Flickr contributor meccanohig has a set of photos taken by an anonymous pipeline engineer working in Egypt in 2002.  It features a lot of old Armstrong guns from the late 1860's through the 1870's.  They are quietly rusting away in the desert, would be nice to take care of them as there aren't that many of them left.  The British occupied Egypt in 1882 on the pretext of supporting the Khedive against the Nationalists, and weren't persuaded to leave until 1956. Meccanohig is a keen meccano modeller and collector and his photos of meccano sets and models are quite stunning.

Abandoned cannon in Egypt

Richard Gatling didn't give up after he perfected the Gatling gun.. he kept at it. In 1893 he obtained a patent on an electrically driven 8 barrel Gatling gun, firing a theoretical 3000 rounds per minute, predating the rifle caliber mini-gun by 70 years. The Germans made a 12 barrel gun driven off an airplane engine in WW1, but it was pretty rough on spent brass and wasn't pursued.
One thing working against Gatling was a workable power source. In fortress type installations a generator could have been set up and many were for early DC electric distribution, however, if the enemy got past the big guns, a Gatling gun would just put off the inevitable demise of the fort.
In order to make the gun mobile, you needed some kind of battery. Electric batteries, while invented in the late 1700's and into the 1800's, weren't available commercially until 1896. Until the Edison battery came along in 1901 the gun didn't have a chance, but all these batteries suffered from lack of capacity, charge retention and they cost a fortune. Richard Gatling's invention was just a bit before it's time.

Patent for Electric Gatling Gun