What a build review checks
A build review is a systematic check of a finished arrow against the bow it will be shot from and the job it has to do — geometry, mass, spine, component fit, and consistency across the dozen. It is a pre-flight checklist with arithmetic attached, run after the arrow exists and before you trust it. It is not a score, and it is not a substitute for shooting the thing.
The value of doing it in one pass is that arrow problems are almost never isolated. A build that is 40 grains heavier than intended is also slower, also lower on the sight tape, and possibly weaker in dynamic spine than the shaft you selected. Reviewing one number at a time hides those relationships. Reviewing the whole build surfaces them.
The five checks, in order
| Check | What it answers | How you confirm it |
|---|---|---|
| Geometry | Cut length, full arrow length, balance point, FOC. | Measure. Balance the finished arrow on an edge; do not trust a calculated balance point. |
| Mass | Total weight, and where that weight sits front to back. | Grain scale on the finished arrow, and again on each component if the total disagrees with the spec. |
| Spine | Whether the shaft's stiffness suits this bow with this point weight at this cut length. | Dynamic spine estimate first; paper, bareshaft and broadhead second. Only the second one is proof. |
| Components | Whether the parts actually fit each other and the bow. | Nock fit on the string, insert concentricity, collar fit to shaft ID, vane clearance past the rest and cables. |
| Consistency | Whether the twelve arrows are the same arrow twelve times. | Weigh and sort the dozen. Check the shaft's straightness tolerance class. Index nocks. |
Geometry and mass
FOC is defined by the Easton ATA standard and it is worth writing out, because a surprising number of published FOC figures use a different denominator and are therefore not comparable:
FOC % = ((balance point − L/2) / L) × 100, where L is the full arrow length measured from the throat of the nock to the end of the shaft, and the balance point is measured from the same origin.
Total weight is the sum of shaft mass (GPI × cut length), insert or collar, point, nock, vanes and wrap. If your scale disagrees with your spreadsheet by more than a couple of grains, the discrepancy is real and worth finding — glue mass, a heavier-than-catalogue insert, or a wrap nobody counted. Chasing that difference is how you learn that your "500-grain build" is a 517-grain build, which is a different arrow on a sight tape.
Energy and momentum
Two derived numbers, both trivial arithmetic once you have weight and a measured velocity. The canonical forms:
Kinetic energy (ft·lb) = (arrow weight in grains × velocity in fps²) / 450,240
Momentum (slug·ft/s) = (arrow weight in grains × velocity in fps) / 225,400
Note what those formulas do differently. Energy scales with the square of velocity, so it rewards speed heavily. Momentum scales linearly with both, so it weights mass and speed evenly. That is the entire reason the light-and-fast versus heavy-and-slow argument never resolves — the two camps are optimising different quantities, and each is right about the quantity it chose.
We deliberately do not put threshold numbers on this page. Minimum-energy and minimum-momentum figures are game-class policy calls, not derivations, and they belong on the pages where the reasoning behind them is written out rather than floating loose on a build checklist.
Static spine is not dynamic spine
The number printed on your shaft comes from a standardised bench test: support the shaft at a fixed span, hang a fixed weight in the middle, measure the deflection. It describes a shaft lying still on a table. It does not describe what happens when your cam system dumps energy into that shaft with a broadhead on the front.
Dynamic spine is the estimate of that behaviour, and it moves with cut length, point and insert mass, nock and fletching mass, and the bow's draw weight. A build review reports it so you know whether you are near the middle of a window or out on the edge of one — and being on the edge is not automatically wrong, it just means you have less margin when you change a point weight later.
Consistency across the dozen
An arrow that flies well is worth little if the other eleven do not match it. Two published shaft properties govern how much sorting you will have to do yourself.
| Straightness tolerance | What it means |
|---|---|
| ±.001" | Tightest common class. Maximum deviation from true along the shaft. |
| ±.003" | Standard for most quality hunting shafts. |
| ±.005" | Value class. |
| ±.006" | Entry class. |
Two honest caveats. Straightness is a manufacturing tolerance, not a promise about flight — a component installed off-axis will out-do a tenth of a thousandth of shaft runout every time. And a tighter tolerance class only matters if your build discipline is tighter than the tolerance: there is no point paying for ±.001" shafts and then installing inserts by eye. Squaring the shaft ends and seating components concentrically buys more than a tolerance class upgrade does.
Weight sorting is the other half. Weigh the finished dozen, sort into matched sets, and shoot the tightest set at distance. For reference, arrows we build in the atelier are matched for spine and weight within half a grain — which is achievable at a bench and tells you what the ceiling looks like.
The parts nobody checks
Nock fit on the serving is the most under-inspected variable in archery: too tight and the arrow drags off the string, too loose and it can fall off at draw. It should hold the arrow's weight and release with a firm tap. Insert and collar concentricity decides whether your point spins true — spin-test every finished arrow and reject any that wobbles. Vane clearance is a yes-or-no question answered with powder spray, not an opinion, and a vane striking the rest or the cables will out-shout every other virtue in the build.
A review is a prediction until you shoot it
Every number above is calculated from what you told the tool. Confirmation happens on paper and then with bareshafts and then with broadheads, in that order — the sequence is on our tuning reference. A build that reviews perfectly and tears badly is telling you something the arithmetic could not: usually clearance, nock indexing, or a bow that is not in spec. Believe the paper.
Common questions
What should a finished arrow build be checked for?
Geometry (cut length, measured balance point, FOC), mass (total weight and its distribution), spine suitability for the bow and point weight, component fit (nock fit, insert concentricity, vane clearance), and consistency across the whole dozen.
How do I calculate FOC?
FOC % = ((balance point − L/2) / L) × 100, where L is the full arrow length from the nock throat to the end of the shaft. Measure the balance point on the finished arrow rather than calculating it — glue, wraps and component tolerances all move it.
What is the difference between kinetic energy and momentum for an arrow?
Kinetic energy scales with the square of velocity, so it rewards speed disproportionately. Momentum scales linearly with both weight and velocity, so it weights them evenly. They are different questions, which is why fast-and-light and heavy-and-slow builds can both look good depending on which one you quote.
Does shaft straightness tolerance actually matter?
It matters less than component installation. A tolerance class only pays off if your build discipline is tighter than the tolerance itself — squared shaft ends and concentric inserts buy more accuracy than upgrading from ±.003" to ±.001" does.
Why does my build weigh more than the calculator said?
Usually glue, a wrap that was left out of the spec, or a component that is heavier than its catalogue figure. Weigh the finished arrow and reconcile the difference rather than ignoring it — the extra grains change your velocity and therefore your sight tape.
Should I check nock fit on every arrow?
Yes. The nock should hold the arrow's weight on the string and release with a firm tap. Too tight drags the arrow off the string at release; too loose is a safety problem. It costs seconds and it is one of the most common causes of an unexplained flyer.
Can a build review replace paper and bareshaft tuning?
No. Everything a review reports is calculated from your inputs. Paper, bareshaft and broadhead tests are the only things that measure what the arrow actually does leaving your bow.
Build Review is part of The Forge by Sparrow Expeditions. Every formula and constant used above is documented on our Methods page. Built by archnerds, for archnerds.