Well, I ran the data in Quickload using generic cartridge/bullet lengths and although the pressure in QL doesn't match well (at all) with revolvers, it does show a velocity difference of about 60fps between a 7.5" and 8.75" barrel. I would suspect that the real difference in a revolver would be less, due to the gap, but considering that you've got an additional .2gr of powder, I'd say that the book numbers and your numbers are actually in excellent agreement.
Subtracting the 60fps from your numbers put you just 3fps off their numbers.
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