I dont know how I noticed this but on my m1a when the bolt is held open I can hear something sliding slightly back and forth when I tilt the gun foward then backwards. Does anyone know what is moving back and forth. I dont notice it when the bolt is closed however.
Location: Third bunker on the right,Central Virginia
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Cattledog nailed it. Gas piston. When the bolt is closed, the piston is pushed to the forward limit of movement- no movement, no noise. But when bolt is open, piston can slide back and forth- unless your rifle REALLY needs cleaning.