What is the test for a jury to convict a defendant of possessing only one firearm?

The trial court erred by instructing the jury that, if it found defendant possessed one firearm, then it could find defendant guilty of possessing six firearms. The trial court's error denied defendant his right to have the jury find every element of the charged offenses. That instructional error "is serious constitutional error that impacts a defendant's fundamental right to a jury trial." (People v. Merritt (2017) 2 Cal.5th 819, 821.) The failure to properly instruct on an element of an " 'offense is amenable to harmless error analysis.' " (Id. at p. 827.) We examine "whether it is clear beyond a reasonable doubt that a rational jury would have rendered the same verdict absent the error." (Id. at p. 831.)