Field Notes · v140 launch companion
My 500-Grain Eland — What Ashby Missed
There's a hunt that has stayed with me for years now, and the math behind it didn't add up the way the paper said it should.
500-grain arrow. 15% FOC. Grim Reaper Carnifour up front. Clean tune, dialed bow, ethical broadside shot on a 2,200-pound eland bull.
On the Ashby chart that's not a "should it pass through" question. It's a "how cleanly does it pass through" question. The mass is there. The FOC is well past Ashby's old 12% threshold. The head is a respected mechanical from a category-defining brand. Run the numbers and you walk away expecting to find your arrow buried in the dirt on the other side.
It almost passed through. Almost. The arrow stopped inside the off-side ribs.
The bull went down. The shot was lethal. But the paper was wrong, and that's the part I want to walk through, because it's the reason Sparrow Forge exists.
What Ashby's model doesn't model
Dr. Ashby's penetration work is a foundational contribution to bowhunting. I don't want anyone to read this and think I'm dismissing it. Heavy + high FOC + clean cutting head was a genuine corrective to the "fastest possible arrow" arms race that dominated the early 2000s. He moved the conversation in the right direction.
But the model anchors penetration to arrow specifications — mass, FOC, head type — and treats the arrow's flight quality at impact as if it's a given. In a controlled gel test, where you can hand-launch an arrow on a perfect axis from three feet away, it kind of is.
On an eland at 40 yards with a 1.5° asymmetric fletch and an indoor tune that didn't survive a desert temperature shift? It isn't.
What the model doesn't capture:
- Yaw at impact — how much the arrow is "tail-kicked" sideways when the broadhead first contacts hide
- Cutting-plane angle of fixed or mechanical blades to the bone surface they hit first
- Speed-vs-mass tradeoff in flight recovery — a heavy arrow with launch yaw has less drag-time to dampen before impact than a lighter, faster arrow with the same launch yaw
- Mass-class-specific bone density in the largest African game — eland bone is structurally closer to cape buffalo than to plains game, and the model treats them all as one curve
Every one of those variables shifts penetration by ten or twenty percent. Stack three of them, and you get the difference between "buried in the dirt" and "stopped inside the off-side ribs."
What changed in how I think about builds
After that hunt, I stopped optimizing for FOC the way I used to.
I still want a heavy arrow on the largest game. I still want a single-bevel or COC fixed-blade head where the species and shot scenario justify it. I still build above 12% FOC, because the geometry advantages are real for spine-on-spine alignment.
But the variable I weight first now is tune-quality at the moment of impact — because nothing else matters if the arrow is fighting itself when it touches hide.
That's also why every page on this site about "what to choose" leads with bareshaft testing, paper tune, and the new yaw-quality input on the build score. The Ashby chart is still useful as a directional input. It is not the answer.
What this means for the Forge build calculator
The Sparrow Forge Score weights measured tune-quality at 40% — more than mass and FOC combined. That number isn't arbitrary. It's anchored on hunts like this one and others I'll write up over the next few months — including a 700+ pound stag where a clean-tune 460-grain mech did catastrophic joint damage that the same head at the same weight could not have done on a yawed arrow.
Mass matters. FOC matters. Head choice matters. But none of them save you from a 6° launch yaw that you didn't catch on the practice block.
Check your bareshafts. Measure your fletched-vs-bareshaft drift. Use the photo-detection tool in the Range Log if you want a number on it. And next time you're staring at a chart that says your build "should" pass through, ask yourself the question the chart can't: will this arrow be flying clean when it gets there?
That eland taught me to ask it.
The math behind those corrections is in the methods page § 12 — every formula, every calibration anchor, every region of the model that's still under-determined.