Exactly.
And, since the energy in the spring is only retarding things during extraction and ejection, ie, not helping pull the case from the chamber, all the energy required to do this must come from the bolt carrier's initial velocity it has gained during the first three-tenths of an inch movement.
1. During the full cycle, this is not a negligible amount. However, you can model drag friction as a constant, as friction usually is, and you can model the hammer over-run drag on the hammer-to-carrier geometry and the hammer spring constant.
What I did is construct the model based on the known M4 data, then play with 'correction constants' until the model reacts to match empirical data. Then you can make your desired changes to buffer weight, carrier weight, spring constants, etc and get reasonable results. Unless you make gross changes, like changing the cartridge to .50 BMG, you will stay in the ballpark.
2. Technically, each coil of the spring (except the last two) are moving backwards with some velocity. The coils in contact with the buffer are moving at the speed of the buffer, and all the way in the back, the last two coils are stationary. You can do the calculus and figure the exact amount of energy lost, but that is about like measuring the height of a mountain with a micrometer. The tolerance on the spring constant is around 4 or 5 N/m, that alone will swamp the these losses in this case. If you had a very heavy spring, and/or a very high velocity, like in a car suspension, that energy loss might be a factor.
3. The gas pressure vs time is as shown in the graphs many posts back, from that you can calculate the force vs time, momentum vs time, energy vs time, etc.
Remember, this is a mathematical model of the system, not a 1-to-1 scale model that reacts exactly like the real thing under all possible circumstances. This is not intended to calculate the exact location a spent case will land, plus or minus an inch, the variables are too many and the tolerances of the parts too wide for this level of accuracy. Go back the the first graph I posted, it notes the time to bullet exit. Do you think ever bullet shot from that barrel take the exact same length of time to exit? And how about all the other 14.5 inch barrels out there? No, but on average they will probably take that long.
What you are looking for are trends: I wish to delay unlocking by 1/2 ms, if I increase the buffer weight, do I get and delay in unlock time? Not really, at least no where near what I want. Okay, I need to look somewhere else to get that 1/2 ms delay I want. What happens if I reduce my piston size? What happens if I reduce the gas port diameter? And so forth; from these results I can figure out the best way to get from where I am now, to where I want to be, with the least amount of fabrication and testing.
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