Just make fire button == rapid fire all the time, no delay.
It works pretty well - no need to worry about making things too easy, because the slowness of the turret movement is still the main problem.
Then tune the number of points lost per bullet until even at the start of the game, with little on screen, it's not too easy.
IIRC, the CGA version used +5 points for killing someone, +50 points for hitting their parachute. So..skill shots on parachutes were worth doing because they refilled your ammo fast (as well as earning points
Of course...you HAVE to play the all time classic clone: Ganja Farmer. "The FBI have sent special agents to Mexico to destroy your weed plantation. Only you, your rasta dreadlocks, your trusty multi-colour camper van...and a machine gun stand in their way".
Your machine gun is mounted on the roof of the camper van, you're standing in the middle of a field of cannabis, and GI's are parachuting in, with the odd "airstrike" which drops napalm on your crops. All crops dead == game over.
After each level, the Rastafarian God comes down from heaven on a pink cloud and rejuvenates a few of your crops.
If you do really well, he also brings "gifts" like a better gun, a rocket launcher, and even "the mighty bong".
Ah, the good old days...gan