Sbm.
I'd want to understand if he was ordered to surrender firearms specifically or some broader class of weapons. Air guns do not use gun powder and, per ATF, are not classified as firearms. Tranquilizer darts are commonly shot from air guns.
Also of note, it is a class 1 misdemeanor per Colorado law (section 18-12-103) to knowingly be in possession of a firearm whose serial number has been defaced. The AA pg 7 mentions a camouflage patterned high powered rifle whose serial number could not be found.