Adding to the info on velocity variation:
I found that in my Steyr LG110 and Walther Dominator the spread in velocity was reduced by respectively 7 and 8 fps (between 2 series of 25 test shots) when I selected the pellets (JSB Exact 8.4) for weight. The selected pellets were within 0,008 gram or 0,12 grain.
So the weight differences between pellets can cause a variation of 7 fps.
That means a measured 7 fps spread may mean that the rifle itself has no spread at all! Good news fot the Mk4.