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My 331 is set up with a 27.5" draw length, and 70 lbs. limbs maxed out.

The experiment started as a result of the fact that I just put together some new Easton XX75 Platinum Plus aluminum arrows (size 2213) for 3d/target shooting with my Ross. The total arrow weight for these is 378.4 grains, which is 5.406 gpp.

Previously, I was shooting 45/70 CarbonTech Cheetah's with a total arrow weight of 345.2 grains, which is just a *hair* (5 grains) under the 5 gpp minimum at 4.929 gpp. This is fine given my short draw length of 27.5" and it's inherrently reduced power stroke.

Here's where it gets interesting... I knew from a previous experiment that the fps generated by shooting my Cheetah's did not seem to change at all regardless of whether I was shooting these arrows tipped with 100 gr. or 125 gr. field points... With both setups, the arrows impacted identically elevation-wise at 40 yards. Further, shooting the Cheetahs with 125 grain tips put my arrow much closer to the 5 gpp minimum, and the arrows seemed to group better. The speed of the Cheetahs is 280 fps.

This kind of stumped me though given that it *should* be physically impossible two arrows are 25 grains different to impact at the same elevation... I figured either it had to do with diminishing efficiency with with the lighter arrow, or possibly had something to do with the difference in FOC %.

Here is where it gets REALLY interesting. Yesterday I printed up a new sight tape for the new Easton XX75's using the On Target Tapes and Charts program. This program had accurately predicted my fps with the Cheetahs, resulting in a dead-accurate tape from the very first one I printed, so I figured the same would hold true for my heavier XX75 arrows. The Tapes and Charts program calculated that my speed with these new arrows would be 264 fps, so I printed up the corresponding tape.

In going to shoot these arrows yesterday, I started noticing that they were all impacting high at the corresponding yardage marks for 20, 30, and 40 yards. To experiment, since I had both tapes attached to my sight at this point, I decided I would try using the old tape for the Cheetah's instead. Low and behold, the 33.2 grain heavier XX75's impacted IDENTICALLY to the Cheetahs!

Now, I realize this should be physically impossible, and I haven't chrono'd the new arrows to verify speed, but accoring to point of impact, the speed with the heavier arrows would appear to very close, if not identical to that of the Cheetahs!

The only two things I can figure again are that this has something to do with picking an arrow weight that maximizes the efficiency of the Ross bow, or the different FOC % of the Aluminum arrows, which is right around 8%, versus the 13+% FOC of the Cheetahs.

If this is the case, it's a no-brainer to shoot the heavier arrows given that the trajectory appears identical, and they're going to have KE given then added weight.

I should also add that this bow is perfectly group tuned and punching bullet holes with bow shafts, no adjustments needed depsite the differing diameter of the shafts.

I realize what I am experiencing isn't physically probable... but does anyone have a better explanation of what might be happening here?

My hypothesis is that it seems that there would appear to be a diminishing level of effiency when shooting progressively lighter arrows out of a Ross bow, which is really pretty cool if you think about it, since max efficiency with heavier arrows = max kinetic energy!