Bullet drop is modeled on gravity. Not sure how accurate it is but bullet trajectory isn't new math. I've always had to calculate bullet drop on long shots in reference to the ground regardless of my plane's pitch or bank situation. This means that on inverted shots you have to account fro the bullets dropping in the opposite direction to what you're used to.
Don't forget bullet scatter is modeled too. When RoF was relatively new, a lot of people complained about "sniping". I remember you could fly a Dr.1 (which at that time was faster also), and a plane like a SPAD could be killed almost every time because you could turn around so fast and begin shooting at him from long range, and hit him enough times to slow him down, eventually catch up, and kill him. That pissed people off!! You have 1,000 rounds to waste, if you want to.
Later, they increased the bullet scatter, so that now it's pretty much a waste of time to fire at somebody who's extended far enough away in the horizontal; and with planes like SPADs and S.E.5as, that's almost instantly.
There is one weird effect that seems to still occur. That is, if you're firing at somebody from long range, and you hit him with even one bullet, it seems to single-stage wound the pilot. This will spook or panic some people enough to get them to start jinking wildly, which slows them down and enables you to catch up and sometimes slow them down more.
One clue they give the attacker is that you can sometimes clearly see a ricochet tracer bounce off the victim, even at long distance! Then you KNOW you're hitting him! It's a cool effect that's fun to see, but probably not very realistic.
Like Barton said, the one thing that's probably extremely correct, is the gravity drop (trajectory). A .303 round mass and propellant charge is well known, and there's a lot of pretty sophisticated processing that goes on in RoF. Actually overall, I feel that it's very well modeled in all aspects.
If you really want to be impressed, look into how the synchronizers are modeled! The different types on each plane are accurately modeled, and they're very different plane to plane, how fast they fire at different RPMs! This is also a very impressive feature that they added later which had a big effect on how effective each plane was.