...off the top of my head but I know that out of a short barrel the difference is about 20 fps maybe a little more but not much. I reload and I've tried to reach better velocities compared to good factory ammo. In a short barrel you can add bang and recoil but it doesn't get that much velocity. You can pack in the powder and the reward is small. More important may be just having a round that isn't so raucous that you can hold and shoot follow ups better.

I have some small guns, especially an alloy .38 that is uncomfortable to shoot standard and downright painful to shoot +P out of.

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