Powder burns differently in different case sizes.
So how do the programs(I.E. Quickload) determine a starter load for a given case/powder?
Is it based on the bomb testing results, with a fudge factor?
Is it based off a "standard deviation" that powders exhibit in different cases?

Part two:
Since it is generally accepted that not all primers are the same, why do companies(I.E. Hornady/Speer, et. al.) interchange primer brands randomly during load developement.
I have written several letters for clairification, but never recieved an answer.

It is common to read something like: Primers other than 7 1/2 changed at different intervals resulted in jammed bolts/blown primers.

Is that because that paticular combo was WAY over pressure, and you can get away with less powder for the same vel??

What do you guys think?

I believe that powder, and primers are just like any other component, if you change brands, lots, size, type, etc, ya need to drop the charge back a ways and work back up ..... every rifle-bbl-chamber-leade length, etc is different and will affect the same change in a different rifle differently....

That's correct. Change any component, then back the charge down and work the load back up.

As to pressure computations, they are based on somewhat complex mathematical models in which powder burning rate is just one of several powder characteristics used in the calculation. QuickLOAD's moldeling uses eight different ones, the burning rate factor being just one of them. It's model assumes the mildest possible primer is used, and does not have an adjustment for that. You can always shrink the case volume some to compensate if your chronograph tests show the program is underpredicting.

Thanks Nick
Been wondering for a while...