Bidspirit auction | Unknown? US Large Bore Rifle

Unknown? US Large Bore Rifle Grenade Barrel - Inert, safe and from the Jim Geibel collection. It is unknown to us. It is aluminum, has flat black anodized finish and has a rifled bore. The barrel is 11 5/8” long and overall it is 13.25” long. The caliber appears to be 16 ga. It has taken a 16 ga. shotgun shell and fits well. The rifling appears to do one complete twist for the 11 5/8” barrel. Jim Geibel had this in the grenade launcher wing of his collection but we have none of his notes.