Scope Mounting with a Digital Protractor: Pro-Tip

C.a.R Firearms is a small shop that specializes in custom work and related services (think build vs. assemble) located south of Seattle. Every time I’m there I make a mess, drooling over fantastic, precision-built guns — either their creations or customer-owned.

But mostly, I enjoy visiting C.a.R Firearms because the proprietor, Fred Hastings, and his staff are solid guys. They provide one-to-one attention, sweat the small stuff, and work hard to deliver the very best for any customer, large or small.

Fred Hastings is a combat veteran with a professional background in CNC machining R&D. He started C.a.R. Firearms as a way to treat his PTSD. It’s wonderful to see him help himself by helping others. I believe that’s what truly drives his business: the satisfaction of helping others.

When Fred found out I’d never used a digital protractor for firearms, he was more than willing to sit down and share how to utilize the measuring instrument to satisfy a shooter’s thirst for correcting alignment and accuracy.

True to the culture of his shop, he had no qualms about allowing TTAG to tag along. (Note: This is not a marketing initiative by C.a.R. Firearms. I asked them if they were willing to teach a “pro tip” and they agreed.)

A digital protractor measures angles, with most measuring a range of 0° to 180° or 360°. The Fowler Mini-Mag measures a total of 360° by way of its four sides, each providing 90° of inclination and declination data, either relative (by zeroing off of a surface) or absolute. On Fred’s recommendation, I picked up the Fowler Mini-Mag (above) for a few pennies over $60 on Amazon.

At a basic level (no pun intended) a digital protractor serves the same purpose as any bubble-level set. It simply provides a much more precise, digital reading.

I’ve used the above Wheeler bubble levels for the past few years with decent, sometimes “perfect” results (based on double-checking with the digital protractor, of course). It must be said: in very rare instances, a clamp-on bubble level may be your only option for ensuring proper scope alignment.

The Mini-Mag has an alloy housing, 3.5lb-pull magnets on three of four sides, a large readout screen, and your standard functions (zero, hold, calibrate, etc.). Onscreen icons indicate tilt direction (up or down), low battery, and the type of reading displayed (absolute, relative, or hold).

I wish the battery panel had an O-ring seal. Anyway, in addition to a digital protractor, Fred’s scope mounting process calls for the following tools:

Aside from a vise (not pictured), the above tools are Fred’s current preferences; there are some suitable substitutions, of course (noted below). From left to right in the photo:

– Torque Wrench: Vortex Optics Torque Wrench (tube case, wrench, bits in bag)

– Digital Protractor: Fowler Mini-Mag

– T-Wrench: Schultz & Larsen Red Handle T-Wrench (optional if you have a toque wrench)

– Upper Receiver Block: The Device Manufacturing AR-15/M16 Upper Receiver Fixture (2 pieces)

Regarding suitable substitutions, a Wheeler F.A.T. Wrench (above, left) or even Fix It Sticks Kit (above, right) (if you have the correct torque bit) will work just as well.

Certainly not every home shop warrants a $300 The Device Manufacturing Upper Receiver Fixture.

Fred was quick to recommend the D.P.M.S. “Panther Claw” block (above) as an alternative. I use the Panther Claw at home and I can attest that The Device’s block has far superior lock-up inside the upper receiver. It also provides a very nice rail section for mounting scopes to rings, absent a firearm.

For this process a Geissele Reaction Rod was not an acceptable alternative from Fred’s perspective. It allows for a variance of play, considering that the differences in chamber lugs and wear of those lugs can lead to a variety of tolerances.

The Scope Mounting Process

The steps outlined below assume you have already double-checked the compatibility of your optic/mount/weapon combination, lapped your rings (should you choose to do so), and mocked-up the optic on your weapon to determine proper placement with eye relief as a central consideration.

It also assumes that your optic has an appropriate vertical or horizontal flat spot from which to take a reading with the protractor. This demonstration utilizes an AR-15 style rifle, the Armalite M15A4 SPR Mod1, but is applicable to many weapon systems.

Step 1: Firmly mount your firearm to a rock-solid vice by way of an upper receiver block or other appropriate device. Ensure neither the firearm, mount, nor vice can inadvertently shift out of place during the scope mounting process.

