powder flask; powder horn
Dated term for a container used to carry gunpowder, typically for muzzle-loading firearms. Now mostly encountered in historical contexts.
博物館で古い火薬入れを見た。
I saw an old powder flask at the museum.
Compound of 火薬 (かやく, 'gunpowder') and 入れ (いれ, 'container'). The derivation is straightforward.