According to a leading 3-D printing researcher, the only way to prevent printed guns from shooting a new loophole in the law may be to control the gunpowder you need to fire them.

“Perhaps the only way forward, if we choose to try and control this, is to control the gunpowder — the explosives — and not the actual device,” Hod Lipson, a Cornell University professor of engineering and an early pioneer of 3-D printing, tells Danger Room. The reason, Lipson says, is that it would be the remaining “controlled substance” in a field that’s otherwise uncontrollable, regardless of the shape or size of the firearm that you’re using — or printing. It is the “unifying material everybody would need, and it would be a good target for regulation if people choose to regulate it.”

This is because while gunpowder can be manufactured at home, it is largely unregulated for small arms ammunition. (Although making it in your basement can be difficult and dangerous.)