I saw an Ashby quote somewhere the other day that mentions arrow weight comes before foc.
But before I saw that quote, the other day after a long shooting session, in which I can get a little sloppy, yes I should stop to avoid picking up bad habits but I didn't. Any way, I missed the target with a 30% foc 600gr arrow and hit the 3/4 plywood on the side of my foam target at 30 yards. I do this to often here lately, the weather has just been so nice lately and I do over do it. Any way, the 300gr fieldpoints on these arrows never go all the way through the plywood, just halfway.
Now I also have a setup that weighs 659grs but is only 25% foc. Well, you can guess what has happened here too, have a I mentioned the weathers been so nice :D well guess what, these arrows shot from the very same bow go through that 3/4 plywood every time. I was really surprised when it first happened and now, I am convinced that over 650grs has merit and probably is a bone breaker. I am so convinced of the penetration potential, that my elk arrows will weight over 650 this year.