I'm try to find the simple logic in the mysterious world of reloading.
in reloading for my hunting rifles I select my bullet for terminal success, either small groups for varmints, or predictable expansion on game animals, then I search for the most consistent load; consistent point of impact, small ES on the chrono, hopefully with similar results at summer temps and winter temps.
Once finding a good load, and I eventually run out of powder, I try another jug of powder, sometimes the same type/new batch, sometimes a different type (less temp sensitive, or one of these new "clean burning/less fouling" types). I use the same rifle, same bullet, same cases, same primer trying to achieve the same velocity I had success with. Once there I get the same consistency only about 50% of the time. It feels like starting load development all over again with every new jug of powder.
My question:
Why wouldn't the best consistency of any rifle/cartridge/bullet combo be the same with any powder that can achieve the same muzzle velocity?
As others have stated, acceleration of the bullet to the muzzle (this varies with the powder used, dimensions of the bore and/or bullet and start pressure). Case fill is another consideration (>95%, avoid 100% as some cases will be compressed and some not, and generally < 105%) so two zones for case fill. Seating primers in a consistent manner helps ES ( I pre-load the cup to the anvil) the thinking here is mitigation of an inconsistent firing pin strike. I get better / lower ES with cases once fired from annealing ( this points to my inconsistent annealing processes). Concentricity of the loaded round must be 0.000" run-out, if not , find out where in the load process that induces run-out, (this will lead you to using a body die and a Lee collet neck size die).
Depending on whether the cartridge is over-bore, like a 22-300 Weatherby as a gross example, barrel erosion must be accounted for. You will see this as velocity dropping off a bit, groups opening up.....essentially losing your tune. An increase in powder usually help here. Bottom line here is that your load over time must be dynamic to the changing 'mechanics' of your rifle and environment that it is operated in.
A LabRadar is a great tool for tracking velocities, my dedicated bench guns are velocity recorded for nearly every round. This gives me a better over-all picture as to what is going on , state of tune and how the environment affects things.
The study of QuickLoad answers a lot of questions about the 'mysteries' of internal ballistics. If you go that route I've found printing off the manual helps immensely in this regard.
The same regime is followed for my hunting loads, the only difference is that a light factory crimp is applied for the sake of reliability in the field.
