The MK 108 Rheinmetall-Borsig cannon had the reputation of being an inaccurate weapon in combat, but this was the result of its low velocity, not of barrel length or any inherent inaccuracy of the design.
It is a common misconception that a long barrel makes a cartridge more accurate. In fact, all a long barrel does is add velocity to the shell, decreasing bullet drop at long range, thereby making it easier to hit the target. A long barrel does nothing to help a caliber group its shots better.
In fact, short barrels tend to group slightly better, especially at distances of 300 yards or less, because the barrel tends to be stiffer. Barrels have individual harmonics, much like those of a musical instrument or tuning fork, which come into play when a bullet is rotating down the barrel. The shorter the barrel, the less impact those harmonics have on the ability of the barrel to shoot accurate groups.
The velocity of the MK 108 is variously given as being between 1640 fps and 1770 fps. This hovers around 60% of the velocity of the M2 Browning .50 caliber machine gun. To put this velocity into perspective, the MK 108 round is approximately 600 fps slower than the Winchester 30-30 carbine round and is 200 fps slower than the round of the .30 caliber M-1 carbine. In fact, the velocity isn't much better than that of the .22 long rifle.
A shell launched at that speed and sighted to be dead on at 200 yards range would be about 6 inches high at 100 yards and nearly 30 inches low at 300 yards.
Now imagine installing this unit in the flexible wing of an aluminum skinned fighter, and trying to hit a target moving at speeds of approximately 400 feet a second, or executing a tight, high-g turn. The attacker armed with this weapon would have to be extremely close to an enemy fighter in order to score a hit.
Thus, one must conclude from these facts that the MK 108 was intended to be a point blank range weapon for use against the large, lumbering Allied bombers.
Regards, Shuckins
