world war one - What was this 1917 shell casing used for

Bought this shell from an antique store. Would like to know the history behind it, as in what it was used for? (The casing is about 16 and 1/2 inches in length)

WW2 German, Soviet, Allied militaria, uniforms, awards, weapons history. War relics forum

Were there really piles of spent artillery shells littered across the battlefields of WW1? If so, what'd they do with them all after the Armistice? - Quora

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Were the howitzer, about 100mm brass shell cases from WW1, reused or just wasted after having been first fired? - Quora

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Original France WWI French “Souvenir de la Guerre - 1917” Austrian Ber – International Military Antiques

1917 Ww1 Trench Art Shell

world war one - What can I learn from these WW1 shell cases? - History Stack Exchange

Engraved shell case : Battle of Romani, 1916

German WW1 Rhinemetall Shell Case 1917

German Brass Shell Casing WWI Aug. 1917 Patronenfabrik Karlsruhe 622 SP255

world war one - What's this 1917 shell casing, more exactly? - History Stack Exchange

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military - What kind of rifle used this shell case? - History Stack Exchange

WW1 German 7.7 cm Feldkanone 96 neuer Art Shell Case in Ammunition