Introduction

Ok, so you have a rifle or handgun that will not shoot to point of aim because your original factory rear sight, or newly installed rear sight won’t adjust high enough or low enough.  This means you will probably need a higher or lower front sight.

Adjusting a front sight works the opposite of how you would adjust the rear sight.  To raise the point of impact you need to make the front sight shorter.  To lower the point of impact you need to make the front sight taller.  This also applies to windage; to move the shot to the left you need to move the front sight to the right.  To move the shot to the right, move the front sight to the left.

Factory Rear Sight
If you have a fixed factory rear sight that you cannot adjust you need to adjust the front sight.  I developed an Excel front sight height calculator you can download by clicking here. 

If you have an adjustable rear sight, you don’t want it to be at the extreme bottom or top of its travel to hit a target; you want it to be somewhere in the middle.  This gives some adjustment for different distances.  Now fire a few shots with your selected load at your selected distance until you get a group.  This will tell you how much and in which direction you need to adjust the front sight.

Let’s say you have a single-action revolver with a rear sight that is just a groove in the top strap, and you want to adjust your point of impact by 6-inches at 25-yards.  At this point it doesn’t matter whether the adjustment is high or low.  Because you are going to adjust the front sight in inches, all measurements must be in inches.

Step 1.      Enter the shooting distance in inches.  One-yard is 36-inches, so 25-yards is 25 * 36 = 900-inches.

Step 2.      Enter the change in bullet impact in inches.  For this example, we want to change the bullet impact by 6-inches.  Again, up, or down doesn’t matter right now.

Step 3.      Enter the sight radius in inches.  This is the distance between the rear sight and the front sight. 

Step 4.      Read the change in front sight height in inches.  If you want the impact to move up, you need to remove 0.050-inches from the front sight.  If you want the impact to move down, you need to add 0.050-inches to the top of the front sight or replace the front sight with a taller front sight.

Many single-action revolvers have a tall front sight to allow for adjustment, which usually requires filing the sight.  This tall front sight causes the shots to be low, so it needs to be filed shorter to raise the point of impact.  I don’t do any filing until I find a load that I like, then I file the front sight to match that load.  I’ve seen many shooters that don’t want to alter their guns, then wonder why they can’t hit a target.

Replacement Rear Sight
Please note much of this information came from the Marble Arms web site.  Some of the material in this article is very technical based on angles and legs of right triangles. 

(Photo and chart courtesy Marble’s Gun Sights, Inc.)

I replaced the rear, non-folding leaf sight that came with my 1947 Marlin 39A with a Marbles #95 folding rear sight.  I chose this sight because it folds down and has both windage and elevation adjustments.  This will be a backup to the tang peep sight I installed.

Marble Arms provides instructions on how to calculate the required front sight.  I developed an Excel tool to do these calculations; you can download it by clicking here.  You can enter data only in the green cells.  This is how the tool works:

Step 1.      Measure the diameter of the barrel at the point where the front sight bead is going to be located.  Enter that value in cell C2.  My Marlin 39A measured .616”.

Step 2.      Measure the diameter of the barrel at the point where the rear sight is going to be installed.  Enter that value in C3.  My 39A measured .870”.

Step 3.      Enter the bottom height of the rear sight you plan to install in cell C4.  .320” was the bottom of the rear sight as found on the Marble’s web site.

Step 4.      Enter the top height of the rear sight you plan to install in cell D4.  .420” was the top of the rear sight as found on the Marble’s web site.

Step 5.      Enter the thickness of the ramp in cell D8.  This is measured from the bottom of the ramp to the bottom of the dovetail.  The ramp on my 39A measured .125” thick.  If you don’t have a ramp enter a value of 0.  Read the front sight height in cell D9.

Step 6.      Enter the distance, in inches, the bullet will cross the line of sight in cell D10.  See below how to find this number.

Step 7.      Measure the distance between the rear sight and the front sight.  Enter this value in cell D12.  My 39A measured 20-inches.

Step 8.      Read the new front sight height in cell D14.

The value in cell D9, .355”, would be the height of the front sight if the barrel were parallel to the ground, i.e. the front sight height is even with the rear sight height.  There should be enough adjustment in the rear sight to get the rifle sighted-in at pretty much any distance out to 100-yards or beyond.  But I wanted the rifle to be sighted-in at 50-yards with the rear sight set 1/3 up from the bottom setting.  If the barrel were parallel to the ground, gravity would cause the bullet to start falling as soon as it left the bore.  To hit a target at a certain distance the barrel must be angled up.  This can be accomplished by either raising the rear sight or lowering the front sight.  I wanted to install a lower front sight, but I need to know that angle, called the angle of departure.

