I feel like the one thing that probably does make it much easier to hit than an airplane is it's larger size and slower speed (though I'll admit I have no idea what altitudes dirigibles are able to fly at)
Dirigibles can fly at many thousands of feet. At even a thousand feet it's a lot more difficult to hit something in the air with small arms than across an equivalent distance over land (due to bullet drop) unless you have specialized anti-aircraft guns.
At 1000 feet, the drop of a rifle bullet is pretty trivial (a few yards) compared with the size of an airship. Considering how slow they move, I don't think a moderately competent rifleman would have much trouble hitting them.
The higher the angle you shoot, the more energy loss due to gravity. Shooting at 60 degrees is considerably different than 0 degrees, for the same range. That's why AA guns need to be so big, just to get that bullet up there with enough energy left to do some damage.
