Most ammunition manufacturers define 4 inch and longer barrels as "service" length. GD Short Barrel was specifically designed for 3-3.5 inch semi-auto barrels; the .38 Special 135 grain +P was designed for the 2 inch revolver. If you look at Speer's performance charts, the 9mm standard velocity GD fired from a 4 inch barrel has the exact same energy as the 9mm +P Short Barrel ammunition fired from a 3 inch barrel.
Like said above, the .38 Special +P SB ammo was designed specifically for a 2" barrel snub nose revolver. The bullet was designed to expand at as little as 800 fps so the additional velocity achieved from the slightly longer barrel couldn't hurt. The NYPD for which it was designed for is using only that 1 .38 Special +P round no matter what the barrel length is.
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