September 23, 2010
This is an obvious exaggeration of reticle cant, but it quickly proves how difficult accurate shooting can be when the reticle is not perfectly perpendicular with the axis of the bore.
I was on the range when my cell phone started buzzing. It was Marc Bartoskewitz, a good friend and wildlife biologist from South Texas. He had been preparing for an upcoming caribou hunt--that meant a new rifle, new scope, and lots of long-range shooting practice--and I had been coaching from afar. Surmising the call might be important, I put down my rifle and picked up the phone.
"Man, I have a problem," Bartoskewitz said after exchanging pleasantries. "I just started shooting out past 300 and 400 yards, and my groups are way left, like 6 or 8 inches."He went on to explain that there was no wind, and the shots were from a bench. Bartoskewitz is pretty handy with a rifle, so I went through the checklist of possible problems.
"Who mounted the scope?" I asked from the top of the list.
He said he had installed the scope himself but without the aid of a leveling kit. We had the problem solved then and there. The riflescope's reticle was slightly canted--not perpendicular to the axis of the bore--and that was causing the strange windage shifts at longer ranges.
Most of us cant a rifle one way or the other when we shoot and usually introduce an error into the relationship between the scope's reticle and the axis of the bore trying to correct the problem. Since the rifle is tilted a few degrees left or right, most folks simply tilt the scope the other way so that it looks level when the rifle is shouldered. It takes someone who holds the rifle level or cants the opposite way to spot the problem.
A canted reticle or rifle is no big deal inside of 100 yards, but the longer the range, the bigger the problem. Unless it suffers from undue influences like wind or yaw, a bullet falls to the ground in a straight line. The vertical stadia of a ballistic compensating reticle or adjustments made via target turrets will not follow the same path as the falling bullet if the scope or rifle is canted. How bad can the problem get? With just 6 degrees of cant, a shooter induces over 36 inches of error at 1,000 yards. It is easy to see how Bartoskewitz was 6 to 8 inches off target, enough to miss a caribou at 300 to 400 yards.
To further illustrate the point, I loosened the rings on my Remington Model 700 Police and rolled the once perfectly perpendicular Leupold Mark 4 6.5-20X 50mm LR/T almost on its side, so the reticle was at a near 45-degree angle. I placed a target at 200 yards and shot the square, first making a 2-minute adjustment up, then 2 minutes right, 2 minutes down, and finally, 2 minutes left. As the photo of the square illustrates, a little cant goes a long way. Even sighting-in was a challenge, since corrections drifted along unusual paths all over the paper. Add conditions like wind or mirage to the equation and placing a bullet precisely becomes almost impossible.
The Leupold Mark 4 6.5-20X scope still shot the square perfectly when canted, but adjustments were anything but correct--the longer the range, the larger the error.
That is why most long-range competitors and snipers have some sort of cant-prevention device, usually a spirit level, on their scope tubes. Competitors in previous centuries added a spirit level under their front sights to accomplish the same thing. Of course, mounting the scope correctly is the first and most important step. Bob Hart, the son in Robert W. Hart & Son, Inc., builds hundreds of precision rifles every year and uses his rifles to compete in a variety of long-range competitions. I asked Hart how he prevents canted reticles.
"You definitely want the scope mounted perpendicular to the axis of the bore--it's another piece of the accuracy puzzle," Hart said. "First double-check that action holes are in line with the bore. I draw a line down a center of the barrel and run a tap into the base screw holes, since the base screws are usually not tall enough, and check for misalignment."
Hart uses an alignment tool to make sure the rings are square and then laps them to remove any stress on the tube. After setting the eye relief for the shooter in the middle of the scope's power range, Hart puts the rifle in a rest and places a spirit level on the action rails, making sure the rifle is perfectly level. He then places a level on top of a turret and rotates the scope until it too is perfectly level.
It is important to keep the levels in place when tightening the ring screws to the manufacturer's recommended specs since the rings can pull the scope out of level. I tighten the screws like lugs on a spare tire, working on opposing corners of the ring until the torque wrench tells me each screw is cinched down correctly.
There are quite a few different tools available to assure a level reticle. A quick search through a Brownells or MidwayUSA catalog will turn up a dozen options. I use the Wheeler Level-Level-Level quite often. It is simple and costs just $20. If you have a spirit level lying around the house and can slide it across a flat on your rifle's receiver or, even better, set it on the action rails, it will work just fine too.
Bartoskewitz called back the next day to say his rounds were dropping into the bullseye with reassuring regularity. He had leveled up his scope the same evening after we talked, and that made all the difference. It reaffirmed the need to test a rifle thoroughly before heading out on the hunt of lifetime. With a level scope, those caribou are in trouble.