Step 2: Turn on your digital protractor and place it on a flat spot on the upper receiver (not the handguard, unless you have a monolithic upper…then maybe). This process uses relative leveling so zero-out the protractor. Check several spots along the upper receiver to ensure the integrity of the rail.

If you are using the Fowler Mini-Mag remember to keep it moving from this point on as it will reset after five minutes of no activity.

Step 3: Mount the bottom half of your rings to the rail and lock them down (consider blue Loctite 242). If you have horizontally or vertically-split rings, for kicks, take a look and see if they are level with each other. The Leupold rings above were off slightly – certainly not enough to cause worry or warrant corrective action.

Notice that Fred likes to use the protractor upside-down. He does so because the top surface of the protractor is the side with no magnets, and the buttons are easier to manipulate on top.

Step 4: With the t-wrench, snug your optic into the mount so that you can barely rotate it. When placing your optic in the mount, be sure to take into account your pre-calculated eye relief. Again, consider blue Loctite 242.

Step 5: Set your zeroed digital protractor onto the flat spot of your optic (most likely a turret) and slowly rotate your optic until you get triple-zeros. Your optic is now level with your upper receiver. If taking your measurement off of a side turret, simply use the side of the digital protractor – do not turn the protractor 90°.

Step 6: Keeping your protractor in place to monitor change, ensure your torque wrench is set to the appropriate specification and begin slowly tightening the rings evenly and in a crossing pattern.

First, tighten until you feel resistance at each screw. Then use several passes of limited rotation to make certain the gaps between the halves of the rings are equal, until you click-off on each screw (indicating you’ve met the torque specification).

If your protractor indicates a change in the optic’s relative position to the upper’s rail, you’ll need to adjust your approach for how you tighten each individual screw, adjust the scope within the rings to compensate for the movement, or consider lapping your rings and trying again (presuming the rings are uneven and pushing the optic out of alignment).

Once you’re locked-in to specifications, remove your protractor from your optic, and then set it back down in the same spot to double-check the reading.

You can triple-check using an available pre-qualified section of rail. Look into your optic and line up the reticle with a right angle (like a door frame) and conduct a visual assessment of your reticle for proper alignment of cross-hairs or other markings, relative to the upper receiver.

The straight-forward, simple process complete, once everything checks out satisfactorily you should be set to re-assemble the firearm (if necessary) and head out to the range to dial-in the optic.

Other Useful Employments

Fred, being who he is, instinctively checked the gas block on each of the three firearms I brought in that day. Above, the Armalite’s gas block was off slightly so he took the liberty of bringing it back in-line.

A digital protractor also works really well for timing muzzle devices. Many devices have a flat spot at the 0°, 90°, 180°, and/or 270° positions from which you can take your reading. When muzzle devices are absent a flat side, you may need to get a little creative, as demonstrated by Fred in the photo above.

If you spend more than a few hours a month working on firearms, a digital protractor is well worth the investment. As soon as you have one in-hand you should block out the next day or two because you’ll most likely be checking and re-mounting many of your rails, optics, and muzzle devices.

And if you’re like Fred Hastings, it will feel like the right thing to do. If you’re not, that’s fine – your arms may still run. But why did you read this article?


  1. avatar Roadking says:

    Fred does the best work around. He worked on Marlin 45/70 for me that was so rough from the factory that the lever would lock up, and when you had the hammer back it was like someone had stuck a dremmel tool in there and just went to town. It was chewed up, rough, and just plain ugly. I don’t know how the gun got through QC. Not only did he have to use a file, and smooth it out by hand which took hours he went through the rifle, and smoothed out everything so it is like butter now. While I was there I had his engraving guy laser etch my initials into the receiver. I get more compliments on that rifle than any other. I wouldn’t let anyone else work on my guns.

    1. avatar BCG says:

      Why is it that gun makers consistently send rough builds out the door. Many other industries would have a customer revolt if their products were substandard. But in the gun world there is corporate welfare protectionism. I expect perfection in my guns and am damn mad when crap flows off the assembly line.

      For instance, I was looking for a Ruger wheel gun. I went through half a dozen of the same model at various stores looking for one with solid lock up and smooth action. Finally found one, and had to pay $80 over the lowest price. Yea, I bought it, but still why are gun makers allowed to get away with shipping junk? Are we scared to diss them outside of their, cough, cough springfield, beliefs?

