How to Install an AR with Scope and Iron Sights: A Guide

That is the order in the diagram. I put the tools on the bench, mount the iron sights, clamp the scope base and rings, level the tube, torque the screws, and bore sight so both sighting systems start near the same point of aim. The live group on paper is the last check. Until that group is where I want it, I treat the turrets as a rough setting.

I am Matt Rice. I mount these setups at our shop in Tigard, Oregon. Clear the rifle before you touch a sight screw. Magazine out, bolt locked open, chamber empty. If your state restricts rifle features, read the rules for your state before you configure the gun.

What you need

Lay these out before you clamp anything. I work on a bench that holds the rifle still. A gun vise or a padded rest is enough. If the upper rocks while you turn a screw, the mount walks off the recoil shoulder before the torque wrench clicks.

• The bits the sight and mount screws actually use. Many rail clamps take a Torx bit. A hex key that is one size off will round the socket. Fit the bit in the screw by hand before you put a wrench on it.
• A torque wrench that reads inch-pounds. A foot-pound wrench is the wrong scale for ring screws. If the wrench starts at 10 foot-pounds, it cannot set a ring screw.
• A small level, or a plumb line you can see through the scope. A bubble on the turret cap is a start. The flat of the scope tube against the flat of the mount is the check I trust when the cap is not square to the reticle.
• Gun solvent and a clean rag. Oil on the rail lets a clamp slip under recoil. Wipe the rail slots, the mount feet, and the ring saddles. Leave any factory lube the mount maker tells you to leave on a dovetail.
• The maker's torque card, or the sheet that shipped with the mount. I torque to that card. A number from a different mount is not a substitute.
• A bore sighter, or a clear look down the bore with the bolt carrier group removed. You also need a safe backstop and a paper target for the first live group.
• A paint pen for witness marks after the screws are torqued. The mark shows whether a screw turned on the drive home.

Thread locker is optional and only when the screw maker calls for it. A drop of oil on the threads changes the clamp load you get at the same wrench reading. I leave the screws dry unless the card says otherwise. I also match the ring diameter to the tube before anything goes on the rifle. Common tubes are 1 inch, 30mm, and 34mm. A 30mm ring on a 1 inch tube is too large, so it sits loose and cannot grip. A ring smaller than the tube will not close. Forcing that smaller ring marks the tube. Over-torque marks a tube even when the diameters match.

Install the iron sights

On a flat-top, the sights and the scope base share one rail. I seat the sights first so the mount does not cover the slots I still need. This step only clamps the sights square. Windage and elevation for a real zero wait until you are on paper.

1. Wipe the rail where the sights will clamp. Oil in a slot lets the clamp creep even when the screw feels tight.
2. If the front sight is part of the gas block, leave it on the barrel. Do not unstake a front sight base to move it for this install. The gas block position is set by the gas port, not by the scope.
3. If the front sight clamps to a rail, set it as far forward as the sight maker allows. Drop the cross bolt fully into a slot. Square the sight so the post is vertical when you look from the rear of the receiver. Tighten the clamp until it does not shift under hand pressure, then torque it to the maker's value.
4. Clamp the rear sight on the rearmost slots of the receiver rail. That keeps the aperture close to your eye and leaves rail in front of it for the scope base. Push the sight forward into the rail slot so recoil drives it against the shoulder, then tighten it the same way. The sight should clear the charging handle when the handle is pulled.
5. Look through the rear and the front. You want one sight picture, not a canted rear. If the rear is twisted, the front post sits off to one side of the aperture even when the bore is pointed at the target. Loosen the rear clamp, square it, and torque it again. Do not bend a post to hide a crooked clamp.

I do not publish one torque number for every iron sight clamp. A folding rear and a rail-mounted front use different screws. Read the card that came with the sights. If the card is missing, stop and get the number from the sight maker before you lean on the wrench.

