The legal limit for paintballs is around 320 -330 fps, it's the same limit that applies to air weapons/rifles and is determined by the weight of the projectile in this case paintball x velocity must not exceed 12ft/lbs force,
UK Legal Limit
To calculate the power of an airgun you need to use a chronograph to measure the speed of the pellet (in feet per second) when fired, and you need to know the weight of the pellet in grains. Once you have that information you perform the following calculation:- speed(ft/sec) X speed(ft/sec) X weight(grains) 450240
This gives you the result in foot pounds force (ftlb).
As mentioned the legal maximum for an unlicensed air rifle is 12 ftlb which from changing round the above formula, gives the approximate values as follows:-
A .22 pellet weighing 14.4 grains, maximum permissible speed is 612 ft/sec
A .177 pellet weighing 7.9 grains, maximum permissible speed is 826 ft/sec
The corresponding figures for a pistol are 433 ft/sec for a .22 and 584 ft/sec for a .177
The pellet weights used in the above calculation are typical weights for the sizes of pellet but you must always check the actual weight of your pellet before performing your own calculation.
Most painball guns can exceed this limit easily which is why sites limit use to 280fps or 300fps for those that like to live on the edge.