Based on this data, it would probably end up with a velocity around 2540 fps. Seems like you lose about 50 fps per inch, pretty consistently. The 9.2" barrel of the Kel Tec has been reliabily chrono'd at 2600 fps, so subtract 50-60 fps for the 8" barrel of the RR.
Thanks for the link.
Interesting project.
I don’t think I would have been able to do that to one of my guns.