Arrow GPI: Grains Per Inch and Finished Arrow Weight
GPI is grains per inch of raw shaft weight. Multiply GPI by shaft length, add insert, point, nock, and vanes, and you have your finished arrow weight. Every conversation about kinetic energy, momentum, and penetration starts here.
Typical GPI by shaft family
| Shaft | Spine | GPI |
|---|---|---|
| Easton 5mm Axis | 340 | 9.5 |
| Easton FMJ 5mm | 340 | 11.3 |
| Easton 4mm Axis Long Range | 340 | 10.8 |
| Gold Tip Hunter XT | 340 | 9.3 |
| Gold Tip Kinetic Kaos | 300 | 12.1 |
| Gold Tip Airstrike | 340 | 8.2 |
| Victory VAP TKO Elite | 350 | 8.7 |
| Victory RIP XV | 350 | 8.5 |
| Black Eagle Rampage | 350 | 9.3 |
| Black Eagle Spartan | 300 | 11.7 |
| Carbon Express Maxima Sable RZ | 350 | 9.1 |
Building to a weight target
Common hunting targets are 425 to 450 grains for whitetail, 475 to 525 grains for elk, and 550 grains and up for moose or heavy bone. Example: a 28.5 inch Easton Axis 5mm 340 at 9.5 GPI weighs 271 grains bare. Add a 20 grain HIT insert, 8 grain nock, 24 grains of vanes, and a 125 grain broadhead and you finish at 448 grains. Sub 400 grain hunting arrows raise real Ashby style penetration concerns on big game.
Momentum, not just kinetic energy
Kinetic energy scales with the square of velocity but momentum scales linearly with mass. Two arrows with the same 80 KE can have very different penetration because momentum is what actually pushes the shaft after impact. This is why elk hunters trade 8 fps of speed for 60 more grains of arrow weight. Ranch Fairy videos on YouTube and Ashby’s original test summaries (bowhunterspgh.com hosts the archive) walk through this in detail.