Ok, I'm going to try and take the scientific approach to this. In this scenario, like Marc said, the bow has too much mass for a 50 lb bow. The limbs can only move so fast with a given load. If the bow is shooting the same speed with with two different loads, then essentially the bow is at dry fire speed. With the lighter arrow, energy is being wasted, the energy has to go somewhere, this will be in the form of limb vibration and hand shock. This may be a terrible analogy, but put a piece of gravel in the back of a diesel pickup, and floor it, record acceleration, keep adding rock and eventually acceleration will start to slow at a noticeable rate, this will be your max efficiency. But when you add only a few lbs of rock, the pickup doesn't even notice it, just like you bow doesn't notice the added arrow weight. Too much engine for what your hauling. So with this bow, you would keep adding arrow weight until your arrow speed started to slow. To say the bow shoots like a 70 lb bow, sure, if you want to say that. It's hard to make that statement scientifically. Ideally, you would remove mass until the bow shot the 500 grain arrow at the point where any larger arrow would start to accelerate at a slower rate. This maybe hard without reducing any draw weight. Every bow is so different, each will have its optimum arrow weight to achieve max efficiently, all affected by design. BUT, I may be wrong on all this. Maybe someone can find some error in what I've said. Or maybe I've said too much....
Eric