Install the scope

After the rail is clear, I clamp the base on the upper receiver, set eye relief, level the tube in the rings, and torque the ring screws. The bore sight comes after the rings are tight, not before.

The drawing shows those three moves and nothing else. The base goes on the rail first. The tube is leveled before the ring caps are torqued. The bore sight is a rough alignment, not a live zero.

Base

1. Pick the mount before you torque anything. Ring height has to clear the charging handle. The optic should touch only the rings. Fold the rear sight down, and the front sight if it folds. If the front sight is fixed, choose a ring or mount height so the objective clears it.
2. Clamp the base or one-piece mount on the upper receiver rail only. Do not bridge the mount across the gap onto the handguard. A handguard can flex when you load a bipod or a barricade, and a bridged mount moves the scope with it.
3. Drop the cross bolts into the slots. Push the mount forward so the bolts bear on the recoil shoulders. A cantilever mount points the overhang toward the muzzle. Finger tight is only a hold. The wrench comes after the mount is seated.
4. Torque the base clamps to the mount maker's inch-pound value, in the order that card prints. According to Leupold MARK AR and MARK IMS mounting instructions, the keeper screws and the 1/2 inch keeper nuts are secured at 65 in-lbs after the mount is pushed forward into the rail.

Per Brownells, the Tech Tip example tightens the cantilever mount bolts to 65 inch-pounds with a torque driver, and the writer tells you to verify the number on your own mount. I treat 65 as the figure those two writeups print for the base clamps they show. If your card prints a different base value, the card wins.

Rings and eye relief

1. Take the ring caps off. Set the scope in the bottoms. The rings must sit on the straight tube. Keep them off the turret housing and off the tapered bells. Leave room to slide the scope for eye relief before you decide the ring spacing is final.
2. Set the stock to the length you actually shoot. Start with the scope as far forward as the tube allows, then slide it back until the picture is full and the black ring around the edge is gone. Do not crowd the eyepiece. On a variable scope I set that relief at the highest magnification, because that is the setting with the tightest relief on most of the scopes I mount. Check the low power too so the picture stays usable when you dial down.
3. Lay the ring caps on and start the screws by hand. If a screw binds, back it out. Cross-threading a ring screw is how a cap gets ruined before the rifle ever leaves the bench.

Level

Level the reticle before the ring screws are final. A bubble on a flat turret cap works when the cap is square to the reticle. It is not always square. I also look at a plumb line through the scope and line the vertical stadia up with the line. Another check is a flat reference under the tube against the flat top of the mount, with light pressure holding the scope down while you snug the caps. Keep the rifle upright in the vise while you do this. If the receiver is canted and the scope is level to the room, the reticle will be crooked to the bore.

Torque

Snug the ring screws in a cross pattern, the same way you would walk around lug nuts, so one side of the cap does not pinch the tube first. Keep the gap between the ring halves even on both sides. Then bring each screw to the ring maker's inch-pound value in that same pattern. According to Leupold, those MARK AR and MARK IMS ring screws are tightened evenly to 28 in-lbs, and the instructions say no more than 28 in-lbs should be applied to the ring screws. Per Brownells, the ring screws in that Tech Tip are torqued to 25 inch-pounds, with even gaps left on both sides. Use the number printed for your rings. I mark each screw with a paint pen after it clicks so I can see movement later.

Bore sight and confirm on paper

A bore sight only gets the reticle near the bore. It is not a zero. I do it after the rings are torqued so I am not aligning a scope that still slides in the saddles.

1. Clear the rifle again. Magazine out, bolt carrier group out, chamber empty. Set the rifle in a rest that does not move when you look away from it.
2. Per Brownells, the visual method in that Tech Tip uses a target at 25 yards. Look through the barrel and center the bore on the target. Without shifting the rifle, look through the scope and move the turrets until the reticle sits on the same spot. A muzzle bore sighter does the same job if you cannot get a clear look down the bore. Follow the sighter's own instructions for how it seats.
3. Put the bolt carrier group back in. Confirm the rifle is still clear, then load only when you are on a range with a safe backstop. Shoot a three-shot group from a rest. If the group is on the paper, adjust the turrets from the center of that group toward the point of aim. If the group is off the paper, stop dialing. Recheck the bore sight, the ring torque, and that the base is still hard against the recoil shoulder.
4. Write down the yardage and the final turret setting once the group is where you want it. I do not call the rifle zeroed until that group repeats. One lucky shot is not a confirm.

