Maybe I misunderstood something, but I thought that this theory predicted that heavier return springs would reduce recoil in pistols, as they slow the slide gradually. But evidence is mounting that the opposite is true. It all started with my wife, who has an old Walther Q5. It still has the weaker old spring that Walther does not sell anymore ("red" spring). The current spare is a standard PPQ spring ("silver" spring). She refuses to shoot her gun with the stiffer spring, and claims that the "red" spring reduces the recoil. In addition, I noticed that users of DPM system report that softer outside spring reduces the recoil, while stiffer one tends not to be perceived so. It is reported by those who actually experiment by changing the outside spring (the DPM kit comes with 3 of them). It can also be seen from reports by those who just use the stiffest outside spring outright, and then complain that there's no recoil reduction. Does anyone understand just why this is happening? At this point I think it's not some kind of selection bias on my part. I gather that Ian's ideas are informed by Jim Sullivan, the designer of Ultimax and other such guns. It was demonstrated in independent testing that his designs work. But could it be that the constant recoil principle only applies to rifle caliber guns?