So, it is quite difficult to work out what the drag coefficient of your projectile might be, let alone worry about how it changes as the velocity of your projectile changes. From wikipedia's simple coefficient of form approximation, I got a (relatively high, for a bullet) drag coefficient of 0.5, but that's not a bad number for a supersonic projectile. I found a paper looking at base-bleed artillery shells (https://iopscience.iop.org/article/10.1088/1742-6596/2054/1/012013/pdf) which measured a drag coefficient of ~.274 for base bleed off and ~.238 with it on... but that's a rather more sophisticated projectile than you're considering.
I'll go with a drag coefficient of .3, which doesn't seem that great but keeping it at both sub- and supersonic speeds is pretty good. You've quoted a 6.5" calibre, and that, unfortunately, is going to make life difficult for you. As Zeiss Ikon pointed out elsewhere, a good reason for having a bullet-shaped bullet is that you can squeeze more mass into the same cross-sectional area without unduly affecting drag or aerodynamic stability. Your projectile has the mass of a 6.5" diameter iron sphere, not a 6.5" diameter iron bullet. This results in a lower ballistic coefficient, which in turn leads to worse performance.
So, here are some trajectories with various elevation angles, showing maximum possible range. The 0-vertical-height line shows the altitude of the firing airship.
As you can see, you can't realistically reach even as far as 300m... that's clearly a maximum range, but it isn't the same as an effective range.
As you didn't define effective range, I can't really give you a definitive answer. In the real world it depends on your ability to hit your target, and once you've hit it, actually do damage. I'm not going to tackle accuracy here, but the striking power of your projectile is going to be strongly influenced by its speed.
Here's a plot of projectile speed vs the horizontal distance travelled:
I've generated similar charts in one of my answers to your related previous question, and as you can see it doesn't look great. It would be up to you to decide how fast a projectile has to be in order to be going fast enough. Your ~16kg round travelling at ~100m/s has ~80kJ of kinetic energy, so it will still pack quite a punch against softer targets.
If 100m/s was taken to be the standard of effectiveness, then an initial elevation of 0.5° will give you an effective range of 135m, with a projectile drop of only 15cm (so the trajectory is basically flat).
The conclusion I came to in the other question was that you shouldn't be firing balls or even bullet/shell shaped things, but much more aerodynamic things if you want long range or high impact energies. Rocketry might also be a good idea, but that is definitely a subject for a separate question.