First, I needed to determine where the bullet crossed the line of sight if sighted-in at 50-yards.  I used Sierra Bullets Infinity Suite v6 to calculate the trajectory of Winchester white box 40-grain copper plated hollow point ammo.  I purchased a bulk box of this ammo and this is what I plan to shoot.  You can see in the above table the bullet crosses the line of sight at 13.25-yards.  I entered 13.25*36 = 477-inches in cell D10 of the tool.  The angle of departure is calculated in cell D11

Now I need to find how much lower the front sight needs to be than the rear sight.  A lower front sight will cause me to tip the barrel up so I can see it in the rear sight.  With the rear sight 1/3 of the way up I want the shots to hit at 50-yards.  Enter the distance between the rear sight and the front sight in cell D12.  My 39A measures 20-inches.  The new front sight height can be read in cell D14.

Cell D13 shows the required change in the front sight.  I could either lower the front sight by .033”, or raise the rear sight by .033”, either would work.  Raising the rear sight by .033 would raise it to .386” which is well within the range of that sight.  (Remember, the range of the rear sight is .320” - .420”.)  .320” bottom height + .033” 1/3 up + .033” change = .386”.

I purchased a Marbles green fiber optic front sight #31MR P/N 603126 from MidwayUSA.com #448808.  This sight measures .312” high by .340” wide.  I fabricated a 0.010” shim and placed it under the front ramp to get the sight up to 0.322”.

Laser Bore-Sighting

Now that I have new sights on the rifle, I need to adjust them.  I used my Bushnell universal laser bore sight to do a rough adjustment of the sights.  This bore sight comes with various arbors that fit in the barrel depending on the caliber.  The arbor screws onto the end of the bore sight and expands to fit tightly in the bore.  I don’t like the in-chamber bore sights because they only work for one caliber, and the reflections off the bore cause interference and make the dot difficult to see.

Using the new sight height and angle of departure I used my Bore Sight Calculator which you can download by clicking here, to find that I should adjust my sights so the red laser dot is 0.28-inches above the front sight at 25-feet.  Yes, this is 25-feet!  I do this inside my house where 25-feet is easy to obtain.  With the lower front sight, the laser dot should already be 0.28-inches above the front sight with the rear sight 1/3 up from the bottom.  This adjustment should get me on paper at 50-yards, but I will have to go to the range to make the final sight adjustments.

Sight Adjustment

So, let’s work the problem backwards.  Let’s say I want to sight in my rifle at 100-yards using my .322” front sight.  This time the bullet crosses the line of sight at 5.33 yards.  This makes sense because the rifle must be tipped up higher to hit at a longer distance.

The angle of departure is now 0.224-degrees.  Using the formula SIN(0.224) x 20 = .078”.  This means the rear sight would have to be .078-inches higher than the front sight.  The front sight height is .322” sight + .125” ramp + .308” front barrel radius = .755-inches, so the rear sight height has to be .755” + .071” = .826-inches.  The maximum height of the rear sight is .420” rear barrel radius + .453” max sight height = .873-inches.  .826” is within the adjustable range of the rear sight.

Now I want to find out what the maximum distance I can attain with this sight setup.  The total front sight height is .755-inches, and the total maximum rear sight height is .873-inches, so the rear sight is .873” - .755” = .118-inches taller than the front sight.  This would make the angle of departure ARCTAN(.118/20) = .338-degrees.  So if the front sight is .755-inches, and the angle of departure is .338, then the bullet should cross the line of sight at .755 / TAN(.338) = 128-inches or 3.6 yards.

Using Sierra Infinity Suite v6 and setting it up so the bullet crosses the line of sight at 3.6-yards, it would be sighted-in at a little over 151-yards.  Probably beyond the effective range of a .22LR cartridge, but I plan to sight it in at 50-yards, which should do just fine for me.

All these numbers and calculations are just an academic exercise anyway.  They don’t take into account the effect of altitude, temperature, air pressure, wind, and other factors that could affect the flight of the bullet.  The goal is to get the shots on paper, but again, the rifle would still need to be sighted-in at the range.