The simplistic example would be shooting from dead six with no closure at 300mph exactly 1500 yards away. Suppose the bullet can travel 1500 yards then vanishes.
If bullets travel a
fixed absoloute distance then this gives a big advatage to the bomber. If they travel a
fixed time, the bomber still has a little advantage, but not that significant.
Relative to each other (fighter - bomber) the bullets come out at the same speed and need to cover 1500 yards. The problem looks symmetric. However! because the bullet max flight distance is relative to the ground, we have to look at the speeds relative to the ground - the plane's bullet adds the plane 300 mph to its ground speed, while the bomber bullet subtracts 300 mph from its ground speed and is slower relative to the ground. The result is that the fighter bullets covers its 1500 ground yards quicker and is removed from the game, while the bomber bullets still travels - it covered less ground distance, but the same fraction of the relative fighter-bomber 1500 yards as the fighter round did, except that the bomber round is still in the game and continues to travel and hit the fighter.
If on the other hand the bullets are removed from the game after a fixed time (instead of fixed absolute distance) then the fighter and bomber bullets will be removed together and if the bomber bullets can reach the fighter, so can the opposite - almost.
The "almost" comes from the wind resistance. The air is stationary and relative to the air, the forward traveling fighter round is faster (adds 300 mph) and suffers more drag, than the slower backward traveling bomber round. In the fighter--bomber frame of reference, the fighter is shooting upwind and the bomber is shooting down wind, which breaks the symmetry. So, the bomber still gains a little extra reach and hits a bit harder than the fighter. The wind effect exists anyway and adds even more to the advantage of the bomber in the case of "absolute flight distance".
I believe that in the game bullets travel a given ground distance and not a given flight time. This is a bit unfair towards the fighter, but I am sure that there are technical reasons why it is implemented in this way.