Unlike an ammunition belt, a feed strip is a rigid strip of metal upon which cartridges may be placed.
A strip magazine. Click on the image to enlarge.
Image licensed under the Creative Commons Attribution-Share Alike France 2.0 license by rubin16 at wikipedia
The gun has an oscillating arm that feeds a new cartridge into the chamber each time it moves.
Feed strips were mostly used by machine guns made by the Hotchkiss company. It is interesting to note that while the manufacturer, Hotchkiss et Cie, is a French company, it was actually founded by an American, Benjamin Hotchkiss. Mr. Hotchkiss got his start working as a gunsmith in Connecticut, but after the American Civil War, the US government lost interest in funding new types of weapons. Due to this, Mr. Hotchkiss moved to Europe and founded a factory in France. His company produced a revolving cannon and several machine gun models. Many of his machine gun models were built to use strip magazines and were used by the French as well as Allied forces in World War I.
Indian gun crew from the 2nd Rajput light infantry regiment at the battle of Flanders in Belgium. Public domain image.
In the above image, we see a Hotchkiss M1909 machine gun (the Mark-I model) being used by an Indian gun crew in Flanders, Belgium. Note the feed strip sticking out of the gun on one side. As soon as the strip was used up, the gun would lock back and the feed strip would automatically fall out. The loader would then insert the next feed strip on the side and the gun would be back in action again.
The M1909 was followed by the Hotchkiss M1914, which was successfully used by the French military from about 1914 to 1940 or so and also saw service with Americans, Japanese, Russian, Chinese, Greeks etc. While the gun was very effective when used by a three or four-man crew, the feed strip system was not so effective when used by a single gunner from the inside of a tank.
US machine gun team from 1st Division operating a Hotchkiss machine gun in Froissy, Oise, France.
Click on the image to enlarge. Public domain image
By the time Hotchkiss came out with the M1922 model, feed strips were falling out of popularity for various reasons. First, feed strips cannot hold as many cartridges as ammunition belts. Different models of feed strips could only hold 15, 24 or 30 cartridges at most. An additional problem with feed strips is that they do not protect the ammunition from the weather or from dirt and they are prone to damage. This is why box magazines were preferred because even though box magazines are heavier than feed strips, they offer protection from dirt and are more durable as well. Therefore, several of the Hotchkiss machine gun models were built with different receivers to use ammunition belts or box magazines, instead of feed strips.