Me 410 A-1/A-2 U2/U4 fixed forward firing armament configuration

Me 410 A-1 and A-2 fixed forward-firing armament (starre Schusswaffen) comprised two MG 17 machine guns (ea with 1000 rounds ) and two MG 151 20 mm cannon (ea with 350 shells). Additional MG 151 (with 400 shells) or two Mk 103 ( ea with 100 shells)  and of course the larger BK 5 cannon (A-1/U4, 22 shells) could be mounted in the bomb bay. In the case of the MGs spent ammunition casings/belt links are ejected downwards and out. These are represented on the new Airfix kit by two small gaps near the wing roots  - do not fill them. 

Note the blast tubes/forward vision slots underneath the cockpit floor/pilot's seat. Both ammunition boxes sit approx behind the seat, one in front of the other - see diagrams below. The two trays connecting to the weapon and feeding out into the open air are marked by the red arrows.  In the next images there are exterior views of the chute openings. Note also the 'cut-out' in the bomb bay doors - the ejector openings for the weapons packs fitted into the bomb bay

 overview of the MK 103 installed in the bomb bay (below, empty) 

Matt at 'Model Minutes' covers this neatly in his recent YT build. Thanks Matt!