Dirt911-
I think these guys have hit it on the head. You're experiencing some basic, common problems. It isn't the lethality of the rounds. Those are set at more than adequate strength. Look at this clip to see what effect the guns have if you do three things right. Set your convergence correctly (or at least effectively), shoot at convergence range, and actually hit your target in a vulnerable area long enough. I'm doing those three things in this clip. The F4U has enough ammo for about 10 bomber kills, and has the same guns as the P47 (except only 6 guns...). My convergence is set at 275.
http://www.mediafire.com/file/czdvwnnnzyi/Buffs.ahfI'm sure you think you're hitting the target enough to get quick kills, but I highly doubt you are. The hit sprites can be misleading. It doesn't take a lot of hits to light up a target pretty brightly. The sprites are "huge" when compared to the actual hit, especially when it's something like a .50 round from a P47. You need to hit one area long enough to do adequate damage. The .50's just punch holes (they don't explode like cannon rounds do) so it takes a concentrated stream (of long enough duration) to do the job.
The good news is that your "problem" can be fixed. Proper set-up, practice, and experience will have you knocking those buffs down fairly easily...
Look up a trainer if you'd like some 1 on 1 help.