Mmmm, sorta difficult to explain but try it like this.
Let's say I have 2 little pipes......one about the diameter of a straw and the other the size of a golfball.
Now, if I take a big deep breath, fill my lungs and then blow it will take much longer to empty them if I blow through the straw than if I blow through the golfball sized hole, right?
Now imagine you then find 2 pieces of wood the same weight than are the diameter of the pipes and sorta lodge them in so you can blow them out.
You'll find you run out of breath far more quickly with the golfball sized one than the straw size.
So lets now increase the mass of the golfball diameter timber so it exits at the same speed as the straw sized piece.
You'll find that a large diameter bore can push heavier projectiles at the same speed as a small bore pushes light ones.
The best way of examining this is to look at the available wildcats in the '06 family.
Start with 30-06 and look at 338-06 and 35 Whelen.
A good common bullet mass is 200gr.
Check out the increased velocity:
- The 30-06 can push a 200gr to 2580fps with 53.7gr of 4350.
- The 338-06 can push it to 2729fps with 62gr of 4350.
- The 35 Whelen can push it to 2807fps with 63gr of BL-C(2).
Now, there are other factors....there ALWAYS are.
Some people here will swear their 338-06 can punch them out over 2800fps which is true but that is using well over 60,000 PSI.
But when it comes down to it a larger hole and same weight pill equals a faster possible MV.
For the record this is why the 8x57 with a smaller case but similar pressure can toe up to the 30-06 with reasonable hope of success.
In the world of ballistics even 15 thou counts.
Same gap between the 8x57 and the 338 Fed.
Ironically it's also 6mm difference in case length between the 338 Fed and the 8x57 and 6mm between then 8x57 and the '06.