[Johnny .45]

Bf 109G's were armed with only three guns: the two 13mm MG 131 machine-guns in the cowling, and a single 20mm MG 151/20 or 30mm MK 108 firing through the propeller hub. The MG 151 was the weapon that replaced the two wing-mounted MG FF cannons of the Emil version.
The fact that it was a single gun rather than 2 wasn't as important as it seems, since a centerline-mounted gun is far more accurate and effective than a wing-mounted gun, and the MG 151/20 enjoyed a greater rate-of-fire and muzzle velocity than the MG FF. The superior weapon/mounting made a single MG 151 more effective than 2 MG FF's.
However, the Luftwaffe soon found that the Bf 109/MG 151 combination was sadly lacking in bomber-destroying power. In response, later Gustav's were fitted with a single 30mm MK 108 in place of the MG 151. This fired far more powerful shells that could easily bring a B-17 down, but it had very low muzzle velocity, which required the fighter pilot to get very close before firing. It also traded anti-fighter capability for anti-bomber capability...although a single 30mm hit on an enemy fighter would invariably destroy it, the slow projectile and extreme drop made hitting the enemy at all an uncertain thing.
Now, taking the German tendency to create "modular" armament for its' fighters into account, I have to wonder whether the two weapons were interchangeable. Could one remove the MG 151 from a Gustav and install a MK 108, or vice versa? Or were MK 108 planes built that way in the factory? It would seem to make sense to build to fighter to easily hold either weapon, like a "universal" wing on a Spitfire. I'm curious, since I can't see any obvious relation to the specific type of plane and it's weapons; I've seen G-6's that are armed with 20mm, and G-6's with 30mm guns. And G-14's carried MG 151's, even though the version came after they began using 30mm's in most planes.
Anyone know anything about this? If a specific type, like a Bf 109G-6 or G-8 can be armed with either gun, than it must be possible to interchange the guns. If it was built to hold one type of gun "permanently", than why wouldn't they denote it's armament in the planes designation? I mean, it would make more sense to title an MG 151-armed plane as a "G-9" and a MK 108-armed plane as a "G-10". Or at least put a suffix onto the designation, i.e. "Bf 109G-6(A)" or "Bf 109G-6(B)".
Oh well, just curious.