Yep, cannon rounds will drop more, it's very noticable on planes with .50's and 20mm. I notice it on the Spit9, for instance. The 50's have a totally different ballistic path than the 20mm. You'll be hitting with one, but not the other after about 400. The thing is, you want to make sure you hit with the 20mm as they are far more effective. Don't fire the .50's unless you are doing some ranging or trying to scare the other guy.
Always remember that the 20mm will be your best weapons for doing damage, the rest are supporting guns. In a way, the primary and secondary armament are backwards, and you should try and think of it that way.
I consider close as within 175, too close as within 75 and too far as 600. I almost never fire at anything outside of 600 unless it is flat and level and trying to run. I'll always try and hold fire until I'm within 300.. but that's just me.
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Soda
The Wrecking Crew