cannon, (kokuji)
熕 is a Japanese-made kanji (kokuji) that centers on a cannon, a large firearm that uses fire to launch projectiles.
熕 is a kokuji created in Japan by combining 火 (fire) with 貢 (tribute, but likely used here for its sound コウ to approximate the cannon's boom). The exact historical development is uncertain.
Fire 火 on the left and 貢 on the right: imagine a cannon firing a tribute of flames and iron balls.
For おおづつ, picture a large (大, おお) tube (筒, つつ) that booms like a cannon: 大筒 -> おおづつ.
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artillery