This is one of the bigger problems with multiplayer FS2 right now and eventually needs to be addressed in some way. If you aren't familiar with the issue, for players other than the host, it can be quite difficult to hit fast moving targets like fighters with primary weapons. This is basically caused by a combination of lag and the low object updates, as the fighter's true location is somewhere in front of what the client sees. It can be partially mitigated by firing in front of the target, so that the shots appear to pass in front of it on the client side, but this becomes hard to do when the ping is more than about 100 or is simply varying a lot.
I can think of a couple of possible solutions:
1: Modify the target lead indicator on the HUD so that it not only takes the target's speed into account, but also the current ping time, and interpolates the fighter's true location from that. This wouldn't help with hitting fighters that are turning a lot, but it would be a major improvement over what we have now. This is probably the easiest solution to implement.
2: Do all primary weapon calculations on the server side, not only the hits but also the movements. Almost all commercial FPS games do it this way. There would now be a slight delay between when you fire and when your weapons actually appear, but they would be more accurate after they show up. Note that secondary weapons in FS2 are already done like this and don't suffer from the whole issue.
3: Do all primary weapon calculations on the client side. This is not done in most games since it's more vulnerable to cheats and gives an edge to people with bad connections over those with good ones, but it's one option.