One of the most common problems we hear about concerning scopes is that the scope has run out of adjustment and will not sight in.  If this happens to you it does not mean that the scope is broken (99% of the time), it just means that the scope ran out of adjustment.  The problem lies with what required the scope to use so much adjustment.
 
The most common culprit is the mounting hardware (base and or rings).  Poor quality rings and bases or improperly installed rings and bases can cause the scope to be so severely misaligned with the bore that the scope does not have enough internal adjustment to compensate.
 
There are several techniques that can help you determine if it’s the rings or bases.  If you are using "Weaver" style rings and you run out of adjustments left or right make a note as to the location on the target where you ran out, then remount the rings 180 degrees from the way they were mounted originally.  Sight it in again, and if you run out of adjustments the opposite direction you initially did, then it’s the rings.  Usually when this happens it is possible to turn one of the rings back around and get the gun sighted in.  This is a band-aid fix and not recommended as a method to solve the problem.
 
If you are using "Redfield" style rings and you run out of adjustment left or right it is most likely because the rear rings is not centered on the base.  If it is centered try the same technique used for "Weaver" style rings mentioned above.
 
If reversing both rings and single rings in different combinations does not yield any different results it is most likely not the rings.  Next you will need to diagnose the base.   Some one piece and most two piece bases can be reversed, try this first.  If the different combinations do not reveal an issue it is most likely not the base.  If you can not reverse the base(s), try using a different base from either the same manufacturer or preferably a different manufacturer.  Many times we run into a bad lot of bases so switching brands is most effective.
 
If the problem is with the rings or bases you can sometimes solve the problem by lapping the rings, shimming the base and or rings or use Burris Signature rings, which have interchangeable plastic inserts that act as shims.  The best solution would be to replace the poor quality mounts with quality mounts that require no shimming.
 
The second most common culprit is the firearm its self having the mounting holes drilled off center.  This is very difficult in most cases to detect without some precise measuring equipment.  In gross cases you can simply “connect the dots” and extend the lines beyond the receiver and see the problem.  Not very common years ago but becoming more common today.
 
The third most common culprit is the scope but it is not running out of adjustment that is the problem.  It’s the adjustments getting stuck, not responding to external adjustment or simply jumping all over the place.  You can very easily diagnose the scope by first centering the reticle then visually watch the crosshairs move by using a boresighter.  Look up the scope’s specs and see if it moves the stated amount in each direction.  Keep in mind that a scope with 40 inches of travel means that it goes 20 inches in any direction from optical center.   If it does not move at all something has happened with the gear assembly, if it moves but not the entire adjustment range then something is causing it to bind (bent tube or rings over tightened).  If it is jumping all over the place the reticle retention spring is defective.
 
Replacing the scope that ran out of adjustments with a different scope that was able to be sighted-in is not a valid method to determine if the first scope is defective because different scopes have different internal adjustment ranges.  The higher the magnification the less internal adjustment a scope will have and additionally different manufacturers use different size internal lenses which will cause them to have more or less internal adjustment.  IE: A Schmidt Bender 3-12 will have substantially less internal travel than a Swarovski 3-12 as would a Leupold 3-9 vs. a Leupold 8.5-25.
 
The rarest culprit is also related to the firearm its self just like the miss-drilled mounting holes but this problem is not always a manufacture defect.  We have seen rifles with bent barrels and this takes the longest to diagnose because it rarely happens and it’s not the first place you start looking.  I’ve seen one that UPS ran over and we did not discover it until we were boxing the rifle back up to send to the manufacturer and notice a tire track on the shipping box.

If your rifle has a bent barrel or miss-aligned mounting holes you need to send it to the manufacturer of the rifle for repair or replacement.

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