An apparatus for retaining an object in a gun tube of a weapon system, the gun tube having a longitudinal axis, a chamber and a breech ring with an opening therein, the apparatus including a plunger that reciprocates in the opening in the breech ring and the chamber; a housing fixed in the opening in the breech ring, the housing holding the plunger; and means for reciprocating the plunger in the opening in the breech ring and the chamber. The object to be retained is propellant. The reciprocating means is automatically operated via electrical and/or mechanical connections to the weapon system.