It depends on lots of factors. One general rule of thumb is the larger the powder charge....and the faster the velocity in the full-length barrel....the more loss there will be with a shorter barrel. As such, magnum rounds tend to lose much more velocity with shortened barrels than non-magnums. Having said that, I will concur with bigcountry on his numbers....at least for many non-magnum cartridges. For magnums, the loss is often more like 50-85 fps/ per inch.