I had a well-used 3" short shroud Century re-import for awhile and a few problems aside, i was a real decent sixgun. I did not own a chronograph at the time so I can't help you with actual velocities.

http://www.thesixgunjournal.net/dialing-in-a-fixed-sight-ruger-da-the-repatriated-ruger...

I don't have a GP100, but I do have a stainless Speed Six with a 2 3/4 barrel. It's a dandy sixgun, and one of the last guns I'd ever part with. I typically load it with the Speer short barrel 38 spl load.

a 3 inch M-66 ("Treasury Gun")
a 3 inch Python. Not a 2.5 inch.
and currently have a 3 and a smiggen inch SP-101.

Never chrono'ed 'em but the do hit hard and are LOUD.

Although my ears still say a 4 inch Python is the loudest of the loud.

What strikes me is that the M-66 and Python were overly bulky for normal uses where a 3 inch might be chosen over a 4 inch.

....and have chronographed most of them. Bottom-line -- they are generally a little bit slower than the 4" guns, but the cylinder gap is generally more important than the 1" in barrel length (i.e. I have several 3" sixguns that are faster than their 4" brethren).

the one thing it is touted for ; concealment, it is actually worse at. Get the Match Champion: it is smooth, easy to tote and really hides well. I prefer the balance and handling of the MC: it is more akin to a Security Six or my L frame Mountain Gun than a factory GP100. The 4" barrel, as I have stated many times, helps press the gun into your side and carry more comfortably.
The only thing about those short barrels guns is the look sorta neat.

--
Of the Troops & For the Troops

15% off, I couldn't walk away.