Can someone explain to me...

[Blammer]

in the nosler reloading book in 8mm Mauser between the 180 gr and 200 gr bullets

why with a 20 gr bullet difference, using the same powder and powder charge you get pretty much the same velocity?

for example

IMR 4350 52 gr max, vel is 2545 with 180's and 2698 with 200's? it is hardly a difference when talking 20 gr of bullet wt.

 

[live to shoot]

Different powders burn at different rates, Look at the same powder and different bullet weights and you will see a drop in velocity.

 

[Ol` Joe]

I think if you check you`ll find in most cases the wgt/velocity differences run in the same percentage range.
The 180 gr bullet is 90% of the wgt of the 200. The 2545 fps of the 200 gr is ~94% of the 2698fps of the 180.
This ratio (bullet over bullet = velocity over velocity) doesn`t always run true but seems to be found true usually, in what I`ve seen.