The barrel wasn't any longer, it was an ordinary MG-FF. Just like the barrel of the MK108 on the Bf 109 didn't extend all the way from the cannon to the end of the spinner, it was much shorter. Blast tube of 5cm radius was installed from the end of the cannon barrel through the engine and through the spinner. The case was the same on the Bf 110.
EDIT: Franky beat me to it
