Ritz had the right idea but he plugged in the wrong number values.
Use this chart to determine FPS, the base is 350fps on .20g bb's each weight equals too 350fps on .20g if any number goes over the fps value for that weight than you know it shoots harder than 350 on .20g
And your mossberg fps dilemma is highlighted in red, goes over those two numbers than it's too hot.