Erich even the prototypes or the single K2 were fitted with MG131s, there were simply no space to fit such a large weapon as the MG151, 15mm or 20mm since there were nearly the same length (just the barrel and a few internal parts were differents).
The MG151 was fitted as a cowl gun on larger aircraft, the Do 335 for instance, as there were enough space. Moreover it was more desirable than MG131 in that case since the ballistics of the 15mm MG151 were quite close to those of the MK103 used on thsi aircraft.
The engine mounted variant of the MK103, the MK103M was still in development when war ended as there were quite a bunch of modifications to introduce over the original design.
As an engine mounted weapon the K-4 relied on the MK108, which did not have excellent ballistics but a high RoF, which is desirable on a fighter. The K-4 could as well be equipped with under wing gondolas but this was rarely if ever carried as they had a detrimental effects on speed and handling and a very different ballistic compared to MK108.
The MG151/20 gondolas had their use when the 109 were still relying on a 20mm engine mounted cannon but with the introduction of the /U4 aircraft equipped with MK108 and the K-4 it was not more needed. Indeed the firepower was sufficient to down a bomber much more easily than with gondolas and without suffering from maneuvrability/performances losses.
The MK108 gondolas were tested on a few aircraft but were not usued operationaly as they had a very detrimental effect on perf and had only 35 30mm shots each. But the next in line in the Kurfurst family was planned as a three MK108 equipped aircraft with one being mounted behind the engine like the K-4, and two in the wings in seme recessed gondolas which were much more aerodynamic.
Olivier
still a bit tired from yesterday...
