Ruger 10/22 Adjustable V-block
Installation is fast and easy and will allow you to compensate for any barrel droop to ensure that your barrel is on the same line as your receiver rail.  If your barrel droops when the V-Block is tightened, it's possible for the barrel to be so low that the scope does not have sufficient range to compensate.  1.  Place the barrel in the receiver hole.  2.  Place the Adjustable V-Block in the barrel cut-out with the setscrew orientated so that the setscrew can be adjusted while the V-Block is on the gun.  3.  Install both cap screws through the V-Block into the receiver and tighten until they just have MINIMAL torque resistance.  DO NOT TIGHTEN THEM.4.  Stand the receiver up on the rear of the receiver with the barrel pointing toward the ceiling.  Ensure that the extractor cut is correctly orientated to the extractor on the bolt.  5.  With the receiver and barrel pointing toward the ceiling, push the barrel tightly against the receiver.  This is the position that you want the V-Block to hold the barrel when the cap screws are tightened.  While holding the barrel against the receiver, turn the setscrew in the Adjustable V-Block until it is pushing on the barrel and begins to minimally push the barrel upwards.  Back the screw off until there is no upwards push on the barrel but the setscrew is still in contact with the barrel.6.  Tighten each capscrew to snug the barrel to the receiver.  Do not over torque the screws.  Overtorquing the cap screws can strip the aluminum threads in the receiver and will render the receiver not usable. Verify that the extractor on the bolt is lined up with the barrel and installation is complete.