Velocity loss or gain? If you take a 24" barrel shooting 3000 fps and cut it down to 18" then add a 7" suppressor what's the final fps?
It would be a loss. If you keep the barrel length at 24" + the suppressor you might possibly see an increase in velocity.
For what it is worth >>> http://www.thefirearmblog.com/blog/2015/11/16/suppressors-affecting-velocity/