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In the case of a semiauto (and a derringer), some of the barrel length is taken up by the round itself, which would seemingly reduce the effective length.

Contrast that with a revolver where all of the barrel is ahead of the round.

As an example, my Bodyguard 380 claims a 2.75" barrel, which would equate to around 2" ahead of the round.

Is this a correct assumption, or is it factored into semiauto barrel length specifications?

I started thinking about this when watching videos about Bond Arms 410 derringers after noticing chambered shells seemed to take up almost all of the barrel.

george wrote:you are correct. This would seemingly give the 4" revolver a velocity advantage over a 4" semi, but the gas loss at the barrel to cylinder gap in the revolver tends to even things out.

I was thinking about this very thing. However, I wonder just how "even" it gets. I would think that the gas loss at the gap has a greater effect on velocity than the "shorter" barrel does in a semiautomatic.

I don't have the necessary guns to test it but a s&w governor that was loaded with 45 acp could be tested against another 45 with roughly the same length barrel through a crono, same would go for a 9mm revolver vs a semi.