The Model 1877 was something of a watershed in the Gatling lineage. The improvements included a completely new feed hopper, and most significantly, new gears with a faster pitch which produced a maximum rate of fire of about 800 rpm while the effective rate of fire was about 550 rpm. This was the last Gatling model the Army bought that featured the oscillator mechanism. The Army bought eleven Model 1877 Gatlings (serial numbers 191-193, 196-201, 225, and 226. The gun below is on display in Oklahoma City, OK.
The Model 1875 featured further improvements over the Model 1874. These improvements included a fixed front sight, a revised ejection port on the left frame rail, and a feed hopper with a more open left side. The Army bought forty-four musket length (serial numbers 107-146 and 163-166) and four Camel (serial numbers 159-162) Model 1875 Gatlings. 1875 was the last year the Camel gun was manufactured.
More photos from Guatemala. Model 1871 Camel gun fitted with a Broadwell drum feed hopper. I'm unsure if the disassembled gun is the same as the one on the carriage. The serial number written on the photo does not reconcile with published Gatling serial number data. Note how the front and rear barrel plates are scalloped between the barrels. This was eliminated sometime between the manufacture of these guns and the introduction of the Model 1874. Caliber is probably .43 Spanish.
The Model 1876 was the first Gatling to have the socket for the magazine moved to the gun's centerline instead of offset to the left as in the Model 1874 and 1875. This was to improve the feeding and reliability of the gun. There were some internal improvements as well. The Army bought nineteen Model 1876 Gatlings (serial numbers 170-188).
The Model 1879 (there is no Model 1878 as the U. S. military didn't buy any Gatlings that year) introduced the aiming lever method of elevation and traverse. The gunner could aim the gun with his left hand while firing the gun with his right. Variations of this were used on all Gatling models from 1879 to 1903. The Model 1879 Gatling was essentially the same as the Model 1877 except for the aiming lever.
Below: Model 1875 displayed at the California Military Museum
Scanned from 35mm prints these pictures are from an American collector who traveled to Guatemala in the 1980s to view and possibly purchase and import the Gatlings still present in the armories there. He was unsuccessful in the purchase of these guns and presumably they are still in Guatemala today. These photos also included an 1879 37mm Hotchkiss revolving cannon.
Contrary to what is written on the one print this is not an "1864" Gatling. Caliber is .58 rimfire.
The Model 1874 exhibited many improvements over the Model 1871 Gatling mainly in the internal parts and in the improved oscillator mechanism. This model was the only Gatling model to have a folding front sight. The US Army purchased fifty-six Model 1874 Camel guns (serial numbers 1 to 56 inclusive) and eight "Musket Length" Model 1874 Gatlings (with 32 inch barrels) (serial numbers 57 to 64 inclusive, and number 105).
Below are Model 1874 Camel guns and the last photo is a Model 1874 musket length gun with a feed hopper for the Broadwell Drum and the adapter to allow use of the Bruce Adapter feed, a rare combination.
By 1878, all but one of the musket length Model 1874 Gatling guns had been withdrawn from active Army service and issued to various State militias.
The Model 1880 Navy Gatling completely changed the appearance of the gun. While all previous models of Gatlings had a steel frame and an exposed cluster of barrels, the Model 1880 Navy has its barrels enclosed in a bronze housing. The only portion of the barrels that is visible are the muzzles. This was carried forward with the Model 1883-1889 and Model 1901 Navy Gatlings. The 1880 Navy used the 40 round gravity feed magazine. The gun below is on display in Fairbury, Nebraska. The serial number is unknown to me.
Below: Model 1875 displayed at the Cody Firearms Museum, Cody, Wyoming
Model 1866 Colt Gatling at the Rotunda Museum in Woolwich, England. Caliber is 1 inch. Note how the frame cross-member is arched up to solve the problem of bullet strikes. .50-70 and 1 inch caliber introduced in 1866.
The very first production Gatling guns were manufactured by Miles Greenwood's Eagle Iron Works in Cincinnati, Ohio. Eagle Iron Works produced six guns but the foundry and the guns were destroyed by fire in 1862. Richard Gatling was able to bounce back from this disaster and contracted with the Cincinnati Type Foundry to build thirteen guns. These guns were all sold in 1863, twelve were purchased by Union General Benjamin Butler and one by Union Admiral David Porter.
After the war ended in 1865 Gatling moved manufacturing to the Cooper Firearms Manufacturing Company in Philadelphia. Cooper built the Model 1865 guns, before Gatling again moved manufacture of the Gatling gun to Colt's Patented Firearms Manufacturing Company in Hartford, Connecticut. Colt's produced all Gatlings in the US from 1866 to the end of production in 1903. The other countries that produced "Gatling guns" were Austria, China, Russia, and the United Kingdom.
More photos from Guatemala. Again these are not Colt built guns but Cooper guns. Note that in one of the photos there is a bullet strike on the frame crossmember. This defect was corrected in the 1866 guns by arching the frame crossmember up instead of down. Also note that both guns are missing their feed hoppers.
1865 Gatling gun
A Model 1871 Camel gun at the Connecticut State Library. This gun also has the feed hopper for the Broadwell Drum.