Modern day tests run on blackpowder cannon/culverin revealed that there is such a thing as too long a barrel. Past a certain length the expanding volume behind the projectile will actually drop below atmospheric pressure, so outside air-pressure will start pushing against the projectile's velocity. Same should apply to modern guns (maybe even moreso, blackpowder weapons have notoriously loose rounds.)
That said, I have no idea how shell diameter optimally relates to barrel length, and I have no clue about the real proportions of the 140mm compared to the model.
To anyone interested in the above test, it's in John Guilmartin's fascinating and highly readable "Gunpowder and Galleys." IIRC it's in one of the appendices, I'm not at home to double-check.
That said, I have no idea how shell diameter optimally relates to barrel length, and I have no clue about the real proportions of the 140mm compared to the model.
To anyone interested in the above test, it's in John Guilmartin's fascinating and highly readable "Gunpowder and Galleys." IIRC it's in one of the appendices, I'm not at home to double-check.