5whiskey,
For a given case length and a given bullet sectional density, a straight wall case has the most rapid expansion. That is, the bullet doesn't have to go as far down the barrel to double the volume the powder is burning in. For a given pressure and bullet sectional density, the acceleration will match, but a bottleneck case will experience less expansion, giving more time for the powder to burn and evolve gas. Thus the bottleneck case will be loaded with slower powders. The caliber here doesn't matter, just bullet sectional density and pressure do.
Faster powders, as normally used in a straight wall case, tend to be less problematic when reducing loads than slow ones do. Faster powders tend to ignite more consistently at lower pressures and temperatures because they lack the heavy deterrent coatings of slow powders. This allows them to work where there is a lot of empty space in the case because that empty space makes it harder to achieve the pressure and temperature level a slow powder needs for good, consistent ignition. At the same time, if you've ever blown air through a regular drinking straw and then tried to do it through a cocktail straw/stirring stick, you know the small hole makes it a lot harder to relieve the pressure in your lungs. In an overbore cartridge, if the pressure from a reduced load is too low to make a good job of sealing the case against the chamber, that means there is more time for pressure to leak out around the case. So, bottom line, yes, overbore cases are less suitable in that there is a narrower band of pressure over which they will work well with the reduced load.
Tobnpr,
Send me the information you have by PM and I'll let you know if I need more detail to create a good QL estimate.
