June 16th, 2008

Stupid Scope Tricks — How to Mess Up Your Zero Big-Time

Your editor wins the dunce of the week award, but he learned an important lesson. This weekend I shot the Varmint Silhouette match at Ojai, California. In preparation for the match, I (wisely) went to a range on Thursday to chronograph my match load and confirm my 100-yard, no-wind zero. All went well there, and after three sighters I put three rounds into 0.260″ right at point of aim. So far, so good.

Editor moves his scope AFTER zeroing. End result? Zero shifts 2 MOA up and 11.5 MOA right. Yikes!

But, while testing, I noted that the scope, having been positioned for prone shooting, was much too far forward. It needed to be moved back about an inch for comfortable shooting at the bench.

No problem I think… I take the gun home, get my Leupold Torx wrench and loosen the rings. Now, I use Burris Signature rings, the ones with the plastic inserts that provide elevation preload: +10 in the rear and a -10 in the front. These are net values, created with pairs of +05 (thicker) and -05 (thinner) half-ring inserts. In the back the thicker +05 insert is on the bottom and thinner -05 on top, while in the front the thicker insert is on the top with the thinner on the bottom.

First thing I note when moving the scope is that it was hard to set the inserts in exactly the same spot (though I tried). I also note that the inserts seemed to flatten out or squeeze down more when I re-tensioned the ring bolts.

I figured I might have to make a minor correction to my zero at the range, so I wasn’t too concerned. Boy was I wrong.

Windage off Six Feet at 600 Yards
At the match, I dialed in my 600-yard come-up and aimed at a steel gong we use for sighter shots. Normally I can “certify” my 600-yard zero in three shots. This time, my spotter couldn’t even see the hits. “WTF”, I think… Then another shooter notes that my bullets are impacting about SIX FEET to the right of the Gong and about a foot too high.

I had to dial in 46 clicks (11.5 MOA) of left windage to get back on the Gong. At 600 yards, one MOA is 6.282 inches. My 600-yard Point of Impact had shifted 72″ to the right — a full six feet! That’s huge. That’s like being off a foot at 100 yards.

LESSON LEARNED: If you make ANY change to your scope, you should re-zero.

SIGNATURE RING LESSON LEARNED: If you are running the Burris Signature Rings, with plastic inserts, the change in POI when you move the scope can be extreme — way more than you’d expect with conventional rings. The plastic inserts take a different “set” each time you tension the rings, even if you do your best to maintain the inserts’ orientation in the rings. No matter how careful you are, those plastic inserts aren’t going to hold the scope in exactly the same place.

So, if you don’t want to win the Dunce Award like I did, make sure you re-test your scope’s zero AFTER you move the fore-and-aft position, and EVERY time you re-tension Burris Signature Rings. It’s worth noting that the ring set in question is five years old and it held zero perfectly until I decided to move the scope back. This was done because my club recently built benches at the 600-yard line so we now shoot from the bench rather than prone, F-Class style.

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