  2. avatar Isaac says:

    This is just a waste of time. The scope does NOT have to be level to the rifle.

    It should be based on personal preference.

    The optic has to be level to the world, that is the only way it matters. The only way to do this is:

    1. Scope level (easiest for beginners)
    2. Feel.

    If you are going by feel, then having the scope level to the rifle will cant the scope when the rifle is canted to fit you.

    Lastly, the outside of the scope has nothing to do with anything that has to be true. The reticle and the tracking are the only things that need to be true. Often the outside of the scope is far enough out of alignment from these to notice a difference.

    I shoot PRS matches. I am not that good but I have shot against plenty of good shooters that would agree.

    1. avatar Jeremy S. says:

      When shooting at extended distances, you have to be sure that your scope is level to the earth. Using the horizon or random objects is usually a bad idea. So, shooters use anti-cant devices (typically a bubble level) attached to the gun or scope.

      If your anti-cant device is mounted to the rifle such as on the Pic rail or built into the scope rings/mount, then your optic does have to be square with your gun. If your anti-cant device is mounted to the tube of your optic (or actually built into the reticle like the digital level system on my SIG Optics TANGO6) then, yeah, it doesn’t have to be square with the gun and your tracking will track properly as long as the scope is level to the earth (as in, perpendicular to gravity). But…most people level their scope-mounted anti-cant devices by squaring them with the elevation turret (or turret cap) anyway, so using a digital protractor like this is a great way to do a very good job of that. If your reticle isn’t square with the flat top of the turret/turret cap, then the digital protractor is still very handy. Square the reticle with something known to be perfectly vertical (like a door frame), zero that angle out on the protractor, then move the anti-cant device to match. The bottom line is that, no matter how you’re choosing to level and square things up, this method is more accurate than eyeballing a general “centered” status with a bubble level, which will leave you with some +/- fudge factor…

      1. avatar Isaac says:

        Apparently something ate my last follow-up comment.

        Yes, it is better than eyeballing if you want the scope cap/turret level with the gun.

        It would do little to help you hit targets at distance.

        You are matching two things that do not need to be matched together. This is showing how to level up an arbitrary surface with another arbitrary surface. Neither of which maters at all.

        Even your examples are just leveling up the scope with the anti-cant device. The fact that the rifle might be true too does not matter.

        The scope/rifle should be set up by personal preference. The scope to the world should be held true.

        I know how to hit targets at distance. I am not saying I am good at it, but I know the how.

    2. avatar Jeff says:

      If your optic isn’t vertically plumb with gravity AND directly vertically centered above (or below) the bore when the rifle is fired, long range ballistics will be problematic to some degree. This of course becomes even more of an issue if you use various “holdover” points along the vertical crosshair for aiming. Anything other than having the optic plumbed along with the rifle may result in varying (and larger) degrees of cant when fired, also creating LR problems.

      But if you manage to plumb only the optic when firing, AND that associated degree of rifle cant results in the bore being directly below and aligned with the vertical crosshair, every time, it will work. If the crosshair winds up off to one side of the bore, but is vertical, it doesn’t work so well.

      Note that since most mounting systems only allow for the center of the scope to be mounted directly over the center of the bore (assuming vertical rifle), having the scope canted means you cannot also have it vertically over the bore, when the scope is plumb.

  3. avatar Rick_in_NH says:

    Note: A vice is a bad habit. A vise is a tool with jaws to hold an object. I don’t correct comments on an article, but any TTAG article should use proper English.

    1. avatar Conner W. says:

      You are absolutely correct! A simple error, however – not lack of knowledge. Thanks for letting us know; it has been corrected.

  4. avatar IN Dave says:

    I use a digital protractor as well, even same make and model. However I dislike using an upper reciever because of difficulty in getting it held tight and putting torque on it. I use a section of picatinny rail on a mill table that I know is level. Almost everything I have is set up with picatinny and I do have some optics that get changed to different guns and so I don’t want a optic that is leveled to a particular gun, I want an absolute level optic that can go on any gun. I am not saying his way is wrong, his work has been proven, in fact I may give his way a try a couple times to see how I like it on a optic for one particular gun.

  5. avatar gargoil says:

    yyyeeeaaaahhhh not necessary.

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