I've taken some grief on my opinions about arrow weight, so maybe I can clarify my views a bit.
I view momentum and KE both as parts of the same puzzle though, in my view, momentum is the key. For a given bow, both KE and momentum increase as arrow weight increases. However at a certain point, arrow weight will get too heavy for the bow. KE and momentum don't increase as much and trajectory goes completely down the tubes. The arrow has gotten out of the bow's efficiency range.
Try as they might to change things, arrow weight selection is still a matter of tradeoffs and balances. You can't have one thing without giving up something else. To get best speed and flattest trajectory, you sacrifice KE and momentum. To get highest KE and momentum, you must sacrifice speed and trajectory.
Lets say a bow's safety and efficiency ranges run from 5 grains per pound of draw weight to 12 grains per pound. Then you can draw a line chart like this:
Light ----------------------------------Heavy
Trajectory ---------------------------- KE & Momentum
5 gpp-----------------------------------12 gpp
_________________^__________________
On my compound, I like my arrow weight to be from the middle to about 3/4 way up the heavy side, about 8.5 to 10 gpp. I shoot a 60 pound, round wheel bow at 33.5" draw. With my stickbows, I want that arrow weight up around 10-11 gpp. I have a long power stroke, 25" or so, and heavier weights are more efficient. (Think of a golf ball and a pingpong ball. Use a long, hard overhand throw and the golfball goes a long way. The pingpong ball just kinda flings out there a few feet and falls to the ground.) Also, I don't produce extraordinary amounts of KE, so I compensate for that by having increased momentum.
If I was shooting heavier poundage and hard cams, I'd probably slide my selector a bit to the light end, around 7-8 gpp, and still have plenty of oomph for hunting anything in North America.
For someone with a shorter power stroke and heavy enough draw weight, to still produce gobs of KE, they can slide the selector button over to the light side, say 6-7 gpp. (Using the golfball/pingpong ball analogy again, thump the pingpong ball with your finger and it rockets across the room. Thump the golfball, and it just rolls across the table while you grab your bruised finger and howl.[:-]) The lighter arrow is more efficient with the shorter power stroke.
Now, for someone that does not shoot a heavy draw and has a short draw length, they tend to slide the selector button even further to the light side, to get better trajectory. This is fine for a 3D arrow, but a big mistake for a hunting arrow. In order for them to get a deep penetrating hunting arrow, they need to slide the selector button UP the weight scale. They need to do like I do and compensate for their low KE values by maximizing their momentum. The less KE they produce, the further up the scale they should go. Of course, they will make some sacrifices in trajectory but with a low KE setup, they don't have much business taking hunting shots beyond 25 yards anyway. IMO, people would do well to maintain their minimum arrow weight at no less than 400 grains. Weights lower than that should be left to highly experienced people that know EXACTLY what they are doing, and are prepared to deal with the risks they know they are taking.
Then broadhead selection becomes part of the issue. When going less than 40 ft lbs of energy, I'd shoot nothing but cut on contact type heads. I would not shoot mechanicals unless I had 60 ft lbs or better. I'd want even more KE with the wide cutting, multiple blade mechs.
Arrow Weight Selection
