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Loss of Zero - Scope Failure/issue?

27 Jan 2014
@ 04:22 pm (GMT)

Brendan Welch

Hi All,

I have recently had an issue where my scope has lost its zero.
I'm still on the paper but, its moved quite a way. up about 3+ inches and right about 2 inches.

The issue I'm now faced with is that I have run out of elevation (down).
There is still plenty of horizontal adjustment left.

The scope is a Bushnell 5-15 tactical is mounted on a 10 degree picatinny rail on a tikka 7mmReMag.

Its only about 3 years old and fired about 200-250 rounds.
The mounts are sturdy steel. I dout they have moved, but you never know.
What should I do?
Should I install in on a 0 degree rail and run tests on the tracking, or should I send it back to bushnell, or .....?
Any advice on what to do would be appreciated.


27 Jan 2014
@ 05:42 pm (GMT)

Nathan Foster

Re: Loss of Zero - Scope Failure/issue?
Hi Brendan, we have to be a bit careful here as it is not unusual to see POI shifts with the T3 magnums. Recoil is so high that the most minute changes in shooting position, butt pad placement and forend control can cause large shifts. If the rifle has a suppressor, there may be carbon caking in the chamber (case neck area) which has changed the chamber dimensions, increasing pressures. If the base was not epoxied in place, the high recoil could loosen the screws. This can on occasion occur- even with Loctite in place. Some of this was covered in the first book, some will be covered in the next book.

So, your check list:
Base epoxied in place as per first book.
If using suppressor- use oversize bronze brush and solvent to remove carbon and copper.
Shooting technique- no bipod with this rifle, use sand bags or day pack for field testing. Use good forend control. The smallest change will effect POI, important to practice often with the T3 magnum until muscle memory is fully established. Good firm sling (see hold that forend article).
Check torque settings on action screws, set to 35lb (which is just firm). Do not overtighten or accuracy will be ruined and rifle will have to be bedded. If you are worried about the two action screws coming loose, loctite them in place.

Check the above, retest, then decide on whether the scope is actually the problem.

As for the height on paper. Burris rings with canted inserts are the go. You can then cant up using the 10 degree/ MOA inserts (if in NZ, contact Dan at Reloaders supplies).

I cannot tell what the problem is without having the rifle in front of me but I suspect that there is the possibility for this to be recoil related (change in technique over time) or technique plus carbon/suppressor problems- based on the direction of the POI change.

If the above tests fail- time to send the scope back. I have mentioned that the T3 produces very high recoil so no doubt this is hard on optics.
27 Jan 2014
@ 11:24 pm (GMT)

Brendan Welch

Re: Loss of Zero - Scope Failure/issue?
Hi Nathan,

Thanks for the advice.
There is no suppressor so we can rule that out.

I have checked the rings and base. It all seems to be on pretty tight.
I will redo as per your book. There was locktite on all screws but no epoxy between the action and the base.

I have been applying the shooting techniques you recommended previously and have been shooting consistently for a while now. I think I have that under control enough to be confident that something has changed.

I'll re-torque and epoxy everything and then re test.
I will also give it a good clean.

I was actually going to replace the scope anyway, so will do the above and go from there.

Many thanks
02 Feb 2014
@ 05:05 pm (GMT)

Brendan Welch

Re: Loss of Zero - Scope Failure/issue?
So, I cleaned the gun, epoxied the base, torqued and loctited the screws and torqued the action screws.

The scope seems to track as it should with the point of impact relating to the turret adjustments made, but the problem is the same; it simply no longer has enough elevation down to be zeroed.

it grouped sub MOA with factory ammo for the 3 shot group I did as a test.

I think a rail change should address the issue, but the question still remains, how has this happened and what are the chances of it happening again?

03 Feb 2014
@ 02:15 pm (GMT)

Nathan Foster

Re: Loss of Zero - Scope Failure/issue?
It has been said in the past that if a scope is used near its extreme elevation or windage adjustment, that the scope can be prone to suffer changes in POI. I do not know how correct this is because I have in the past used the .308 Win near bottomed out as many folk do when setting up cant for long range. But heavy barreled .308 rifles do not recoil the same as your rifle. So this may be something to consider.

I guess you have three options; Change the rail. Change the rings to Burris with inserts or change the scope for one that has a greater internal range of adjustment which solves both your concerns over the scope bottoming out and your mistrust of the current unit. I am guessing you have about 60 minutes adjustment (you'll need to double check this). If you change out to a Sightron SIII 6-24 x 50 MD, you will have 100 minutes.

Good to see the rifle is nice and accurate.


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