roguetechie said:

If we're talking about the AK and pkm as they currently exist, both designs do in fact impart extra energy to the return stroke from their collision with the rear trunnion. Whether another design would do so or not and to what degree would heavily depend upon the specifics of that design, however in these two cases Max is entirely correct.

No, Max is entirely incorrect.  Max said that the bolt starts with "some initial velocity."  

Even the PKM bolt starts with zero initial velocity.  https://youtu.be/01mrTVUJ_Bo?t=45

Shot 1 of all open bolt weapons begins with just the forward spring power of the weapons spring. 

Shots 2-200 may or may not have additional power exerted on the forward stroke

roguetechie said:

When you consider that it will probably be easier to make lightweight polymer pull out disintegrating links it's not entirely out of the realm of possibility that true next generation belt feds could use pull out links.

An important part would also be to limit the belt acceleration. The Evolys belt movement starts during the feed phase, just after pushing the round out of the link, and this seems to dramatically reduce the belt peak acceleration.

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?