Answer to the questions is "it depends". Burn rate alone is just one factor to consider. Some powders take more space in a case than others. This is where QuickLOAD is very handy. You can input your case capacity, bullet, seating depth, then ask the program to rank the powders according to velocity and fill ratio.
Generally you'll find that ball,spherical) powders take less space in a case. If you're tight on capacity and want max velocity, a ball powder is a good option,though it may not be the most accurate). If you have excess case capacity, such as with the 6.5-284, a long extruded powder will,normally) give you more case fill.
Generally, though not always, increasing your load density does tend to help reduce ES/SD,though there are certainly exceptions to the rule). Getting back to your question--going to a slower burn rate powder will probably give you better case fill--but you really need to analyze this on a powder by powder basis. This is where QuickLOAD can be invaluable.
I was recently talking with German Salazar about his 22 PPC,featured gun of the week). He was using Varget, but found it powder-fouled heavily and he wasn't getting the load density he wanted. He thought he'd have to switch to a much "slower" powder. However we did a QuickLOAD calculation and found that RL15 would fill the case better, plus give him a bit more velocity. He tested RL15, and to his surprise, everything went as predicted. RL15 gives him 100% load density, the powder fouling was reduced significantly, and accuracy was as good or better. He took his new RL15 load to a match and won it convincingly, shooting his personal best score ever. His 597 at 300 yards would have beaten the top score,595) recently shot at the Europa Cup 300m championship.
Just curious. Mike.
