Well, I don't know what happened to my post. Went into some secret internet dimension I guess. But the gist of it was along the lines of what Sylvan said:
First of all it isn't like KE and momentum oppose each other. They go together even better than peas and carrots. If you do something to increase KE you will by default be increasing momentum as well. If you do something to increase momentum you will by default be increasing KE.
The fact of the matter is, light arrows lose out on all counts. The lighter the arrow, the less efficient the bow is, the less momentum the arrow has and the less energy the arrow leaves the bow with. The only thing going for them is the flat trajectory to make hitting game easier at long distance. But then, what happens to performance out there at long range?
In the example used above, the 530 grain arrow carries more momentum which helps it retain it's energy better on it's way to the target. It will actually strike the target at 50 yards with 51 ft lbs, only 1 ft/lb of energy less than the 330 grain arrow has
at the bow! The 330 gn arrow has let it's KE dribble down to a measley 43 ft lbs.
Is it a big deal to worry about downrange performance when you're talking deer hunting and cranking out 70-80 ft lbs of energy? Probably not. Is it a big deal when you're shooting a bow that just barely the legal minimum draw weight for hunting? Not only is it a big deal to worry about downrange KE and momentum, it's CRITICAL!
I am very much in favor of establishing legal 400 grain minimum weights for hunting arrows.