The OBT tool at work: from the measured barrel to the comparison with barrel time

The Long node calculator in use is a spreadsheet that asks for few data and wants them true. The data to be entered are four: the free length of the barrel, from the muzzle to the first attachment on the stock or on the action; Young’s modulus of the steel (between 210 and 230 GPa), its density (between 7.83 and 7.87 kg/dm³) and Poisson’s ratio (between 0.27 and 0.30); the barrel time of the charge under examination, read from the internal ballistics software; and the initial node from which the series is to start. The calculated data are two: from the three elastic constants the tool derives the speed of the pressure wave in the steel, which with E = 220 GPa, ρ = 7.83 kg/dm³ and ν = 0.28 is 5,993 m/s, and from this and from the length it derives the series of quiet times of the muzzle. A correction coefficient, stated in the spreadsheet, allows the wave speed to be scaled for the difference from Long’s reference: in the example case the factor is 0.962 and corresponds to the relation’s reference wave speed of 5,765 m/s; overwrite it with 1 and you have the original relation. The factor does not scale the speed inside a propagation calculation: it multiplies the node times directly, as Box 12.C shows.