You have to temporarily put the plastic cover in place to make sure the spacing is proper. The plastic cover should be snug to snap in place with no space between the back end and the front of the receiver and no space between the front of the plastic cover and the back of the picatinny block. Remove the cover and put your levels in place, tighten the four grub screws in the receiver that hold the barrel in place. Then while making sure the picatinny rail is level also tighten the grub screws in picatinny block that hold the barrel. Check to make sure your magazine slides in and out.
Thanks
Charlie Frear, Georgia Airguns