The weakness of the breech-loader was the obvious problem of sealing the breech. All guns are powered by the explosive conversion of a solid propellant into gas. This explosion propels the shot or shell out of the front of the gun, but also imposes great stresses on the gun-barrel. If the breech—which experiences some of the greatest forces in the gun—is not entirely secure, then there is a risk that either gas will discharge through the breech or that the breech will break. This in turn reduces the muzzle velocity of the weapon and can also endanger the gun crew. "Warrior"s Armstrong guns suffered from both problems; the shells were unable to penetrate the 4.5 in (118 mm) armor of "Gloire", while sometimes the screw which closed the breech flew backwards out of the gun on firing. Similar problems were experienced with the breech-loading guns which became standard in the French and German navies.