As-is, I consider Pinlte-mount guns a complete waste of both in-game money and rulebook pagespace. With a +2 Target Number (TN) penalty to hit and needing to to succeed by four or more over the adjusted TN, the weapon has basically a TN+6 penalty to do any damage. All this for a single box of damage! This makes it difficult to damage even the largest planes (TN1+6= TN7), let alone your average dogfighter (TN6+6=TN12)!
My suggested change/house-rule is that the pintle-mount gun retains the +2 TN to-hit modifier and the range of 3 hexes. The "Succeed by 4+ to do damage" is dropped, and the pintle-mount gun does the same damage as a normal .30caliber machinegun. I haven't decided whether it can be only loaded with AP or you can use any ammo type you would prefer.
Essentially the weapon because a rear pintle-mounted .30 cal (a common feature in some late-30s and WWII era heavy fighters, and fighter-bombers) with reduced range and an accuracy penalty. It retains the "Cockpit Cost x2" cost, but now also requires that the cockpit be at least a two-seater, meaning it costs at least $500 to install (base cost with no other cockpit modifiers), which covers the cost of a 30 caliber MG ($180) and the "turret" mounting ($300). The only thing is it doesn't cost weight, although a 30caliber gun weighs 150lbs, the significantly reduced range and accuracy should counteract this.
This should make them at least a some-what valid threat while not threatening Rear-Turret's defensive superiority.