just a few quoted ranges of LW guns taken from 'the luftaffe fighter force''view from the cockpit' by adolf galland
'the me.109 was at this time(1942-43) armed with only 1 mg151(20mm) and 2 x mg 17 (7.92 mm).the ballistics of these weapons did not permit effective fire at ranges greater than 500-600 yards'
there is then list of guns
MG 17 7.9mm 200 metres effective range
MG 81 7.9mm 200 "
MG 131 13mm 400 "
MG 151 15mm 600 "
MG 151/20 20mm 400 "
MGFF 20mm 400 "
MK 103 30mm 800 "
MK 108 30mm 400 "
BK 5 50mm 800 "
as you can see the mk 108 does appear to have a substantialy lower effective range but unfortunately it doesnt explain why so the shorter barrel etc mentioned by the other posts may wll be the cause.
hope this helps a bit...now GIVE US BK 5 HTC