Function default_twist_inches 

pub fn default_twist_inches(
    diameter_m: f64,
    mass_kg: f64,
    muzzle_velocity_mps: f64,
) -> f64

Synthesize a default barrel twist (inches per turn) for a bullet whose twist the shooter did not supply, by inverting the Miller stability formula for the twist that yields DEFAULT_TWIST_SG_TARGET (MBA-1135).

This replaces the mass-blind fixed 1:12" fallback: heavier / longer bullets correctly demand a faster twist. The bullet length is taken from estimate_bullet_length_m (so the whole default is caliber + weight aware), the velocity term is (v_fps / 2800)^(1/3) at the supplied muzzle velocity, and the density correction is 1.0 (sea-level ICAO standard).

Inverting Sg = 30 m_gr * vel_corr / (t_cal^2 * d_in^3 * L_cal * (1 + L_cal^2)):

t_cal = sqrt( 30 m_gr * vel_corr / (Sg_target * d_in^3 * L_cal * (1 + L_cal^2)) ), twist_in = t_cal * d_in.

Sg_target was selected by comparing 1.5 vs 2.0 against common factory twists (velocities: .308/175 gr @ 2600 fps, .223/55 gr @ 3240 fps, .224/77 gr @ 2750 fps, 6.5mm/140 gr @ 2700 fps):

bullet	factory	Sg=1.5	Sg=2.0
.308 / 175 gr	1:10-12“	1:12.9“	1:11.2“
.223 / 55 gr	~1:12“	1:11.7“	1:10.1“
.224 / 77 gr	~1:8“	1:8.4“	1:7.2“
6.5mm / 140 gr	~1:8“	1:8.9“	1:7.7“

2.0 wins: Sg=1.5 gives 1:12.9“ for the 175 gr match bullet (no real .308 barrel is that slow and it under-stabilizes 175 gr), whereas 2.0’s 1:11.2“ / 1:8“ are exactly the standard twists for the twist-sensitive high-BC bullets, and slight over-stabilization is the safe direction for a default.

Guards degenerate inputs (diameter_m, mass_kg, or muzzle_velocity_mps <= 0) by returning the historical 12.0 (1:12“) fallback. Always returns a positive inches-per-turn value.