One of the unfortunate (but worth it, I think) aspects of having a no-longer produced rifle is the need to develop a good load for the rifle in the absence of easily found production cartridges. Fortunately, I have been collecting brass for a while, and I still have my stash of primers.
Powder, however, seems to be "catch as catch can". You never know what will be in stock, and you need to have a good idea of what will work if it is in stock. I "get" that some powders burn faster than others, and are more suited to specific loads, and I understand that programs like QuickLoad can help in narrowing down loads that might work. I just want to make sure I understand what may help start the process.
In this case, I'm going to focus on my 6.5X54 MS rifle, with the 450mm (17.7") barrel. CIP specs will give me a maximum load, and I'm going to stick with one bullet to "make all other things equal". Bullet chosen is the 160 grain Woodleigh RNSP. The only variable I'm going to change is powder, and even there, to keep things reasonable (i.e. not too many choices) is to consider Vihta Vouri powders. Comparisons are done in QuickLoad. If it helps the discussion, Woodleigh lists only 8 powders in their manual, and I haven't seen them in stock in an awful long time.
To be clear: I understand that these are "estimates", and are no substitution for working up a safe load. I also understand that published loads are better, but also know that not every powder company has tested their powders in this cartridge, and published data lists powders that are not always available. Additionally, there are some bullet manufacturers who provide bullets, but leave the load data up to the powder companies. When I run out of my supply of Woodleighs, I may need to switch to Hawks, and then where will I be? Here, we are working only in theory. No one is pulling the trigger on these loads without going through a safe process.
VV lists several powders as being "slow burning and suitable for heavier bullets". But which would be most appropriate?
To condense my notes I'm going to compare N150, N160, N165, N550, and N560 at either maximum pressure (Pmax of 52940 psi) or full case without a compressed load.
Based on the QL estimates, for N150, I can load 37.2 grains of powder and be at maximum load, hits 2208 fps, fills the case to 93.2%, and burning 100% of the powder.
For N160, 39.2 grains hits max load, hits 2308 fps, fills 90.8% of the case, and burns 99.28% of the powder.
For N165, 41.8 grains is below max load hitting 2208 fps, fills 99.9% of the case, and burns 94.32% of the powder. More powder will keep within pressures, but will be a compressed load, and it's not going to burn that powder in the barrel anyway.
For N550, 39.2 grains is max load at 2334 fps, fills 90.7% of the case, and burns 99.2% of the powder.
For N560, 43.2 grains is below max load at 2339 fps, fills 99.9% of the case, and burns 92.21% of the powder. More powder will keep within pressures, but will be a compressed load, and it's not going to burn that powder in the barrel anyway.
So, depending on the powder, keeping pressures safe and speed about where it needs to be, I can fill the case but not burn it all, or I can leave some space in the case and not "waste" powder by sending unburned powder out the barrel with the bullet. Those powders that do not completely fill the case will exceed max pressure with a greater fill.
Looking at the above, my first choice should probably be N150, leaving some space in the case but burning it all, followed by N550 filling slightly less and burning slightly less and tied with N160.
Am I looking at this the right way? Or am I missing something important? Obviously, you don't want "too little" powder in the case, but filling it and sending unburned powder out doesn't make sense either.
