This brings us to the MK 103, which many sources state was used as an engine cannon in some of the later series planes. The truth is that the MK 103 was a massive gun which would not fit in the small space available between the back of the engine and the cockpit. To give some comparative figures; the MK 108 weighed 60kg and was 105cm long, whereas the MK 103 weighed 141 kg and was 235 cm long. About 133cm of the length of the MK 103 was too wide to fit within the narrow tube running between the cylinder banks, so had to be mounted behind the engine, and this was simply impossible.
Despite this problem, the idea of arming the Bf 109 with such a powerful weapon was attractive, so Rheinmetall-Borsig started work on a modified version which would fit, designated the MK 103M. The gas-operating mechanism was slimmed down which enabled the gun to be mounted some 30cm further forward, leaving just enough space to squeeze it into the plane. The MK 103M was reportedly test-flown in a K series, but reliability was poor and the gun was not adopted for service. By this time, however, plans had been drawn up for its installation in the K-8 and K-10 models and these plans later came to light. It was probably this which led researchers to believe that some versions of the Bf 109 were fitted with the MK 103.
Just to back up my statement.
from the page
http://www.bf109.com It is a very common myth, as is the 15mm cowl gun myth, I am not sure where they both come from. I've heard they come from a book published in the 70's. It has been long lived and only during the past few years it's been corrected.
In my opinion it can be easily explained and understood by simply saying this: You can not fit a 2.3 meter, 140kg cannon in a space designed for a 1 meter, 60kg cannon.
The Bf 109 was always designed for "the most powerfull engine fitted in the smallest and lightest airframe possible".
The Mk 108 was IMO the far superior weapon for fighter vs fighter combat and for fighter vs bomber attacks. Of course opionons are like a**holes, everyone has one.
The Mk 108 was lighter, higher rate of fire, powerfull enough to bring down a fighter with a single hit or heavy bombers with 3-5 hits. It was much cheaper and easier to produce.
As I said in my previous post, even the german aces often fired at distances of 100 meters or well bellow. If you look at WW2 fotage of german fighters attacking bombers from a long distance (400 meters or more) they usualy score no hits at all even with 20mm and 13mm guns that are far easier to hit with then a 30mm. Doesn't matter what the dispersion is if the pilots can't hit at that distance anyway.
So what good is it to have a gun that you can kill with from 1000 meters if the pilots can't hit?
The Mk 103 was a terrific tank killer, high velocity, heavy AP shells with a fairly good rate of fire.
