This is not the most user friendly history site out there. You have to dig around even resizing the page to get at sub menus.
When you read the actual D. (Luft) T. 2410 A-1/U4 Teil 8A April 1944 manual, there are only 22 rounds carried onboard in a closed ring self feeding magazine. The navigator has no access to the weapons bay during flight.
http://www.luftarchiv.de/The English translation of the german info for the BK5.
This cannon was designed from the CHP 39, for mounting in the me 410. The barrel, the CAP, pipe reverse and feeding the original tank weapon have been modified slightly for this purpose. In addition, an electrical trigger and the furnaces was installed. With an automatic charger to the rear part of the gun around built 22-shot magazine and the single and also fire could be shot. Travelling with 3 magazines, changed on the fly by the gunner,
was intended for the me 410. The night Hunter pattern Ju 388 J-4 should take even a twin mounting the BK 5 in a tank under the fuselage. Also several were equipped with front workshops he 177 A-3/R1 with the BK 5, to their transport flights from Zaporozhe to Stalingrad and back to fight the enemy on the ground. Are used especially when using the so-called BK/M.Gr. notable successes achieved; so, for example, a group of 53 has me 410 A-1/U4, which were equipped per weapon with 36 shot fired with the BK 5 6 enemy flights during the period from 22 February to 11 April 1944 a total 129 units of B-17 "Fortress" bomber and 4 pieces of the B-24 "liberator" only 9 losses of own. At bottom shot attempts on one he 111 with the Spr.G "M" (0.350 kg) of explosive) the hull with a hit was decomposed into two parts. A hit was enough even for four-engine machines such as the killings of 9 "Fortress" bombers from half a mile away on 9 and 11 April 1944 proved. Data: Caliber 50 mm, Cadence 45 rounds/min, weight 540 kg and length 4342 mm