Based on Jeff's idea, starting a separate thread for Arms Locker oddities and enigmas or interesting unique items or things that ought to be in any locker....
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A kit for adding an under barrel vertical fore-grip to a long arm (easier on the arms and shoulders for long holds on weapons w multiple attachments, scopes, and sighting systems) [from friend in US SF]
A box of sim-munitions that load and cycle slug throwers. Includes primer and powder. Impact slightly harder than a paintball. Goid for realistic training... wear eye protection. Ammo comes in common calibres.
Bolt on bayonet lug for long arm (despite the manufacturer not providing that option)
Launched net grenades. Explode 3-5 meters from target, deploys filament web net to capture. Fits standard GLs. Nastier variants are auto tightening or electrified.
A custom made, silver chased flintlock pistol w wood chassis. Likely used for noble duels. Needs cleaning.
A woiden box labeled "Munitions, 60mm Low Velocity, Incendiary". Label is incorrect, contents distribure leaflets for psyops. 12 Rounds.