Sorry let me try to clarify my "position" on things. To me it looks like we both are "right" but have simply technical discrepancies.
-I believe an AEG will impart the same kinetic energy to each BB reguardless of it's mass (I'm hoping to do some emperical tests to prove this THEORY).
-The BB with the greater mass will decelerate ("lose it's velocity") at a slower rate due to it having the greater momentum (I'm thinking of momentum more as "inertia" rather than strickly m*v).
Kinetic energy (.20 @ 90m/s) = 810 joules -> From this I can calculate the theoretical velocity of .25's (neglecting rotational energy)
Velocity of .25's with same KE = 80.5 m/s
Momentum of .20's = 18
Momentum of .25's = 20.125
Deceleration due to drag = k*v^2/m, where k=constant=coefficient of drag*radius/2 ; v=velocity; m=mass
-You can see here that the deceleration will ALWAYS be greater for the lighter BB fired from the same AEG (theoretically!). Also, you will notice that the heavier BB will ALWAYS have the greater amount of momentum when fired from the same AEG. Hence, why I said the momentum had something to do with the drag and deceleration and straighter flight, etc.
-Of course drag has many parameters such as surface area, fluid viscosity, as well as mass and velocity so by me referring to it’s momentum being the key factor maybe a bit of an oversimplification for our current airsoft situation?
-Drag is also a force (or in the above equation an acceleration assuming it’s correct), and to turn that into negative work (energy “stolen”) you must multiply it by the distance traveled (work = force*displacement….units of energy). Basically technical mumbojumboo that is very confusing.
-This is all assuming that the BB is NOT SPINNING. Once it starts spinning it gets A LOT more complicated (ie. Magnus/Bernoulli effects, rotational energy, etc).
Topic of what to use for calculating how much “hurt”:
-Note that impulse and momentum go hand-in-hand (same units and used together to do impulse/momentum analysis). Impulse is the integral of force*d(time)….which is basically Force*time for simplicity sake (assuming the force does NOT change with time, which usually isn’t the case)
-So again, I have no idea what to use for calculating “hurt” but I’m leaning towards impulse/momentum.