Any military rifle, I'd vote for the French
Chauchat (pron; Sho-sho) from The Great War (WWI). It was notorious for jamming, failure to feed, failure to eject, etc. (It was French after all. I wonder if it came with a white flag?

) It was a shoulder fired full automatich rifle, and when Americans rejected it's use, the Browning M1917 Machine gun was used until the B.A.R. was able to catch up with troops.
http://www.firstworldwar.com/atoz/mgun_chauchat.htm
As for U.S. rifles, the Springfield .30-40 Krag-Jorgenson has the distinction of having the shortest life span as an issue rifle to regular U.S. troops. It is known as the "One War" rifle, the Spanish American War. Although not a bad rifle, with a very smooth action, it was somewhat underpowered compared to the enemy's 7mm Mauser. I own a Krag (1889) and it is a joy to shoot, but not suitable for military use.
http://www.spanamwar.com/krag.htm
The Krag-Jorgenson is featured in the made for TV movie,
"The Rough Riders," a fictionalized account of Col. Theodore Roosevelt's 1st U.S. Volunteer Calvery during the Spanish American War, in Cuba.
One also shows up as a goof in the old John Wayne classic, "
The Fighting Sea Bees." In that film, Big John picks up a discarded enemy (Japanese) rifle to continue the fight, but on close up, it is clearly a Krag.

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