In this post I want to address some very controversial issues among the professional sport shooting world, and try to answer these questions with basic physics. Bear with me as it does require some introduction! Below is an animation of the gas impingement system typical of a modern semi-automatic rifle: http://22.214.171.124/arfcom/gasimpingement.gif (animation) For those not familiar: 1) The spring on the left is the "buffer spring" 2) The reciprocating mass consists of the Bolt Carrier Group (BCG) is the gray thing as well as the Buffer (the yellow part in the spring) 3) There is often an adjustable valve that controls how much gas is allowed back into the system to reciprocate the BCG. In modern professional 3-gun competitions, the standard practice to reduce felt recoil during rapid firing is to do the following things: 1) Reduce the internal reciprocating mass (BCG and Buffer) by as much as 50% 2) Because reciprocating mass is now lower, also reduce the flow of gas from the adjustable gas block so that the BCG reciprocates at approximately the same speed as originally. 3) Because reciprocating mass is now lower, also reduce the buffer spring tension so that BCG returns to battery at approximately the same speed as originally. The conventional wisdom or explanation for these changes is that by reducing the internal reciprocating mass, the overall felt recoil of the gun will be less. The fact that by restricting the gas port, more of the gas pressure will exit from the barrel of the gun rather than being directed back through the impingement system, is typically ignored -- though I don't think that's a safe assumption (more on that later). If one consider the imaginary limit in which one continuously reduces the weight of the reciprocating mass, you eventually end up with a rifle that has no reciprocating mass -- in other words, a bolt action rifle. It is widely agreed that a bolt action rifle has larger felt recoil than a semi-auto rifle. The physics explanation is that the reciprocating action of the BCG absorbs some of the momentum. In other words, the momentum of the reciprocating mass is subtracted from the momentum of the ejected bullet+gas to get at the remaining felt recoil. At this point, it is tempting to conclude that the professional 3-gun shooters are full of BS. The fact they have a lower reciprocating mass necessarily means that their guns must have more felt recoil. Or does it? This is not quite the end of the story, because It is standard practice to put a compensator at the end of the barrel, which is basically a series of open-air baffles that reflect as much of the gasses coming out of the barrel back in the direction of the shooter as possible, with a hole just barely large enough to allow the bullet to continue moving forward. It has been found that an effective compensator can reduce overall linear motion of the rifle by as much as about 75%. So, while it is true that a rifle with low reciprocating mass will have less rearward momentum from the movement of internal parts, it will have MORE gas pressure reflected back towards the shooter at the end of the barrel by the compensator. At this point, my arm-chair analysis of the problem escapes me. I'm not sure how effective recoil mitigation in the form of reflected gas pressure would be in comparison to reciprocating mass. I'm interested in trying to get a better understanding of this question from an intuitive/theoretical (arm-chair) perspective...but how to even begin?