The gun body and the Vertical Gun Rack have two front and rear connection points, but its gun body is not completely fixed on the Vertical Gun Rack, but can slide back and forth. Note that on the left side of this photo, there is a spring under the front connection point. This spring part, called Vertical Gun Rack buffer, is to add a floating skateboard between the gun body and the gun frame. There is a spring in the skateboard. When shooting, the machine gun body floats back and forth on the gun frame skateboard to absorb part of the consumption. Recoil.
This kind of Vertical Gun Rack with buffer has some advantages, that is, the buffer can absorb a lot of recoil, and the impact on the gun frame is relatively small, but there are also disadvantages. This kind of buffer frame is heavy and complex. In addition, the buffer spring will lose its elasticity after a long time, and the buffer slides will wear out, which will affect the shooting accuracy. Speaking of the Vertical Gun Rack with buffers, we can explain what a "flexible gun rack" is. The so-called elastic gun rack is a machine gun Vertical Gun Rack that does not have a spring buffer on the gun rack, but relies on the elastic deformation of the legs of the gun rack to absorb recoil. For example, the tripod of the PKM general machine gun mentioned by the questioner.
This tripod is made by stamping and is very light. The frame and the gun body are rigidly fixed and connected without a buffer. When shooting, the recoil is directly transmitted to the Vertical Gun Rack, and the recoil is absorbed by the elastic deformation of the legs of the frame. This gun frame greatly simplifies the structure and reduces the weight.