From Crosscut:

I-1639 raises the minimum age to buy a semi-automatic firearm from 18 to 21. It also requires buyers to pass a more thorough background check, go through a 10-day waiting period and complete a training course. The suite of new regulations also contains a “dangerous access prevention” law that increases liability for gun owners who don’t securely store their weapons. If a weapon ends up in the hands of someone banned from owning a gun, for example, the legal owner could be charged with a felony.