Some user addons of helicopters I've noticed, don't behave the same way as those that come with the game.
For example, the UH-60 you can shoot the gunner and pilot, and kill them whilst inside. However it seems with some addons. Such as DMX's UH-60 w/m134. I've tried to shoot the gunner and pilot from exposed parts of the chopper. It seems like it isn't possible. The same holds true for the release of the Capt Moore's Littlebird pack.
I was just wondering why this seems to be.