The HK33 is a selective fire weapon with the famous roller-delayed blowback system of operation. It employs a two-piece bolt consisting of a bolt head with a pair of rollers and bolt carrier. Upon firing, the two cylindrical rollers in the bolt head are cammed inward by inclined surfaces of the barrel extension and impart a rearward motion on the locking piece, which also propels the bolt carrier rearward. This built-in mechanical disadvantage delays the movement of the bolt head relative to the bolt carrier which is withdrawing at four times the velocity of the bolt. The rollers soon compress entirely into the bolt head, clearing the "locking" recesses of the barrel extension (though they are never truly mechanically locked), and both parts now continue rearward together, opening the breech and actuating the extraction and feeding cycles. The chamber is opened under very high pressure, thus the chamber received a series of flutes in order to increase extraction reliability and prevent "sticking" of the spent casing to the chamber walls.
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