I would like to point out that both sides in WW1 used lighter than air craft. On the French/German front observation balloons were in regular use - Aircraft were used to shoot them down using incendiary rounds.
From Wikipedia about Balloon Bustingin WW1:
"Although balloons were occasionally shot down by small-arms fire, generally it was difficult to shoot down a balloon with solid bullets, particularly at the distances and altitude involved. Ordinary bullets would pass relatively harmlessly through the hydrogen gas bag, merely holing the fabric. Hits on the wicker car could however kill the observer.
Some things that need to be said about vulnerability regarding recon balloons in WWI.
First of all, they were tethered well behind friendly lines.
Secondly, a standard defense if enemy fighters were giving it too much attention was to simply haul the balloon down ASAP, which was actually a quick thing to do.
Thirdly, balloon tether sites were DEFENDED, often by multiple machineguns(both against air and ground), infantry and sometimes even by artillery and heavy antiair guns.
They were commonly hard to shoot down with light weapons because they were almost always far enough from enemy troops that rifles were shooting in noticeable arcs, so hitting was rather random.
If enemy infantry could get within a thousand yards unseen, a balloon was usually "dead", and if the observers in it were lucky, the ground crew would have time to pull it down before leakages had it crashing down instead of being pulled down.
Often causing a difficult dilemma for the observers, hope that the ground crew gets it down before it crashes, or jump out with parachutes and hope they´re not already too close to the ground for the chutes to deploy.
Neither balloon observer crews, nor especially pilots attacking balloons had much of a life expectancy. Balloons were heavily defended targets, because they were important targets.
For shooting down the zeppelin I would just say that other than angle guns all of the artillery have a very small amount of elevation and would not be that useful for hitting the zeppelin. And if the zeppelin was high enough it would not be reachable and even if it was smoothbores would not pose that much of a threat mostly based off of the fact that zeppelin bombers worked against Britain in WWI. Not well I will admit but they were able to resist a lot of damage.
https://en.wikipedia.org/wiki/Zeppelin# ... orld_War_IEarly offensive operations by Army airships revealed that they were extremely vulnerable to ground fire unless flown at high altitude, and several were lost.Also, it should be very much noted that airships are not entirely easy things to fly high with. When WWI started, the operational ceiling for existing military Zeppelins in Germany was ~6000 feet.
Remember, airships "float" in the air, the higher you go, the thinner the air is and the less effective that floatation is(or you end up with the gas cell expanding to the point where it bursts).
In late 1916 the limit was raised to 13000 feet.
The later so called "climbers" could routinely manage 20000 feet. However, their usefulness was greatly affected by problems caused by the altitude, oxygen deprivated crew, severe cold, frozen radiators, congealed oil, cracked windows, pipes snapping or leaking from cracked seals...
Pretty much no end to the issues. And probably more dangerous to fly than for anyone you try to attack with them.
And in regards to "smoothbore", well you may wish to remember that most of todays tank guns are smoothbore, and there´s no real reason against anyone building that kind of guns with simpler tech.
Getting a good highspeed, long ranged subcaliber shot to work wont be the easiest of things to do, but it´s certainly not impossible.
Also, making cannons so they can be used as antiair weapons? Not hard at all.
