Abstract: In a breech-loading gun loaded by first ramming a projectile into the gun rrel to a firing position therein and then inserting a separate propelling charge, unlimited fall-back of the projectile from the firing position is prevented by means of an annular internal groove in the barrel providing a forwardly-facing shoulder, a rearwardly-facing shoulder carried by the projectile, and resilient means for automatically moving the projectile shoulder outwardly into abutting relation with the barrel shoulder on rearward movement of the projectile from its firing position. In one embodiment, the projectile shoulder is the rear end of a resilient strip extending rearwardly and outwardly from the projectile. In another embodiment, the projectile shoulder is the rear side of a radially-movable spring-biased detent on the projectile.