Why does a lighter spring reduce felt recoil?

[zaitcev]

Ian McCollum espouses a theory of recoil at times that posits: most of the recoil is transmitted to the shooter when the action stops in its rearward position. Therefore, constant recoil systems work by not letting the bolt, carrier, or slide to slam the receiver. Instead, the motion of the action is decelerated by springs alone, which reduces the recoil. This is why the recoil spring in CETME-L and HK Mark 23 has 2 sections: the longer one powers most of the motion, such as feeding, and the short, but stiff part buffers the action slamming the frame of the gun.

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?

 

[browningguy]

A heavier spring transfers the recoil felt in your hand at a faster rate than a soft spring which provides a more gradual transfer of force. It's only milliseconds of course but can be felt by many people. At least that's my understanding of the physics.

 

[WestKentucky]

My take on it is that it’s more about hand position and slide motion than it is anything else. Theoretically the constant recoil system works, but would require a bit of overtravel. Not a huge ordeal, but it’s lengthening the slide and lengthening the duration of force felt. The reality is that rounds hot enough to reliably operate a constant recoil gun are probably still hitting a positive stop or the overtravel is massive. It still boils down to what a person perceived though, a longer and more consistent impulse or a snappy quick impulse. Personally I dislike a long impulse as I perceive those as heavier recoil. I believe that to be a large part of why I prefer manual action handguns. With snappy guns the slide begins closing quickly as it basically bounces off of the stops and slams home.that forward motion is a pull that seems to negate the push of recoil to an extent.

 

[jmr40]

This may not be relevant to your question. But when recoil is calculated you can figure the actual recoil in ft lbs of energy, but the speed or the recoil is also part of the equation as well. A good example. A 300 WM shooting 180 gr bullets has virtually the same recoil as a 35 Whelen shooting 225 gr bullets. But the 300 WM recoil hits the shoulder much faster. While the raw recoil is exactly the same, most people find 35 Whelen to be more comfortable to shoot because that recoil is spread out over a longer period of time.

You see the same with gas operated shotguns. In equal weight guns the recoil of a gas operated shotgun will be spread out over more time and will be more comfortable for the shooter than the same shells in a pump or double. The actual recoil is exactly the same, it just doesn't hit you all at once.

While I'm much less familiar with handguns I can see how different recoil springs could do something similar.

 

[Steve in Allentown]

The recoil spring exerts force forward and backward at the same time. As it compresses it is pushing the pistol backward against your hand and forward against the slide. The heavier the spring the more force it exerts.