FOC vs Penetration: Is It the FOC or Just the Weight?

First four rows: soft-tissue model, 70 lb / 29 in / 340 fps IBO compound, 28.5 in arrow, 3-blade fixed unless noted, 20 yd impact. Last row: Ashby’s measured buffalo data, 40 lb recurve.

How Much of Your Own Arrow’s Penetration Is FOC?

Where Did This Question Come From?

It comes up on every forum, every season. The version I keep going back to is a TradTalk thread asking whether a setup needs more FOC. The best reply came from someone who had tried extreme FOC and went back to a lighter, flatter arrow on a well-tuned bow.

That is the trade this page puts numbers on. FOC probably helps. It costs trajectory. And the grains that create it are doing a second job at the same time.

How Did I Model 280 Arrows?

Every number comes from three engine files the site’s calculators already run: the balance-point model in the FOC optimizer, the virtual-mass speed model from the trajectory tools, and the penetration model behind the pass-through calculator. Nothing was tuned for this article.

The compound baseline is a 70 lb bow, 29 in draw, 340 fps IBO. It shoots a 28.5 in arrow with a 16 gr insert, 9 gr nock and three 7 gr vanes. The grid runs four shafts (6.0 to 11.5 gpi), seven heads from 100 to 300 gr and five broadhead types, plus a 50 lb recurve. Impact is at 20 yards.

Energy and momentum are exact. Penetration is not. The model turns momentum into penetration potential, the inches of uniform soft tissue an arrow could cross, using a broadhead factor and an FOC factor. A broadside whitetail is about 15 in through; an elk about 22. There is no bone in it. The methodology page lists every constant.

All 280 builds are in a free CSV if you want to check a row.

Does FOC Still Help When Arrow Weight Stays the Same?

This is the clean test. Every arrow below weighs exactly 450 gr. I moved weight from the shaft to the head, so FOC runs from 11.1% to 29.5% while nothing else changes.

Speed, energy, momentum and trajectory are identical across the chart. The bow only sees 450 gr. So the 22% rise is purely the model’s FOC factor at work.

It is smaller than the roughly 40% gain Ashby reported for extreme FOC. I kept the site factor modest and capped at about 29% FOC on purpose. A calculator should not crown a winner on its least certain input.

What Happens When You Just Add a Heavier Broadhead?

This is what most of us actually do. Same 8.3 gpi shaft, heads from 100 to 300 gr. Weight and FOC now rise together.

Kinetic energy barely moves: 73.9 to 75.8 ft·lb. A compound hands out roughly the same energy and a heavier arrow takes a bigger share of it. Momentum is what grows, by 25%. I dig into why that matters in momentum vs kinetic energy.

Negative impact numbers are inches below the aim point at 40 yards with the pin set for 20.

So How Much of the Gain Is FOC and How Much Is Mass?

That depends entirely on what you believe FOC is worth. Here is the same 100 → 300 gr change run four ways.

The mass column never moves. Only the belief about FOC does. Rows ignore the site model’s cap so each assumption shows cleanly.

At the site default, FOC gets 45% of the credit and mass gets the rest. Believe FOC does nothing and mass gets all of it. Believe Ashby’s bigger numbers and FOC takes more than 60%. Anyone who quotes you a single figure is quoting an assumption.

How Do Hunters Actually Build a High-FOC Arrow?

There are only a few ways to move the balance point forward: a heavier head, a heavier insert, a weight tube behind the insert, or a lighter shaft carrying the same head. Here they are on the same 70 lb bow.

Elk-class tissue in this model is 22 in through, broadside. Weight tube modeled as 60 gr spread over the first 4 in behind the insert.

Look at the last two rows. The light shaft with a 200 gr head weighs 57 gr less than the heavy shaft with a 100 gr head, shoots 2.3 in flatter at 40 yards, and still models ahead on elk. That result rests entirely on the FOC assumption. With FOC set to zero, the heavy shaft wins on momentum. This is exactly the argument that fills forum threads, and now you can see what each side is betting on.

Whichever route you take, a heavier front makes the shaft act weaker. Check the new build in the dynamic spine calculator before you buy a dozen.

Does Broadhead Type Matter More Than FOC?

By a wide margin. Same 433 gr arrow, same 17% FOC, same speed. Only the head changes.

A 125 gr cut-on-contact build at 408 gr models to 32.1 in. A 250 gr mechanical build at 533 gr, with 9 points more FOC and 15% more momentum, models to 24.5 in and hits 5.0 in lower at 40 yards. My ranking: head design, then momentum, then FOC. The mechanical vs fixed broadheads guide covers the why.

What Does Real Bone Data Say About FOC?

My model has no bone, so I went looking for someone who shot arrows into real bone and wrote down every number. The cleanest set I found is Ed Ashby’s 2008 heavy-bone test on an Asian buffalo. Four arrow sets, 12 arrows each, same 190 gr head, same 40 lb recurve, 20 yards, into ribs about as thick as a big bull elk’s.

Same 190 gr broadhead on every arrow, 40 lb recurve at 20 yards, 12 arrows per set, one young adult bull. Numbers as published by Ed Ashby.

Two things jump out. First, getting through the rib tracked weight, not FOC. Sets 1 and 2 weigh the same and carry the same momentum. One has 32% FOC, the other 12%. Both broke the rib on exactly 6 of 12 shots. Add about 70 gr and both heavier sets broke it every time. Ashby put that step near 650 gr for this setup.

That 650 gr figure belongs to a 40 lb recurve pushing about 0.35 slug·ft/s. A 450 gr arrow from a 70 lb compound carries half again as much momentum, so its bone step probably sits lower. Nobody has published that measurement, so take the order from this test, not the number.

Second, once through, FOC ran away with it. Among arrows that broke the rib, the 32% FOC set went 96% farther on average than the 12% set, and 115% farther on the median. The shortest high-FOC arrow still out-penetrated the deepest normal-FOC one. That is a much bigger FOC effect than anything in my soft-tissue model.

His wider work points the same way. In his 2019 summary of penetration factors, Ashby reports 40% to 60% gains for extreme FOC across tissue tests, and a single tip bend costing 14% on average. Treat all of it with the right caution: one animal per test, low-speed recurve arrows, and one researcher. It is still the best measured data I can find, and it says my soft-tissue number is a floor.

What I take from it: on broadside whitetail lungs, the model above is a fair guide. On elk, bear, hogs or quartering shots, get the weight high enough to break bone first, then build FOC on top, on a shaft stiff enough not to bend at impact.

Does the Answer Change on a Traditional Bow?

The ranking holds. The drop bill gets much bigger.

Recurve speed here assumes the bow delivers the same energy to every arrow weight. Real recurves get a little more efficient with heavier arrows, so the true drop cost is somewhat smaller.

Going from 125 to 300 gr adds about 17% momentum and 17 in of extra drop at 40 yards. Real recurves get a bit more efficient with heavier arrows, so the true drop penalty is somewhat smaller than this. It is still a lot for a hunter judging yardage by eye.

What Is the Bottom Line on FOC and Penetration?

In soft tissue, weight is the part you can prove and FOC is a modest bonus. On bone, the measured data flips the picture: weight gets you through, and FOC decides how far you go after that. So for whitetail, build head, then weight, then FOC. For elk and bigger, carry enough weight and momentum to break bone, then push FOC as far as your spine allows. If two arrows weigh the same, take the one with more FOC.

Start with the FOC optimizer, check the speed cost in arrow weight vs speed, and make sure the shaft can carry the head. My guide to why spine charts disagree helps with that last part.

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