From Pilot_51's Wiki
How much deviance to be added to a weapon due to turning:
x - maximum deviation possible y - how much to add for each frame tilting up and down z - how much to add for each frame turning left and right w - how much to reduce per server-frame (30 fps)
All x-values (maximum deviation) of SetFireDev, SetMiscDev, SetSpeedDev, and SetTurnDev are added together.
Source and more details