Dormus of Austria. Simple blowback The blowback (sometimes referred to as "simple", "straight" or "pure" blowback) system represents the most basic auto loading operation type. In a blowback mechanism, the bolt rests against the rear of the barrel, but is not locked in place. At the point of ignition, expanding gases push the bullet forward through the barrel while at the same time pushing the case rearward against the bolt. The expanding gases push the bolt assembly to the rear, but the motion is slowed by the mass of the bolt, internal friction, and the force required to compress the action spring. The design must ensure that the delay is long enough that the bullet exits the barrel before the cartridge case clears the chamber. The empty case is ejected as the bolt travels to the rear. The stored energy of the compressed action spring then drives the bolt forward (although not until the trigger is pulled if the weapon fires from an open bolt). A new cartridge is stripped from the magazine and chambered as the bolt returns to its in-battery position. The blowback system is practical for firearms using relatively low-power cartridges with lighter weight bullets. Higher power cartridges require heavier bolts to keep the breech from opening prematurely; at some point, the bolt becomes too heavy to be practical. For an extreme example, a 20 mm cannon using simple blowback and lubricated cartridges would need a bolt to keep the cartridge safely in the barrel during the first few milliseconds; furthermore, the average force supplied by the return spring is limited to or the bolt will not travel back far enough to feed a new round. Consequently, the return spring is not powerful enough to keep the bolt closed when the gun is tilted up. In addition, there is not enough energy stored in the bolt to cycle the weapon. Due to the required bolt weight, blowback designs in pistols are generally limited to calibers smaller than 9×19mm Parabellum (e.g., .25 ACP, .32 ACP, .380 ACP, 9×18mm Makarov, etc.) There are exceptions such as the simple blowback pistols from Hi-Point Firearms which include models chambered in .40 S&W and .45 ACP. Simple blowback operation can also be found in small-bore (such as .22LR) semi-automatic rifles, carbines and submachine guns. Most simple blowback rifles are chambered for the .22 Long Rifle cartridge. Popular examples include the Marlin Model