Originally posted by zorstorer 
I have read posts about this for awhile now....what exactly is/was the big deal?  4 guns on 1 trigger, 2 on the other?  Did it go inside matched with outside and the middle gun being the odd one? 
 Yep that was it. 
 Seriously, the load out had 4 guns with the same number of 50 cal rounds and another 2 guns with more rounds (Its late and I am not going to go dig out the precise numbers).
The P-51D could have it's guns set to fire in any groupings by some switches.  So, any setup is correct, but most of the pilots set them to fire with 4 in sequence and 2 by themselves for "plinking".  This is mentioned coincidently in the December issue of Flypast (just got it airmail from England today).
Cheers.