1. What are the dynamics describing the relationship for a given caliber between bullet weight, powder type (slow/fast and any additional variables that I am unaware of), primer (doesn't it just have to energize the powder or is there more to it than that?). I'll try to provide you an answer based on general principles only:
Generally, the less overbore of a cartridge and the lighter the bullet, the "faster" the burning rate of the powder. That is generally speaking only. Use the manuals for specific loading information. For instance, a .45-70 with a light for caliber bullet will use a much faster burning powder like IMR 4064, where the Remington 7mm Magnum with a 175 grain bullet might be loaded with Retumbo or H1000. Again, follow the manual at all times.
A primer's job is actually two fold: #1= Light the powder. #2=Light the powder with enough force to provide complete and reliable ignition.
#1 = standard pistol or standard rifle primers for the average task. Magnum primers should be used for magnum loads and hard to ignite powders such as ball powders. (Win. 748 and Win. 760 come immediately to mind.) Always use the type and brand of primer called out in the manual for the application you are loading for.
2. when you measure the velocity of a bullet with a Chroney, what are you looking for beyond consistency? Is it a slower bullet or a faster one, consistency in speed or something else? Something else, is my answer. My purpose is to be sure I am not exceeding the velocities that I should be by more than a little bit. I am also wanting to see if the load is as consistent as my target says it is. Why? If I am getting velocities much faster than the manual says I should, it is a very good indication my pressures are exceeding what they should be and if I am near the maximum listed load, then I am probably exceeding the safe pressure levels for that cartridge which means I will back off to safe levels. Not only do I not want to have a gun blow up, feeding a gun a constant diet of maximum loaded cartridges is like driving your car with your foot on the floor all the time. Not a good practice if you want longevity.
3. What are your measurement strategies (assuming you have a chroney) in fine tuning and/or evaluating a new round? See # 2 above.
Also, a chronograph is helpful in providing information. During development of test loads, my favorite method is to load 3 rounds of each 1/2 grain increment increase in powder, always starting with the manual starting load. If during the course of fire from starting load to maximum load there is a sudden significant increase in velocity - greater than anticipated or that the book says it should be - I stop immediately. Such is an indication of excessive pressures. Also, if your velocities vary widely in any particular recipe - be it powder, primer or bullet - I'll usually make note of it in my log and move on to something else. That saves me a lot of time and money spent on components. If your rifle doesn't like a load, it will show up on the chronograph. A good example is my Remington 788 carbine in .308 Win. In defiance of all that is holy, it will not shoot BLC(2) worth a hoot regardless of bullet or primer. It shows on the chrono stats, too. Extreme spreads well over 100 fps are common and standard deviation less than 40fps are rare, and even more rarely repeatable.
