Shot #1 or shot #200, does not matter.  Initial velocity of the bolt is zero.

Shot #1 starts with the bolt held in place by the sear:  Bolt velocity = zero.

Shots #2-200 start with bolt rearward motion stopped by the receiver, at which point bolt velocity = zero.

Unless "initial velocity" is being defined as velocity of the bolt at some point other than the point where the bolt's forward movement is initiated?

Elastic.

Inelastic collisions are where the objects stick together in a lump.

stancrist said:

Unless "initial velocity" is being defined as velocity of the bolt at some point other than the point where the bolt's forward movement is initiated?

Id define what Max is talking about as "Spring +" power. 

Shot 1 = bolt traveling forward operating purely on spring power

Shot 2-200 = bolt traveling forward operating on spring power + energy of the bolt 'bouncing' off the back of the receiver 

Personally I think designing the weapon to rely purely on Spring Power, and minimizing bolt impact to improve controllability/reduce shot dispersion is the way to go. But it;s entirely believable that this "Spring +" effect of the bolt bouncing off the receiver does allow for more power for stripping rounds off the links. 

Redefining zero as positive doesn’t make it a valid claim.

Relying on the shear strength of the rear of the receiver to stop bolt travel also seems like bad design.

If your feed design requires the residual energy of the bolt after collision with the receiver to chamber a round, how will it ever fire the first shot to get some residual energy?

Your rationalisation of an incorrect claim is spurious and is decreasing the s/n ratio in this thread.

gatnerd said:

Id define what Max is talking about as "Spring +" power.

I dunno.  He very clearly talked about "velocity" in post #12.

mpopenker said:

...initial velocity of 3-4 m/s...

Regardless of what word he used, he's describing the bolt bouncing off the rear of the receiver, adding a bit more power to the bolts forward stroke then spring power alone. 

Does the PKM pull mechanism shorten the total length by several inches?

yes, as it allows placing the belt feed unit above the barrel breech

Just look atthe Polish UKM-2000 machine gun which is, basically, the PKM mated to MG-42 belt feed

gatnerd said:

Regardless of what word he used, he's describing the bolt bouncing off the rear of the receiver, adding a bit more power to the bolts forward stroke then spring power alone. 

Some buffers can also add power to the forward stroke.  A spring type can prevent solid impact with the rear of the receiver,  if it is not fully compacted at the end of the bolt's travel, then use the stored energy to accelerate the bolt forward.  The amount of energy returned to the bolt would likely be higher than if it had impacted the rear receiver.   A hydraulic buffer can also return energy for forward bolt motion, although by design, a certain fraction of the energy has been absorbed.  Nevertheless, since a hydraulic buffer, like an automotive shock absorber system, can be combined with springs and valved for relatively high damping in compression and low damping in "rebound",  it can return a significant amount of energy to accelerate the bolt back to its original position.

Yeah that one, I'm not an engineer I just play one on tv lol.

I'd love to be better educated than I currently am, working on it but there's so mucg to work on so little time.