You could also use the 303 as a range-finding aid. Just converge them out a little farther than your 40mm's, and hold the trigger as you get close. When your impacts converge on the target, pull the trigger for the 40mm, and with the reaction time delay of the human brain and the extra distance of the 303's convergence, you should end up firing at about the right time.
I think this was done in the real world too.