Keep in mind, the 109K-4 had 30mm, and 13mm rounds, i don't doubt one of those bullets couldn't of set off a bomb 
.
The low velocity HE 30mm would pose no kinetic threat to the thick bomb casing. It doesn't have the energy or the structural integrity to do the job. There is a chance that the exploding shell could set off the bombs detonator, but since the bombs are still "safed" this is unlikely.
The bigger threat is the 13mm AP, which has essentially the same performance as the M2 AP I described earlier. Again, very unlikely.
I'm sure that's why a couple of them could tear the structure (not just the skin) of a bomber into pieces. 
We're talking about somehow setting off the bombs in the bomb bay. 30mm can and did devastate the aluminum structure of buffs, but unless you get what we call a "sympathetic detonation" from the bombs themselves, the bombs go pretty much unscathed. Think of heavy steel pipes being hauled around in a giant Coke can. You can easily blast the Coke can to Hell while barely scratching the pipes.
And the skin
is the structure.
