The trigger mechanism is similar in design to that used in the FN FAL. When set on "0," "S" (safe) or a white bullet in a rectangular box with an "X" superimposed over it, the selector lever's spindle prevents all upward movement of the sear and its nose cannot drop out of engagement with the hammer's notch.
When put on "1," "E" (einzeln = single) or a red bullet in a box, pulling the trigger will rotate the sear down and out of the hammer's notch. When the hammer rotates forward, the sear slips forward and its end drops down off a fixed step. In recoil, the hammer is rotated back by the bolt carrier and catches the sear's nose, pushing it back in contact with the fixed step. After the bolt closes again, the auto sear releases the hammer, which is then held by the sear. Releasing the trigger allows the tail end of the sear to rise and move onto the fixed step. Pulling the trigger again will repeat the process.
When the selector lever is set to "F" (full-auto), or seven red bullets in a long box open at the front end (implying infinity), its spindle allows the sear's tail to rise so high that the sear's nose does not engage the hammer notch at all. The hammer is thus held by the auto sear only. As soon as the bolt carrier moves completely forward the auto sear is released and the hammer set free.
Three-shot burst controls are also available. They consist of an intricate ratchet-counting device fitted to the trigger mechanism, which holds the sear off the hammer until the allotted number of rounds bave been fired. The device ensures that only the correct number of rounds are fired in a single burst and any interruption commences a new count. After each burst the trigger must be released to set the counter back to zero. Although the burst mechanism is reliable when maintained properly, it adds significantly to the number of parts in the trigger mechanism. Furthermore, it is in my opinion a superfluous feature on this weapon, as its 800-875 rpm cyclic rate permits experienced operators to fire two-shot bursts with ease. U.S. Navy SEAL Team 6 experienced a serious accident during a training session as a consequence of improper disassembly/assembly of the 3-shot trigger unit, and discarded this option.
There are now four trigger units available for the MP5 series. All have housings fabricated from a two-piece synthetic molding. The standard "SEF" group provides safe, semiauto and full-auto positions with a selector lever on the left side only. Its pistol grip has finger swells, while the others do not but are flared at the bottom in the front to prevent the firing hand from slipping downward. The so-called "U.S. Navy" group offers the same three positions but uses bullets rather than numbers or letters for markings and is ambidextrous (a selector lever on each side of the housing). There is also the ambidextrous, four-position, 3-shot burst group.
The MP5 SF (single fire) carbine's ambidextrous trigger group has only two settings and will fire only in semiautomatic (this foolishness at the behest of the FBI). In addition, a special selector lever and lockout key are available to prevent the "SEF" trigger group from firing in the full-auto mode. The selector lever on all of these trigger groups can easily be pushed downward with the thumb of the firing hand while retaining the correct firing grip. However, the lever cannot be rotated upward - back to safe - without shifting the operator's grip. This remains as the single legitimate ergonomic criticism of the entire Heckler & Koch series.
