Yeas they did new cartridges but not larger ones. Probably the human race is getting weaker as the years go on.

But a slow big bullet must give more of a shock than a smaller and faster, people that get shot thru the body sometimes react much less than someone that gets kicked, that is for the first moments offcourse. And using solids the only way of increase the shock is to make it bigger and heavyer not faster. If you miss the brain all the speed in the world wont matter, its still a miss.
Then you would want something that is able to couse a large hydrostatic wave to shake the brain, that is if you hit tissue, if only scull is hit you want it to take out as large peace as possible that way giving the most trauma. A 6,5mm bullet is sufficient to kill a elephant but most used larger once, why? probably becourse you had a little more chance of getting it down.