The Messerschmitt BF-109 was designed around a centrally-mounted 20mm "motorkanone" that fired through the propeller hub and a pair of cowl-mounted, synchronized machine guns. Some later versions went to two wing-mounted canons canons and the tow machine guns. It was regular practice for the pilots to fire the MGs first, "walk" the tracers onto the target, then fire the canon(s).
Bob