Some sources describe this as a delayed blowback action but it is actually a locked breech. In the late 19th century, firearms makers learned how to use this mechanical force to create "self-loading" weapons, whether they were in artillery, rifles, shotguns, or handguns. The lower-powered calibers such as .22 caliber rimfire, (example gun) Walther P22 .22 Rimfire handgun were able to self-load using the energy produced by the .22 rimfire cartridges which simply blew the action open to reload a new cartridge. (This is called simple blowback.) As cartridges grew in size and power the amount of recoil in the chamber increased. The energy created by larger and higher pressure cartridges, such as 9×19mm Parabellum pistol cartridge, results in violent movement of the gun's action. In the case of simple blowback (and delayed blowback) the changes needed for a firearm to be able to control the higher amount of inertia and higher chamber pressures resulted in firearms design changes. Some designers handled this problem by making the moving parts of the firearm heavier and the strength of the recoil spring much greater. Firearms such as the Beretta SB950 (.25 ACP) take a great deal of force to chamber a round. Beretta solved this problem with a tilting barrel which allowed loading without having to pull the slide to the rear. Gun makers developed ways to keep the actions from opening too soon by "locking" the breech closed. Using this technique a firearm, such as the Colt .45 ACP Government model handgun was made which featured a grooved barrel and a grooved slide which were cammed together to prevent the breech from opening. The camming mechanism "locked" the breech closed until the entire barrel and slide assembly had moved far enough that the pressure in the chamber was low enough to safely unlock the breech during the process of ejecting the spent casing which was followed by loading a new loaded cartridge. (This is called locked breech.) Firearms that are physically small, such as handguns, use a system of recoil referred to as short recoil. This is adequate for the smaller calibers. In large firearms such as the Browning M2HB .50 caliber machine gun a similar system called long recoil is used. The difference is how far the breech moves after a shot is fired. Locked recoil systems rely on timing to allow safe operation. A very heavy bolt mechanism can be used (such as in sub-machineguns) to slow the rate of movement and reduce the rate of fire.