ORIGINAL: Sylvan

Going to a heavier arrow ALWAYS results in an increase in both momentum and kinetic energy. The reason for this is that, upon release, stored potential energy in the limbs is transferred more efficiently to the kinetic energy of arrows of greater mass than of lower mass. That's just fundamental mechanics and the rule can't be violated.

Sometimes, especialy when the increase in arrow mass is small and/or measurement equipment/setup in testing is not precise, one can obtain results that SEEM to go against the physics. The reality is simply that measurement noise is larger than the effect of the change in mass.

In short, if you are trying to maximize ke/momentum there is no optimum arrow weight. Heavier is always better. However, as some have pointed out, the heavier the arrow the lower the velocity and hence there's the rub. As you gain in penetration potential you lose in trajectory so this is the tradeoff you have to accept.