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Normal Topic Correctly fitting your scope - part two (Read 181 times)
50BMG
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Correctly fitting your scope - part two
Oct 6th, 2018 at 9:21pm
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Scope fitting tips and common errors - part two

I am assuming you have a high-quality sight that is appropriate to the type of shooting you intend to undertake and it is properly suited to your physiological requirements. I know not everyone has the required equipment to undertake some of the steps here. If you do not and you a serious about getting your sight correctly mounted you need to find someone who does. Do not overlook the importance of getting this right. It is not uncommon to halve group sizes by correcting sight mounting faults. What is the point of spending a lot of money purchasing a rifle and developing the perfect load if you take a "near enough is good enough" attitude to sight fitting?

Having prepared yourself to begin mounting your sight you need to consider the final objective. To connect the sight to the rifle so the sight axis is in absolute alignment with the barrel bore axis and will stay that way no matter what. Do not lose sight of this and do not allow compromise for the sake of expediency as so often happens. This is a deliberate and precise process where minute discrepancies will be to major detriment when it counts. Make sure you have a distraction-free, well lit, non-marking work area and plenty of free time. A purpose designed gun vise which will allow you to hold your rifle securely as you work on it is essential as a rifle moving while being worked on is a frequent cause of misalignment.

The first thing to do is to inspect the various items i.e. rings, rail etc. and ensure the requisite screws and small parts are all present and correct.

The next thing to do is to carefully examine all screw holes. Check that they are in not obstructed and have no foreign matter in them. Use a Vernier calliper to verify that hole depth (combined ring/ rail thickness and hole in receiver) marginally exceeds screw length to ensure that the screw can be tightened to the correct torque without bottoming out. I have attended to a large number of rifles with so called accuracy problems that came down to screws actually being loose because they had bottomed out before being able to apply the required clamping force to retain the sight as intended. In every case I was assured that “the screws are really tight” or something like it.  A little less problematic is where just one screw is in this condition and in time the sight gets distorted due to twisting.  You really need to ensure the holes are the correct depth. A quick check is to run the screws in until the just bottom out. They should all have exactly the same amount protruding. This also a good way to test the threads of both screw and hole. It is not common but there can be mismatched thread pitches or insufficient depth of thread. Both cause damage or accuracy issues as well as foul language and premature manual balding.

Once you have confirmed all is well with parts and hole machining it is time to look over the ring bases. Is the mating surface of the ring perfectly flat and are the screw holes on a line perfectly perpendicular to the bore axis of the ring itself? They should also be absolutely perpendicular to the mating surface and parallel with all other screw holes.

Once that is all confirmed as correct it is time to examine the receiver itself. Are the screw holes going to allow you to mount the sight where you will need it? It is not uncommon to find the  location of the tapped holes will not allow the sight to mounted far enough back to allow correct fitting to the shooters eye when the rifle is correctly mounted in the firing position so as to provide the correct relief and sight picture (“without conscious effort or strain”). As with the rings it is important the holes are correctly aligned at right angles to and parallel with the bore axis (as appropriate) and finally they are absolutely centred on the bore axis to ensure the sight will be directly above the bore axis. Anything less requires a trip to an armourer to have new holes drilled and tapped in the correct positions without exception. You cannot accurately mount a sight unless the holes are precisely located as described. The name stamped on the rifle is no guarantee that errors will not be found as errors can occur with tooling and machinists can have a bad day.

The next post will over a general rifle preparation and set up to receive the rings
« Last Edit: Oct 18th, 2018 at 9:11pm by 50BMG »  

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50BMG
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Re: Scope fitting tips and common errors - part two
Reply #1 - Oct 7th, 2018 at 3:48pm
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I forgot to add that the reason for the apparent pedantry is that a.5mm error in this area can translate to up to 10mm error on the target at 100 metres when the compounding "tolerance stack" and load errors are factored in.
  

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