This is what I believe about bullets. I have believed other things in the past but this seems to fit with my experiences. You do not have to believe what I belieive, I just lay it out there and I may be wrong agian. It would not be the first time I was wrong.
A 300 grain bullet traveling at 2000 fps would have the same momentum as a 200 grain bullet traveling at 3000 fps. Now then, if they were solid steel and there was no mashing or distorting of the bullet, then penetration would be about equal.
Here are the momentums for two older styles cartridges from Weatherby
340 Wby w 250gr @ 3000fps= 750000
375 Wby w 300gr @ 2800fps = 840000
The bigger bullet even though slower has 12% more momentum. Your point I believe is that at some point a faster lighter bullet is better than a heavier slower bullet is correct and that is when you get to an old slow 45-70 vs a smaller diameter that is faster and has a bullet to constructed to withstand heavy imnpact on bones. There are some details in here that go back to my joke about the T rex and 17 rem. Even with a solid steel bullet the 17 just aint going to cut the mustard.
We then come to the frontal area of the bullet and the diameter of the hole. Bigger holes kill better obviously but how do we take that into consideration. The Taylor knockout power is one attempt that seems to me to be as good as any. What he did was take momentum and multiply it by the diameter of the hole.
You can go look at it here
If you want to play around with some momentum vs energy and fast light bullet vs slow heavy bullet this JBM ballistic calculator will compute both energy and momentum.
Now then you will notice that my calculations above and the first web site and the second website all have very different numerical values for momentum. This is because momentum is a dimensionless value. Lots of people talk about pound sec etc but it is not correct. It has no units. Consequently, if you divide my numbers by 7000 you will get to one website and then divide the numbers on that website by 32.2 and you will get the second website. (I think- I havenít run the conversions for about a month now so I might not remember all of the correct numbers.)
In bullet momentum just try to recognize that different people convert grains of weight to pounds of weight and then some of us will convert weight to mass. Up above, I did no conversions because we are just comparing two bullets.
There are people who swear by hydrostatic shock but I am not one of them. I swear by breaking bones and penetrating.