Twist Rate & Stability Factor Calculator
The usual twist guidance is a grain-weight rule of thumb, and grain weight is not what decides stability — projectile length, velocity and air density are. This runs the Miller rule with the velocity and atmospheric corrections applied, returns your gyroscopic stability factor, and tells you the slowest twist that still clears a working Sg of 1.5 in the conditions you actually shoot in.
Bore diameter in inches — .224 covers 5.56 and .223 Wylde, .308 covers 7.62 NATO.
Weight decides nothing on its own — it is here because the formula needs a mass.
The variable the grain-weight charts throw away. Measure it, or take it from the maker’s dimensional data.
Enter 8 for a 1:8 barrel, 7 for a 1:7.
From your barrel length, not from the box.
Cold air is dense air, and dense air is what destabilises a projectile.
Thin air stabilises. A load that keyholes at sea level can be fine at 6,000 ft.