OK, so I'm a little late to this conversation, but better late than never. I've been communicating with Tom Holland (Xbowsniper) this week about weighing pellets and setting up a regulator. The epiphany I finally had was that, all other things remaining the same (air pressure, hammer spring and such), a 1% change in pellet weight will see an inverse 1% change in FPS at the muzzle. In other words, if I shoot a 14.3 grain Crosman Premier Dome and it really weighs 14.3 grains (very few actually meet the exact spec) at 800 fps, but the next pellet weighs 14.45 grains, I would expect FPS at the muzzle to drop to about 792 fps. Conversely, if I drop the weight by 1%, I'd expect to see the speed increase to 808 fps.
That doesn't sound like much, until you realize that the last tin of pellets I weighed ranged from 14.16 grains to 14.70 grains, almost a 4% variation. Speeds could vary 32 fps across all pellets in the tin according to the math. Other variations in the gun may mask the pellet weight variations, but it is real. I've had the same experience weighing JSB pellets as well. 3% or a touch more variation across a tin of pellets. 32 fps can effect point of impact,
In larger pellets such as the JSB 34 grain .25 caliber pellets, 1% is a pretty big range of 3.4 grains. In Tom's case shooting 8.4 grain pellets, the 1% range is closer to 0.84 grains. Smaller = tighter tolerances.
It's been an interesting and educational adventure. For serious target shooters, weighing is a must. For most of us plinkers, not so much.