The trigger lifter has a front nose and two side noses. If the front nose will engage, the first barrel will fire. However, the next side nose must then engage.Often, on a well used M 21,it will not slide into place.One may buy a new lifter (Galazan) ,or carefully, file the side noses so that they will slide under proper sear.There should be a small amount of of take-up on the trigger on the first and the second take-up.