Acctually the MP5 is select-fire:single shot,3 round burst and full auto.
There's an optional 2-shot burst trigger available for some H&K and other weapons, preferred by sonme operators over the more common 3-round burst units pioneered by the French, a requirement they had for the Ruger Mini-14 rifles they tested in the mid 1970s and on prior to their own adoption of the FA-MAS clarion.
Around 1977 and '78, the Army SF was using the Ingram M10 quite a bit, particularly for *advisors* in El Salvador, who were not authorized to carry rifles for their own protection. One of the developmental tasks I worked on back then was for a 3-shot burst trigger for the Ingram guns, and by using an escapement mechanism working against an advancing plate with holes in a line to allow the sear and disconnector to interact, it was possible to pick the number of shots to be fired by using easily interchanged [by an armorer] engagement plates with a different number of holes in them, usually 2, 3, or 6.
If that sort of tinkering was still lawful now, I'd be working on a similar burst trigger for the Calico M950 or M100 9mm or .22 carbines, which are available with 50 or 100-round magazines. And I might be thinking of 5-shot bursts for the 9mm, and 10-shot bursts for the .22.
