Miller,
Isacc Newton's 300 yr old laws of motion address your uncertainty about acceleration of the gun vs. the payload. They basically say: if you push harder, the object accelerates harder, and; if there is a force it has an equal and opposite reaction. Pressure is the origin of the force as it pertains to guns. Put all this together and you can see why I conclude that the gun accelerates differently with different pressures.

What a person can 'feel' or what fatigues a particular person more than another is very subjective. So if a person says a particular load is less comfortable to them than another, what basis would I have to discount his 'feeling'?