I found this on another Forum
taken from RyanM on THR forum.
Hm, I'm going to bump this sorta old topic to say that I've chronoed some slugs through a Mossberg 500 Serbu. In an 18.5" barrel, standard Remington 1 oz. slugs were doing 1410 fps. Through the Super-Shorty, 1140 fps. That's less than a 20% velocity loss. Reduced recoil was, coincidentally, 1140 fps through the 18.5" and 920 fps through the Serbu, once again less than 20% loss. It wasn't my chrono, though, so I only shot slugs through it.
The Super shorty is a 6.5 inch barrel compared to an 18.5 inch barrel.
Unless I am mistaken the slugs would be effected by barrel length more than shot.