Ok, I know it sounds crazy and probably splitting hairs BUT. If you have a 10 grain alum. insert and a 190 grain point and a 50 grain brass insert and a 150 grain point is the spine going to change? Thanks. BILL
If both the brass insert and aluminum insert share equal length such as 0.9"s, then you should expect the same arrow flight.
If the brass insert, for example was 1.2"s, then 0.3"s would be acting as a footing thus stiffening the shaft.
Note: Typically, aluminum insert shaft length is virtually identical to a 50 grain brass insert shaft length.
In my experience, not enough for you to know it at reasonable hunting and shooting distances.
I think that if the weight is in front of the main "flex" in the shaft at paradox, it would be serving as forward weight, and would cause the shaft to flex, making it weaker. If it is behind that flex point, it would then be dead weight that absorbs some of the arrow's energy, thereby contributing to the stiffness of the arrow. I have at times put long screws into the backs of my inserts to increase the point weight, and they extended more than twice the length of the inserts. They did just what I expected them to do when I worked on tuning, effectively weakening the shaft.