E.g., if I tell it that a 30-06 load with a 190 Hornady BTSP and "x" grains of H4350, at "y" OAL in Remington brass with a CCI primer got 2640 fps, can it tell me that if I then use "y" grains of RL-19, with all other factors being equal, I will likely get 26XX fps?
If you have known velocity with a given powder, QL will estimate pressure if you input powder charge, bullet and OAL. Or you may setup the propellants table for a given case, bullet combo and you may use every powder between two burn rates to deliver:
1. loads that produce your desired pressure
or
2. loads that will achieve a desired velocity.
QuickLoad will then make a printable table for each powder, between the upper and lower burn rates showing:
Powder (Name and brand)
Density
Calculated charge
Velocity
Pressure
Barrel time
You may export this table to Microsoft Excel if you have it.
It will allow you to do a lot of "what if" modeling.
Regarding the brass, you may make a case file for each type of brass in a given caliber. You need to determine the water capacity for each brand of brass edit the file and save to a new name. It sounds very involved, but it takes more effort to describe the procedure than to do it. All in all, QuickLoad is very good software and very easy to use.
Now, you can go up and change powder type. If the new velocity and pressure are too high or low, change the charge. Theoretically, the program is still recognizing your barrel as it's "Standard."
However, Qload recognizes (assumes the possibility of) a 10% variation or error factor. Not bad given that SAAMI, if I remember correctly, operates within a 9% factor. If you do a lot of load development a program like Qload will save you a lot of time, effort and, eventually, money. But it's not the Holy Grail. On one occasion I ran into an error factor of 18%! So you still must test loads and start low.
Where the program earns it's money is helping you find those powders that are most suitable, offer max velocity with min pressure, and which powders are very close. I consider the program VERY much worth while.
Steve