Scope Installation and Zeroing
I recently purchased a new Marlin XT-22 with the intent of building a training rifle for off-season shooting practice. This economic bolt-action repeater came with iron sights and with the option to mount a scope. Since most of my hunting rifles are outfitted with a scope I opted to purchase one for this training rifle. In keeping with the economy theme I settled on the Simmons 22 Mag 3-9x32mm AO offering.
As I was unboxing my new purchases I thought it might be beneficial for some if I wrote a technical review on how to install a scope, and ultimately zero — sight-in — a rifle. Hence, below is a pictorial account of the decisions made and steps involved when installing a new scope on any rifle platform.
Before we get down to the nitty gritty details I think we should start with a few part definitions:
Receiver: The part of a rifle that contains the operating mechanism or action
Dovetail: A joining method using opposing angles to hold two items together
Bases: Hardware mounted on rifle top to allow the mounting of a scope
Scope Rings: Generally a two piece circular ring that bolts around scope and onto bases, which come in various heights
Scope: A telescopic, tubular optic containing a reticle or dot for sighting a target
Windage and Elevation knob: Adjustment knobs on scope body to change point of aim with reference to point of bullet impact. Windage refers to horizontal adjustment while Elevation refers to the vertical.
In/lbs: Measurement units for tourque commonly used in reference to tightening of bolts
The first decision in mounting a scope is which method of install you will use. Some firearms have the option of mounting a scope on dovetail groves in the top of rifle receiver; however, the most common method is mounting on mounting bases. The photo below shows the top of the receiver with groves for dovetail rings and 4 screw holes for mounting bases.
I chose the more traditional option – using mounting bases – for two reasons, 1) although dovetail rings are sufficient for a low recoil rifle, bases are known to be a more solid option, and 2) there are many more scope ring choices for base applications than dovetail.
Because all firearm actions are different there a hundreds of base configurations. My particular firearm required a two-piece base – Weaver #12 style. If you are considering ordering from a catalog or online be sure to order both!
To install bases the factory screws must be removed from the receiver.
The bases are simply mounted into place securely. These screws should be torqued to between 15-25 in/lb depending on rifle model. Tightening all screws to manufacturer’s specifications will save you a lot of time and ammo at the range – it effectively prevents you from trying to zero your rifle with a loose scope.
Next comes the installation of the bottom half of scope rings. The ring clamps underneath the base with a long screw – which sits in groove of the base — and tightens (20-25 in/lb). The screw in the base groove keeps the scope ring from moving back and forth; hence tightening the screw onto the base holds the scope down and keeps it from moving side to side. This is one of the advantages of the base method over the dovetail. Dovetail mounting relies on sheer downward pressure to keep rings from sliding during recoil, whereas bases have a built in stopping grove.
Next is probably the most important step in mounting a scope – the actual mounting of the scope (please hold your applause). Place the top half of the ring over the scope and set it in lower half. Snug up the screws so that the scope is held firmly but can still be adjusted.
You will need to determine the amount of eye relief required. Eye Relief is the distance from which the scope can be from your eye while your cheek rests comfortably in natural shooting position and which you can still see clearly through the scope. This can be set upby shouldering the rifle and placing your cheek on the stock – the way you would when shooting. If the power of the scope can be adjusted ensure that the lowest power is selected – to give you the largest field of view. If the scope is too far away from your eye a black shadow will appear inside the ocular lens, this is called parallax – and even I’m not qualified to explain that. Next, slide the scope in the rings closer or further away from your eye so that you have a full field of view. At this point you should turn the power ring to the full power of zoom to ensure that the same field of view remains present.
Before tightening the scope in place you will want to level it so that the reticle – crosshair — appears level in each 90 degree plain. Levelling a scope can be as simple as levelling the rifle and using a common level on a flat surface – elevation knob – or placing a level line on a wall and levelling the reticle to that.
At this point you should tighten the scope in the rings – 15-20 in/lb.
Normally you would be done, but in my case I had a minor issue – the bolt would touch the scope when cycling the action. My scope rings were simply not high enough, however, the minuscule height difference required did not merit purchasing higher rings. So, I used an old gunsmithing trick to give me the minor height change required. I removed the rings with scope intact and removed the bases. I then cut small strips out of an aluminum pop can and placed them under the bases creating a shim. After reinstalling everything, the bolt cleared the scope.
Zeroing a Scope
Once at the range I was able to set the gun up in sand bags, which prevent the gun from moving around in the initial stages of zeroing — and can double as a shooting rest later.
I set up a target at 25 yards and began to bore sight the gun. Bore sighting is a technique in which the bore or barrel of the rifle is aimed at a target and then the scope is adjusted to the same point of aim. This can be achieved by the use of a laser in the bore or by the method explained herein. To bore sight my gun I removed the bolt from the gun and looked directly down through the barrel. I then moved the gun so I could see the paper through the bore. Finally, I made sure the crosshairs in the scope were also on the paper. In theory this process ensures that you will at least hit the paper on your first shots as you begin zeroing.
Prior to firing the first rounds I made some adjustments that many ignore when sighting in a rifle — adjusting the scope to your own eye. I focused the ocular lens so the picture was clear and I adjusted the objective lens to focus on the target at the current range.
I settled in behind the gun and began shooting. The first round struck the middle of the paper so my bore sighting had been effective. I finished my 3 shot grouping, and I was surprised when I didn’t see any additional holes in the target. Concerned, I loaded up 2 additional rounds to prove the first wasn’t a fluke. Upon looking at the target I soon realized that all 3 of my first shots had passed through the same hole! Not bad.
As you can see the first grouping was low and right, 2 inches and 1 inch respectively. I spun the caps off the elevation and windage knobs and read the adjustment indicator. One click was equal to ¼ MOA – or Minute of Angle. MOA is another complex shooting term that simply translates to 1” at 100 yards. Therefore one click = 1” inch at 100 yards. Since I was shooting at 25 yards the amount of adjustment required is higher. Moving the bullet an inch at 25 yards would require four times the adjustment than it would at 100. Therefore to adjust my scope 2” up and 1” left I needed to move the elevation knob 32 clicks (4 clicks @ 100 yards x 4 (factor for being at 25 yards) x 2 inches required target movement) and the windage knob needed to be moved 16 clicks.
With the adjustments made I tried a couple 3 shot groups at 25 yards on other targets.
Lastly, I moved back to the 50 yard marker to ensure that any minor adjustments were not required. I was happy with the way the gun performed and the rest of my day at the range was spent plinking!