Maybe it's possible for an arrow shot at an extreme downhill angle to increase it's speed, energy and momentum, IF gravity can give it enough of a boost to overcome the wind resistance. Possible?!? On a level shot, wind resistance is going to guarantee a loss in speed, energy and momentum. But the heavier arrow will definitely retain what it leaves the bow with better than the light arrow.

Two arrows from the same bow, one maybe 350 grains and the other, say 600 grains. The light arrow will have greater speed, only slightly less energy and a good bit less momentum. The heavy arrow will have a good bit less speed, slightly more energy and greater momentum. The sliding scale and trade-off thing I was talking about on the other thread. You gotta give up something to get something else. Increased speed and better trajectory vs increased momentum and better penetration.

The light arrow has less momentum working for it, so it uses up a higher percentage of it's speed and energy just getting to the target than the heavy arrow. The light arrow doesn't even penetrate AIR as well as the heavy arrow. An animal's ribcage?? Well.... I believe flesh and bone is more dense and resistant than air.

You CAN jack up the speed enough to increase momentum to equal the heavy arrow, BUT... The heavy arrow would have to be shot from a 60 pound, round wheel bow vs the light arrow from an extremely high poundage cam bow.

People like to talk energy because it's easier to understand 60 ft lbs vs 80 ft lbs. 6578 pound/seconds... What the heck is THAT? Momentum in that form shows momentum's relationship to how energy is expended over time but it is hard to understand in that form.

I like the old formula I used to see all the time in the gun magazines. I think it's the Hatcher penetration index, but it's been years since I was into handguns and I don't remember too well. Anyway, it's projectile weight in grains times speed divided by 10,000.

A 350 grain arrow at 300 fps and 70 ft lbs will have a penetration index of 10.5.

A 600 grain arrow at 230 fps and 70 ft lbs will have a penetration index of 13.8.

To get the 350 grain arrow up to 13.8, you have to drive that suckah at 394 fps for 120 ft lbs! Will an arrow with 120 ft lbs penetrate better than an arrow with 70 ft lbs? I would HOPE so! But which would you rather shoot on a daily basis?

13.8 vs 10.5 doesn't sound like a great deal, but look at the percentages. Momentum is nearly 40% higher at a 24% sacrifice in speed. Gain in penetration potential vs loss in trajectory.

The ballistic table at Jacksons seems to be pretty accurate at predicting downrange velocity loss. You can plug your numbers in and figure the index at different yardages. It will steadily decrease at each distance, but the heavy arrow will show higher retention of what speed it does have, along with energy.

So, I guess that's the long version of saying I agree with Frank. Your arrow had to have been wibbly wobbly at 20 yards but finally stabilized when it got to 30 for there to have been better penetration at 30. Or you shot a soft spot in the target at 30.

Now that I've posted, I'll go get a cup of coffee and see if I can understand what I just wrote. [:-]