Gidday, in past threads this has been discussed. I find that the enemy AI skill level also affects the range at which the enemy guns start to fire. I've had enemy aces engage with me at greater distances than the "range" value in the gun xdp file.
Another thing I've tried is to edit the "timealive" value, as sometimes there is a big mis-match between the "range" value and the timealive" value. For example for a bullet travelling at 745 metres per second, a time alive value of 1.0 gives a theoretical maximum effective range of 745 metres. I don't know if the game models a slowing of the round/projectile over time? If the physics allow the projectile to slow, then a timealive value of 1.0 would give an effective distance of less than 745.