Steve... have you flown the P-51B? If so... how'd you do in it?
The guns of the B have a different feel, for me anyway. IMO, there is a noticeable difference in the gun packages, specifically in snap shots.
I have little trouble focusing my fire in a snap shot and knocking parts off when flying the D. In the B, these snap shots rarely knock a part off...at least for me.
Sure, in tracking shots one can knock a piece off w/ 4 guns quite nicely, with a bit of a longer burst than the 6 gun package.
Again, I just feel that the 4 .50's are poor for quick bursts and snap shots(which I seem to get more often than a nice lead or tracking shot).
This is merely my opinion. The B is a fun plane, and my congrats to those who have better luck w/ the guns of the B than I.