At 400 FPS using .20's you'll want to be at 327 or less on .30's.
Use this calculator and play with the variables.
The key thing to remember is you want to use the ENERGY of the projectile not the speed of the projectile (I think Illusion or ThunderCactus had a pretty good writeup about this in a post somewhere).
Also note that with GBB systems that the relationship between speed and weight isn't linear. It's almost exponential so you need to use the BB's you're planning on using when you chrono but also do the right calculations for energy.
A GBB pistol shouldn't be over the limits (unless you have a 330 FPS limit for pistols for safety when mercying or at less than 20ft. engagements) but this is a concern for GBBR's which can go way over field limits.