Another version is the alarm clock gun, where a magnifying glass sets off the priming chasrge when the sun angle comes to the magnifier just right. Screwed into a post they will set off the gun at the same sun angle every day.
Willie
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I think you are referring to what's known as a sundial or noonday cannon.
The picture above is one I assembled from a Dixie kit. I have since filed off the bolt shafts flush. Just like any sundial, one aligns the gnomon (triangular vertical blade) to make its shadow fall on the correct time. Then one adjusts the magnifying glass to focus on the vent a the desired time--usually noon Since these were never intended to fire anything but a blank charge the bore is rough cast.
Here is another one I have that would function if I vented it, but the base is a cast polymer material meant to look like scrimshawed ivory, and I was always afraid it would break. It was built by a sundial company as an indoor decorative piece.
I have a 3rd brass and wood decorative model, but don't have a current picture of it.
Most of the black powder cannons that are listed on EBay are perfectly capable of firing a projectile. However, people have learned to simply include a statement that their item is a signal gun only and is not intended to fire a projectile. Basically, as Mike1234567 has already indicated, they are simply lying just to keep their listing from being jerked by EBay
These little CVA cannons built from kits marketed back in the 1970's get posted for sale all the time, and they actually have a .45 caliber rifled barrels.
Of course the sellers always fib about these too just to avoid the EBay police.
Cheers
