You are fixing on killing yourself or somebody if you are bareshafting with a broadhead. All this is point weight and not plucking etc.. is real good advice with fletched arrows, but the best is quit bareshaft tuning with broadheads. They go everywhere without fletches because of what has been stated above even when tuuned perfect.
Plucking, weak, stiff, or absolutely perfect doesn't really matter as they will never consitently tune without fletchings! Seriously, I have tried it. I have read where a high foc will stop this but I don't buy it. For one I have tried it, but only briefly because I was afraid I was going to kill something or somebody inadvertantly, and I have yet to find those arrows. Number two, if there isn't a drag on the rear end of that arrow to keep the broadhead from planing, it's going to go whatever direction if happens to be pointing due to that planing. It simply does not work.
God Bless