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Thinking about the effect of a heavier boolit on the combustion, it reminded me of what my thermo lecturer was talking about with the idealised otto cyle and diesel cycle - modelling the combustion as simple heat addition, a heavier boolit will get more PdV work. So, the otto cycle is more efficient than a diesel cycle ceteris paribus - this is because the ignition of the fuel is so quick it's kinda like isochoric heat addition (i.e. constant volume), whereas diesels are limited by the time it takes to inject the fuel* so are better approximated by isobaric heat addition (constant pressure, so with volume increasing). Since some travel is taking place in the diesel cycle before all the heat is added, there's less area under the P-V diagram so less work is done. *according to my lecturer, non-direct injection diesels are not halal Something similar will happen with a heavier boolit - it will have moved less before all the propellant is combusted, so it's closer to the ideal of holding the bullet in place until all the propellant has combusted to maximise PdV work*. A light boolit, OTOH, will have moved down the tube more when the pressure in the chamber was low, so doesn't get as much work done on it. The rate of combustion is probably proportional to pressure and temp as well, giving an even more pronounced effect. All this is happening completely separate to the not-quasi-equilibrium thingy with the ratio of propellant gas speeds and boolit speed - that reduces efficiency towards the end of the barrel, whereas this reduces efficiency near the chamber where velocities are much lower *maximum pressure limits are for wusses