Run the scope and the sights together

A magnified optic on this rifle can be a traditional scope in rings or a prism with its own mount. The photo is our Rhino 4x magnified optic.

The Rhino 4x uses a one-piece Picatinny mount, so you clamp that mount to the rail. A scope with separate rings uses a base and ring caps instead. In both cases the rail is first and the bore sight is last.

Leave the iron sights on the rifle. They are the backup when the optic fogs, breaks, or the battery dies. Use the scope when you want magnification. Fold the rear sight down, and the front sight if it folds. If a fixed front sight still blocks the objective, choose a ring or mount height so the objective clears it. Co-witness is for a red dot. The height for that optic is in the absolute and lower 1/3 co-witness guide.

If you are pairing a fixed-power optic with irons, I wrote up what that looks like on the range in integrating an ACOG with iron sights.

Troubleshooting

• The scope looks rotated. Loosen the ring caps only. Leave the base clamps tight. Level the tube again, then retorque the ring screws in a cross pattern to the ring card.
• The point of aim jumps between groups. Check the base clamp and the ring screws. Look at the witness marks. A mount that shifted off the recoil shoulder looks like a bad zero. Also check that the mount is not bridged onto the handguard.
• The picture is fuzzy at every distance. Set the eyepiece focus on a plain sky or a light wall until the reticle is sharp, then leave that ring alone. That is ocular focus, not a loose mount. If the target is sharp and the reticle is soft, the eyepiece is the part to turn.
• You see a black ring, or the picture blacks out when you shoulder the rifle. The scope is too close or too far for your stock length. Loosen the ring caps, reset eye relief at the magnification you shoot, and torque the rings again.
• The scope or the mount hits the charging handle. The rings are too low, or the mount sits too far back. Move the base forward on the receiver rail, or use a mount that clears the handle. Do not grind the handle.
• The iron sights block the scope picture. Fold the rear sight down, and the front sight if it folds. If a fixed front sight still blocks the objective, choose a ring or mount height so the objective clears it.
• The tube shows ring marks. The rings were over-torqued, or a ring was sitting on a bell instead of the straight tube. Stop and inspect the tube before you clamp it again.
• Screws loosen after the first range trip. Recheck torque against the maker card. A broken witness mark means that screw turned. If the same screw backs out twice, the mount is the problem, not the ammunition.

When you are ready to pick the optic, start with our AR-15 scopes.

If you want a magnified option for this install, compare the best scope for AR-15 on that same page.

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AR-15 scopes

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FAQ:

Q: Do I install the iron sights or the scope first?

A: Install the iron sights first on a flat-top. They sit on the same rail the scope base uses, and it is easier to seat them before the mount covers the slots you need.

Q: Is a bore sight the same as a zero?

A: No. A bore sight only gets the reticle close to the bore so your first shots land on paper. A zero is confirmed with live fire at a known distance.

Q: What if the iron sights block the scope?

A: Fold the rear sight down, and the front sight if it folds. If a fixed front sight still blocks the objective, choose a ring or mount height so the objective clears it. Co-witness is for a red dot.

Q: Where should I compare scopes for this rifle?

A: Use the AR-15 scopes page to pick the optic, then come back to this install order. The page covers magnified optics for the AR, which is the buying decision this install assumes.

Article Written By Matt Rice, Owner Operator of OZARK ARMAMENT

Matt Rice is the founder of Ozark Armament in Tigard, Oregon. He started the company in 2016 and still mounts the optics he sells.