Concept: If Sentry is placed on inventory block (chest or barrel, for example), it can shoot dispenser-compatible projectile items such as arrows, fire charges, fireworks, etc. that are contained in said inventory. If inventory is empty or contains only items that can't be shot by dispenser, it reverts back to its default bullets.

This allows more formidable sentry-based defense and balanced, because such projectiles are expandable and require player's production and supply.