Why have you got your convergence set so far apart?
300, 275, 250.....from the low to the high, you have a difference of 300 ft in convergence points of your rounds!
I would say that only 1/3rd of your rounds have a chance of hitting, and at that probably only a small percentage of those are actually hitting the target.
Try setting all guns to the same convergence. I use 250 for all guns. I generally get in pretty close before pulling the trigger. Experiment with it to find your best convergence point.
Now, granted, my gunnery sucks. But when I do hit with the .50's I hit hard. Once you are lighting up the target, they are light frikk'n lazer beams.
cheers,
RTR