I could be way off. But my thinking is that a material like foam, that can be compressed… that there’s like some resistance on the tip of the arrow as it enters the foam and compresses the foam as it penetrates. I’m sure you would see some differences in penetration from one arrow to the next but maybe that compressive resistance is enough to hide the effects of friction alone. I don’t know… You would think a heavier arrow would compress more foam… But who knows. Where’s @Milo when you need him?Perhaps. But, being that foam stops by friction, using the same diameter shaft I expected to see some difference if there was any gain by shooting a slightly heavier arrow.