Having known at one time that the gun was in the trunk is not the question.
The statute says "knowingly and willfully did COMMIT".
If he forgot the gun was there, he did not KNOWINGLY and WILLFULLY commit the CRIME.
Even if you argue "knowing", he clearly did not do it "willfully".
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.