InRange has an excellent interview (and range video!) with Jim Sullivan, a designer of the AR-15, Ultimax 100, Mini-14, and other firearms. In it, they get to fire Sullivan’s improved M4, which is more tolerant of sustained fire and employs the “constant recoil” principle used in Sullivan’s Ultimax 100 machine gun. I’ve fired the latter weapon, and I can say without a doubt that Sullivan means what he says when talking about its performance and soft-shooting characteristics. To help illustrate this, here’s a video of my friend Arthur firing the Ultimax 100 taken by a high speed camera at about 1,000 frames per second (due to the lighting conditions that evening, the quality is not as high as I would like, but the effect of the constant recoil mechanism is very evident):

In March of 2008, Small Arms Review published another interview with Jim Sullivan, which carries more detail about his time working at Armalite, Ruger, and in Singapore. Parts I, II, and III are available on the SAR website.