[Unclenick]

I probably should have mentioned why 15 feet is common in published load data. It is the average distance between screens specified by SAAMI for standard velocity measurements. When you see a velocity published for a commercial round of ammunition, for example, it will have been fired from a SAAMI specified velocity barrel, which has a minimum chamber, same as a pressure barrel, and a specified length (see below). Indeed, SAAMI specs allow pressure barrels to be used for velocity testing.


Centerfire Rifle Standard Velocity Barrel Lengths from 1992:

.22 Hornet — 16.000" ±0.010"
.30 Carbine — 20.000" ±0.010"
7.62x39 — 20.000" ±0.010"
350 Rem. Magnum — 20.000" ±0.010"
351 Winchester — 20.000" ±0.010"
44 Rem Mag — 20.000" ±0.010" (rifle only)

All others: 24.000" ±0.010"