Arrow build for the Bowtech BTX 31 (2016) — spine, weight, speed
The short answer. On a Bowtech BTX 31 (2016) at 70 lb and a 29″ draw, the engine builds a 382-grain arrow on 300 spine flying about 312 fps, making 83 ft‑lb of kinetic energy, 0.53 slug‑fps of momentum, at 13.3% FOC. Every number below is this engine's output for this exact bow — not a chart, not a rule of thumb, and not copied from anybody's spec sheet.
The Bowtech BTX 31 (2016) is rated at 335 fps IBO with 80% let-off. That IBO figure is measured with a 350‑grain arrow at 30 inches and 70 pounds — a test arrow, not a hunting arrow. The tables below show what the bow actually does with an arrow you would take into the woods, at every draw weight it offers.
The recommended starting build
- Spine: 300 — engine-picked for 70 lb at 29″ with a 100‑grain point
- Total arrow weight: 382 grains
- Estimated speed: ~312 fps
- Kinetic energy: 83 ft‑lb
- Momentum: 0.53 slug‑fps
- FOC: 13.3%
▸ Open this exact build in The Forge — free, and it loads with every component above already selected.
Every draw weight, side by side
Spine is not a property of the bow. It is set by draw weight, arrow length and point weight acting together, which is why the engine's pick changes down this column. If your bow is not maxed out, read the row that matches your actual poundage.
| Draw weight | Spine the engine picks | Total arrow weight (gr) | Speed (fps) | Kinetic energy (ft‑lb) | Momentum (slug‑fps) | FOC |
|---|---|---|---|---|---|---|
| 60 lb | 340 | 356 | 302 | 72 | 0.48 | 14.3% |
| 65 lb | 300 | 382 | 302 | 77 | 0.51 | 13.3% |
| 70 lb | 300 | 382 | 312 | 83 | 0.53 | 13.3% |
What one inch of draw length costs
| Draw length | Spine | Weight (gr) | Speed (fps) | Kinetic energy (ft‑lb) | FOC |
|---|---|---|---|---|---|
| 27″ | 300 | 382 | 292 | 72 | 13.3% |
| 28″ | 300 | 382 | 302 | 77 | 13.3% |
| 29″ | 300 | 382 | 312 | 83 | 13.3% |
| 30″ | 300 | 382 | 322 | 88 | 13.3% |
What this build is cleared for
We grade against five energy floors and we tell you which of them are Easton's numbers and which are ours. Three come from Easton's game-class chart, which Easton retired in 2012 and which we reproduce from archive captures on the kinetic energy chart by game class. Two are ours, labelled as ours.
| Game class | Floor (ft‑lb) | Whose number | This build at 83 ft‑lb |
|---|---|---|---|
| Small game | 15 | judged | clears by 68 |
| Deer / antelope | 25 | charted | clears by 58 |
| Elk / bear / boar | 42 | charted | clears by 41 |
| Heavy game | 50 | derived | clears by 33 |
| Big game | 65 | charted | clears by 18 |
Energy is a floor, not a plan. It tells you whether an arrow has enough to work with; it does not tell you where to put it. For that you want the target size — broadside vital dimensions for 61 species are on the vital zone size page, with the sourced shoulder heights they derive from.
Bow specifications
- IBO speed: 335 fps
- Let-off: 80%
- Axle-to-axle: 31″
Common questions
What arrow spine do I need for a Bowtech BTX 31 (2016)?
At 70 lb and a 29-inch draw with a 100-grain point, the engine picks 300 spine for a 382-grain finished arrow. Drop to 60 lb and the pick moves to 340 spine at 356 grains. Spine is not a property of the bow — it is set by draw weight, arrow length and point weight together, which is why the table above shows every draw weight this bow offers rather than one number.
How fast does a Bowtech BTX 31 (2016) shoot a real hunting arrow?
About 312 fps with the 382-grain build above at 70 lb and 29 inches. The bow's advertised 335 fps IBO is measured with a 350-grain arrow at 30 inches and 70 pounds, which almost nobody shoots. The gap between 335 and 312 is not the manufacturer lying — it is the difference between the IBO test arrow and a huntable one.
Is this arrow heavy enough for elk?
Yes on energy: 83 ft-lb against the 42 ft-lb elk, black bear and boar floor — a margin of 41 ft-lb. That 42 is Easton's charted figure, not ours. It also clears the 25 ft-lb deer and antelope floor. Energy is only half the question though: penetration on elk is driven by momentum and FOC as much as by kinetic energy, and this build sits at 0.53 slug-fps and 13.3% FOC.
What game does this Bowtech BTX 31 (2016) build clear?
At 83 ft-lb it clears small game, deer / antelope, elk / bear / boar, heavy game, big game. It clears every floor we publish. Two of those floors are Easton's charted numbers (25 and 42 ft-lb), one is Easton's top band (65), one is ours placed between charted anchors (50), and one is ours with thin evidence that we label as such (15). We publish which is which rather than presenting all five as industry standard.
Where do these numbers come from?
The same physics engine that runs The Forge, with no hand-tuning and no chart lookup. Launch speed is IBO-corrected for your actual draw weight, draw length and arrow mass. Kinetic energy and momentum use the Easton method. The spine pick comes from a model validated against the published manufacturer charts — 98% agreement with Easton, 85% with Gold Tip. Bow specs (335 fps IBO, 80% let-off) are the manufacturer's published figures. When you chronograph your real bow, your measured speed replaces the estimate everywhere.
Method, and what we are not claiming
Launch speed is the bow's published IBO corrected for your actual draw weight, draw length and finished arrow mass. Kinetic energy and momentum are the Easton method. The spine pick comes from a coefficient model published in full on the spine what-if page, including its measured agreement with each manufacturer's chart.
What we are not claiming: that this is the arrow you should shoot. It is the arrow this engine builds from a sensible hunting default on this bow, so that the numbers on this page are reproducible rather than editorial. Your draw length, your point weight and your broadhead all move every figure here, and the permalink above opens the build so you can move them.
Built by archnerds, for archnerds — The Forge by Sparrow Expeditions.