“That being said, I am sure we could tweak the 223 to go 100 grains and high pressure”
While you might be able to load a 100 grain projectile into a 5.56 mm case there wold be so little room for propellant you’d never achieve enough velocity to make a effective cartridge. I load 55 grain soft points into 5.56 brass and 24 grains of powder darn near fills the entire case. That give me about 2900 fps or so if memory serves.
The only way to “tweak” that case for a 100 grain bullet would be to lengthen it. Then you’d have to redesign the entire gun.
I did not think of that.
I thought that if burn time was reduced and higher pressure used, we might squeeze out more energy out of the powder burn and more efficiency during the bullet travel time through the barrel. Thus short barrels would require higher pressure but to detriment in handling. Loading a 50Cal can be done dangerously this way if not enough powder is in the chamber because the powder lays flat with a huge surface exposed to the primer spark, making for extremely quick but low energy high power/pressure burns.