Some of mankind’s first applications of the principles of ergonomics were with weapons. It probably started with the smoother rock,and evolved into the club or spear that was easier to grip, to the more balanced sword, down through the ages until eventually we arrived at the blocky polymer grips of today’s popular autopistols. Rifle stocks also, through this process of evolution and engineering continuous improvements, have come to be a good general fit for the average shooter. “Good general fit,” however, has never appealed to the “precision” rifleman. Now, this peculiarity isn’t just about looking cool and basic comfort. The shooter’s physical control over the rifle is dependent on their interface with the action, barrel, optics, and stock or chassis.
When everyone was still having gunsmiths carve wooden stocks, the answer to optics was the Monte Carlo style raised comb, which enabled an improved cheek weld while maintaining a low buttplate position. While he was at it, the gunsmith could also custom carve the cast, cant, and length of pull to fit the patient and affluent customer.
The practice of using old world craftsmen to carve expensive custom stocks to the specifications of an individual rifleman, however, didn’t meet the mass-production or standardization needs of law enforcement or the military, and doesn’t typically meet the needs of today’s impatient and uh… “thrifty” sport shooters. Some early answers were the addition of leather cheekpads, such as one might find on the old M-1C and M-1D sniper rifles, but the eventual answer was to apply components that could be quickly adjusted to one shooter today, and with the crank of a few knobs, fit the same weapon to a different shooter tomorrow. That has led to the development of stocks with a multitude of adjustments that used to be found only on exotic competition guns.
It would be outside the scope of this article to make an exhaustive study of the history of adjustable stocks, but suffice it to say one of the earliest practical applications of multiple adjustments to a modern military-style rifle would have to be the iconic Heckler and Koch PSG-1, introduced in the late 1970’s. The aesthetics of this rifle have stuck with us, and continue to influence modern designs.
But when we circle back to the beginning and talk about adjusting a stock to the individual rifleman, there are only so many ways to skin that cat, namely:
• Vertical adjustment of the cheekrest or comb to align the eye with the center of the optic
• Horizontal adjustment of the cheekrest to coordinate the position of the head and eye with the set “eye relief” of the optic
• Horizontal adjustment of the buttplate to set the proper “length of pull”
• Vertical adjustment of the buttplate to set the proper “drop to heel,” the heel being the upper corner of the butt (as opposed to the “toe,” which is the bottom corner of the butt)
Not all of these adjustments will be found on all stocks, the two typically found are length of pull and comb height, such as on the J. Allen Enterprise Chassis stocks and the Magpul PRS. Stocks such as the XLR Element and KRG Whisky-3 Chassis also allow horizontal adjustment of the cheekrest. Where you start with your adjustments depends on whether or not the cheekrest of your stock has horizontal adjustment. If you can't adjust the position of the cheekrest, that is your starting point, and your eye relief and length of pull will be based on that. If your cheekrest is adjustable, you have a little more flexibility.
Other adjustments such as “cast” and “cant,” which involve the rotation of the stock or optic around the linear axis of the bore… well, it is just my opinion that these adjustments are not advisable on “tactical/practical” guns, as the shooter may need to use their other shoulder to make a shot, or may need to take a shot from a modified field position, in which case they will be handicapping themselves even further. I like to keep the buttstock square with the vertical axis of the rifle.
When adjusting a stock, it is important to realize that some of the factors involved are variables, and some are constants.
The throw of your bolt is a constant you must deal with. The eye relief of your optic is a constant you must deal with. Scope height over bore is a constant you must deal with (assuming you have mounted the optic properly). The length of your neck (and some other physical proportions) are more or less constants you must deal with. The trick to ideal adjustment is to successively add these constants to eliminate the variables.
Step 1: Starting with an UNLOADED RIFLE retract the bolt fully if you are on a bolt gun, or the charging handle for AR-pattern rifles, and ensure that there is ample clear distance from your cheek to the rear of the bolt or charging handle as the case may be.
Once you find that position, you need to check your eye relief. Close your eyes and take a comfortable prone position behind the rifle. We know the length of pull is going to be a bit off, just ignore that for now. Get a proper cheek weld with your eyes shut, and when comfortable, open your eyes and evaluate your view through the optic. If you don’t have edge to edge clarity, you may need to adjust the optic horizontally until you are in the “eye box” of the scope, which is to say where you have edge-to-edge clarity with no shadowing. If you can’t get sufficient adjustment just by moving the optic, you may need to adjust your length of pull and possibly cheekrest position. Also, you should adjust the comb height at this point so you are looking directly down the center of the scope when your face is mashed down firmly on the cheekrest.
A word on adjusting the position of your optic:
Most qualified sources recommend that you mount your optic as low as possible. There are some exceptions to this if you are scoping a magnum rifle or if you are running night vision devices, but generally, lower is better. Qualified sources will also advise you to set your rings as far apart as you can.
To this, I would also add that if you are using a cantilevered scope rail, I recommend trying to mount your rings over the steel of the receiver as much as possible. I don’t like mounting a ring on a section of free-floating rail over the action, or cantilevered out over the barrel. A good starting point would be with the rings centered over the mounting screws at the front and back of the scope rail. Then place the scope in the loose rings with the windage and elevation knobs roughly centered between the rings.
If you need to slide the optic backwards or forwards to establish proper eye relief , make sure that the bell of the scope isn't binding anywhere or contacting the front of your scope rail. Tightening the rings at this point can flex and damage the scope tube. When you are sure your scope isn’t touching anyplace it shouldn't, level it and tighten it down using the manufacturer’s torque specs.
The traditional method for finding length of pull was to grip the rifle and place the butt inside the elbow joint with the forearm held at a 45-degree angle to the bicep. When shooters used shooting slings and bladed 30 to 45 degrees off the side of the rifle, this made more sense, and to this day the average length of pull on a rifle is about 13.5 inches. However, the modern shooting technique favors the shooter’s body being lined up behind and parallel with the line of bore. This results in less length of pull necessary. How much less? Well, if we do the math it looks something like this:
So the average "traditional" 13.5 inches length of pull (X) times .866 (COS 30) = Y or a "modern" length of pull of 11.69 inches. This would be fairly standard for the average adult height of 5'10" but since there really is no equation that can suit every body type the best way to adjust length of pull is by feel. For a scoped rifle, our length of pull is a function of the eye relief of the scope (cheek rest position) and the length of our neck. My suggestion is to start with a length of pull of about 12" and adjust from there.
To find the correct length of pull and drop to heel height, close your eyes again and prone out behind the rifle, getting the heel of the rifle “in the pocket,” on the muscle, not resting on the collarbone. If the heel is on the collarbone, lower it until it is not. Once the heel is correct, move the buttplate backwards until you can comfortably get behind the stock with your cheek on the cheekrest and edge to edge clarity in the scope without straining your neck forwards or backwards.
Once everything is set, get off the rifle, then try mounting up in the prone position as quickly as you can. Check your position on the cheekrest, check eye relief and comb height. If anything feels off, make small adjustments until you can mount up the rifle rapidly and everything more or less falls into place. Once you have found it, I recommend using a paint marker to draw some reference lines so if you ever have to change adjustments or take things apart for maintenance, you know how to put everything back where it belongs.
That is the technique I use for finding optimum adjustment for an adjustable stock. Depending on the type of stock you have, you may need to make some other adjustments, but keep in mind that beyond the 4 critical adjustments we have already discussed, the rest is personal preference.