Brute is right about there not being an exact formula with what you will actually see as a velocity when you change BB mass, but if you want to find out the approximate velocity, or just calculate velocities at equal energy levels, it's a pretty easy conversion. You don't even need to calculate energy.
Here's my formula: v2
= original mass (the mass you measured your BB at)
= new mass (the mass you want to convert the fps reading to)
= original velocity (the velocity you measured)
= new velocity (the velocity that you are trying to calculate)
with the derivation here: http://www.airsoftforum.com/board/FPS-conv...&p=18246289
Pretty much, when I want to run some approximate numbers it's just a matter of if I know the velocity is going to go up (due to decreasing the mass) I'll divide the heavier weight by the lighter weight, take the square root (or if you're using a calculator that doesn't have a square root function, like the scientific version of the built in MS calculator, take it ti the .5 power) and then take that number and multiply it by the original velocity. if I know it's going to go down, I just divide the other way (or if I know I screwed up in dividing, I can just divide 1 by that number). As the formula implies, the velocity is basically proportional to the square root of the ratio of the masses, of course assuming equal energy.
Then, with real world values, you might have a higher-than-calculated or lower-than-calculated based on a variety of factors, one of which can include BB size. Smaller BBs, like say Bioval .27s or Excel bio BBs, are going to have less of an increase, if any, and a larger BB like an SGM will have more of an increase. Then, play with hop up, the tightness of the barrel, etc and the only way to know your actual velocity is to measure it.