I found this the other day. I know that the gatling guns really never made any kind of tactical impact, but I thought this passage was interesting. Its from PVT Oliver Wilcox in the 83rd PA. The letter is dated June 10, 1862 from the Peninsula.
"We have a cannon drawn by one horse that one man can fire two hundred times a minute by merely turning a crank. Every revolution fires one ball. It is a curious Yankee contrivance. The cartridges are put in a hopper, carried one by one round in a cylinder and shoved into the barrel and fired. It makes a noise like the dogs of war let loose. The balls are only a size larger than our musket balls, a regular "Minie ball." Don't you think one of those coffee mills could "weed out" a secesh regiment about as quickly as any tools they have? I understand that every Pennsylvania regiment in the service is to have them. All the Pennsylvania regiments near here have them."
Since the Gatling Gun and the Agar Gun are both mentioned on this thread, it can be pointed out that there is a convergence of sorts between the two weapons on this date.
On this date in 1861, the Agar Gun was demonstrated in WDC to three Cabinet members, five generals, and one governor (It had been demonstrated to Mr. Lincoln on the previous day). Some of the witnesses were impressed with the weapon, but the same can't be said for those running the Army's Ordnance Department. They deemed the gun impractical.
On this date in 1876, George Armstrong Custer met his end at the Battle of the Little Bighorn. Custer had left behind some Gatling Guns because he feared that they would slow him down and allow the Indians he was pursuing to: "Get away."
It is unclear whether or not the presence of a few Gatling Guns would have changed the outcome of that battle. Different historians have different opinions on that question.
What is clear is that Colonel Custer died in a target rich environment. Just perhaps if he had brought those guns, he might have survived the battle. If that had happened, he might have had the opportunity to write more books such as his: MY LIFE ON THE PLAINS: OR PERSONAL EXPERIENCES WITH INDIANS (1874). We just might have seen titles such as: "WHY I BROUGHT MY GATLING GUNS TO THE LITTLE BIGHORN", or perhaps: "MY LONG POLITICAL CAREER."