A lot of folks are asking "What should I expect the velocity in my barrel when the posted load is for a different barrel length?"
There are techniques for estimating velocity and some are better than others. If you want a precise estimate, try using this link: Velocity Estimator. On the other hand, if you want to get a quick idea you can try this chart:
To use the chart, start with the barrel length for which your load is listed, move up to the velocity for that load, then move left or right parallel to the colored lines to your barrel length, and you have an estimate of your velocity. Precision is not as important as it might seem because the differences between chambers and barrels are frequently greater than the precision you use in working the chart!
There will be times when the barrel length is not covered by the chart. In these cases, the link - "Velocity Estimator" - can be used to calculate the approximate velocity.