AV Avenger

Please forgive my rather rudimentary drawing, not to scale either. Looks like I shouldn't have passed preschool, however the concept is similar to shimming a scope, except in reverse. Haha.
1) shows a properly aligned shroud, everything is in alignment. As it should be.
2) Shows how the shroud would align, with the grub screw being too tight.
3) Shows the effect of the grub screw being to tight and the shroud, being "made" to fit.

Now the explanation. As you tighten the screw, you force the barrel lower in the rear shroud support,(or more accurately the rear barrel shroud support higher in relatuon to the barrel)now as the barrel is lower, the shroud support, becomes higher in relation to the middle barrel shroud support and the barrel band. When you make it fit, this forces the barrel to be facing in a downward angle, which is very small but perceptible in your point of impact and measurable in your shroud. Measure the space between your barrel shroud and the air tube at the back, near the breech and again near the muzzle, if it's the support, you will notice that there is a definite change in gap. Unlike a scope, which wants to be facing downward to increase your elevation, your barrel needs to be pointed up to get the same effect.
Hope this explanation helps.
As a small edit, the rear shroud support does not affect where the barrel starts, as much as where it ends. I know it sounds counterintuitive, but if you have a fixed middle point and you raise the rear, the front drops and vice versa. The barrel rear point remains constant, the barrel shroud however does not, and it is the barrel shroud and support that forces the muzzle down.