Looking at pictures online, they look exactly the same. Some of the off-brand darts even have four or five-star reviews. Despite all of that, Duma says the darts are very different.

While both are made of small foam cylinders, its the tip where you can see the biggest difference. The Nerf brand darts are squishy and deflate when you press down on them, while the off-brand darts are made out of a tough, plastic-like rubber that has little to no give. That's why Duma says these knock-off darts have a higher risk of injury.

"In this case, the Nerf dart comes in and deforms and spreads out the load on the eye," he says. "If the tip of the dart is rigid or more rigid, it doesn't deform. You get higher loads and higher stresses and you have a higher risk of eye injury."

Duma is an expert on injury biomechanics. He's been leading the Virginia Tech research on helmet ratings for years. But it was a different kind of research that played a role in this study on Nerf guns, it was research that he started back in the mid-'90s to evaluate eye injuries for soldiers.

He says he was able to use some of the same techniques he used in that study to evaluate the toys and the difference in the darts.

"We developed a headform for the military to evaluate the eye and face injuries," says Duma. "We shoot the dart at the headform and measure eye response and calculate risk in that way."

Shooting darts at the headform makes the difference between the brand darts and the knock-offs clear. The tip of the Nerf brand dart seemed to collapse and the dart fell, not having a big impact. Shooting the headform with the off-brand dart made a loud popping noise and the dart ricocheted off.

Duma says sensors embedded in the headform revealed the off-brand darts more than doubled the risk of injury compared to Nerf brand. He says that research, along with measuring the darts' mass, velocity and area of impact to calculate a value called normalized energy both revealed the higher risk of injury.

Duma says his own house is often filled with Nerf guns and he hopes this research will help other parents avoid the risk that comes with those off-brand darts. He says this is a case where brand matters, and buying Nerf-brand darts can significantly cut down on the risk of injury.