Powder basics
 

Guys this may seem like an uneducated question and maybe it is but why is it some rifle powders work well in some cals and others do not? I shoot alot of 30 cal at 2600-2900 fps and 243 at the same speeds and have used alot of different powders and some have worked well and others not so well. what works better with the above listed, slow or fast burning powders and which are slow and which are fast burning powders?
Thx in advance for all the replays.

Here's a burn rate chart so you can see which powders are faster or slower:

http://www.imrpowder.com/burn-rate.html

The burn chart is just for relative comparasons every powder burns differently in diferent cases. The 30-06 will use faster powders then the tighly necked 243win while a strait case like a 45-70 needs very fast powders. Only use listed powders from reloading manuals.

4350 medium/slow burn rate type powders works great in a 30-06 while the 243win should work with a slightly slower powder. Just for example the 35whelen likes IMR4320, the 30-06 the IMR4350, and the 270win IMR4831. Same case with different neck diameters.

The more overbore the cartridge, aka smaller bore to larger case, the slower the powder it will like. The closer to being a straightwall case, the faster it wants. The heavier the bullet for a given caliber, the slower the powder, usually, from my limited experience. My 300RUM prefers the slowest of powders, and its heavily overbore. The 7mm rem mag is also fairly overbore, pretty big step down at the shoulder, and likes powder almost as slow. The 375H&H likes powders in the same range as an '06, and if you look, their cases are similarly proportioned, albeit bigger with the 375.