This is a rough estimate, but I remember seeing some 9mm data that showed about 8% drop in muzzle velocity going from a 5" to a 4" bbl and another 8% going from a 4" to a 3". The 8% stuck in my head. YMMV.
The .40 and 9mm lose very little velocity going from a 4" to a 3" barrel. 25-75 f.p.s. average depending on the bullet. Unlike the .45 which was designed for a 5" barrel and loses lots in a 3" barrel. In fact in Ammolabs tests only 3 loads expanded reliably from 3" guns. Taurus Hex, Cor Bon Powerball, and Winchester Ranger +p. The 9mm and .40 are usually tested out of 4" barrels and are high pressure rounds. They hit their peak pressure fast. Good in a short barrel. In fact many people recommend 9mm and .40 over .45 in sub 4" barrels because they still perform well.
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