Among the various types of rifles, bolt-actions tend to produce the highest level of accuracy due to their general superiority in the areas of barrel-to-receiver rigidity, action stiffness, breech lock-up and minimum manufacturing and operational tolerances. Another benefit of the bolt gun is its straight-line ignition system which gives uniform primer indentation and, with the proper trigger mechanism, allows a fast lock time and a light, crisp trigger pull. The aspects of design that affect trigger pull quality are the subjects of this article.
Before we get into a discussion of trigger design, let’s first define a few terms. The term “trigger” is often used to refer to both the entire system that releases the firing pin (the trigger mechanism) as well as the individual part of that mechanism that is directly pulled by the finger. That double usage will be followed here, except when a distinction must be made between the two meanings to avoid confusion.
The firing pin, also known as the striker, is a thin rod, often, but not always, having a round cross-section that is contained within the bolt body and which is powered by the mainspring. When the firing pin is released and driven forward, its small rounded tip impacts the primer, causing ignition.
The cocking piece is attached to the rear of the firing pin, and does two things. First, it engages a helical cam—or in the case of a few designs, two or more such cams—in the bolt body, designed to retract the firing pin and compress the mainspring when the bolt is turned. In virtually all modern commercial actions, this is accomplished when the bolt handle is lifted, known known as the cock-on-opening pattern. Second, the cocking piece engages the sear. In some designs, there is no separate cocking piece; the rear portion of the firing pin is shaped to perform the same function.
The sear is the component that retains the cocking piece in the cocked position and is acted upon the trigger piece, or an intermediary lever in three- or four-lever designs, to release the cocking piece and fire the gun.
Lock time is the interval from trigger actuation to firing pin impact on the primer. The shorter the lock time, the greater the potential accuracy, particularly in long range shooting. This is because in almost all rifle shooting situations the gun is moving slightly, even as the trigger is pulled. There is less time for movement with the shorter lock time.
Lock time depends upon a variety of factors, such as mainspring rate and compression, mass of the firing pin and cocking piece, length of firing pin fall and friction. As a general rule, lock time goes down with a heavier mainspring, a lighter firing pin/cocking piece, and shorter firing pin travel. Most military bolt rifles of both World War I and World War II featured heavy firing pins, in an attempt to ensure ignition reliability even under battlefield conditions of dirt and neglect. The lock times of such rifles are thus relatively long, ranging from about five to nine milliseconds. By comparison, modern commercial bolt guns from makers such as Browning, Remington, Sako, Savage, Weatherby and Winchester generally have lock times from two to three milliseconds.
Speedlock kits can improve the lock times of both military and commercial bolt guns. These kits usually consist of a heavier mainspring and lighter firing pin; some also shorten firing pin travel. The reductions in lock time obtainable from such kits can be dramatic—as much as 40 percent with some military bolt rifles and 30 percent or more with some commercial models.
In many cases, lock time will decrease simply by increasing mainspring weight alone. The use of a heavy spring with a heavy firing pin, however, can increase the vibration created by firing pin impact. This vibration can be transmitted to the barrel and thus affect accuracy. Thus, for best results it’s usually preferable to install both a heavier mainspring and a lighter firing pin. Tests have shown that the snappy firing pin impact this combination produces is more reliable in igniting primers than a heavier but slower blow.
In addition to lock time, there is also barrel time or action time, which is the interval between firing pin impact and the bullet’s departure from the muzzle. The duration of this interval—on average, one and a half milliseconds or less—is determined by factors such as powder burning rate, case capacity and barrel length. Alterations in a gun’s trigger and ignition mechanism do not affect action time.
Classic military bolt rifles such as the Mauser M98, Japanese M1905 Arisaka, M1903 Springfield and the M1917 Enfield feature direct-pull or double-draw triggers. In such designs, the front of the sear is attached to the receiver by way of a pivoting joint; in the rear, the sear’s vertically protruding nose engages the cocking piece. The trigger pivots on the sear. As it does so, rounded lumps or lobes on its heard bear against the underside of the receiver, camming the sear nose down and out of engagement with the cocking piece. Generally there are two lobes, the first allowing a small initial amount of trigger movement to take up the slack with a light pull, the second requiring a heavier pull to finally disengage the sear.
This pattern yields the two-stage pull characteristic of military rifles. Though direct-pull triggers tend to produce heavy, often creepy pulls, they are well-suited to military use, as they are rugged and simple, and their pulls are appropriate for the rigorous conditions, rough handling and nervous trigger fingers of wartime.
Set triggers add a secondary mechanism to the basic trigger pattern. When this secondary mechanism is set by pulling an auxiliary trigger or pushing the single trigger forward, the sear can then be tripped by a light final pull. Though popular in Europe and found on many European guns, set triggers have never really caught on with American shooters, possibly because the override triggers available here for both military and commercial rifles provide a pull as light as any set trigger. In addition, because set triggers require two mechanisms to trip the sear instead of just one, they generally have longer lock times than override triggers.
The most common type of trigger found on commercial bolt rifles is the override trigger, so called because the cocking piece overrides the sear when the trigger releases the sear. Most commonly in such a trigger, the sear pivots at the front or rear, and is supported by the trigger piece against the pressure of the spring-powered cocking piece. When the trigger is pulled, the sear is no longer supported, and is easily pushed down and out of the way by the cocking piece.
Aspects of trigger pull
Probably the most salient characteristic of a trigger pull, certainly the aspect most folks focus upon, is pull weight. Pull weight, usually measured in pounds or ounces, is the force that must be applied to the trigger piece to release the sear. Most folks find that a heavy trigger pull makes both a surprise break and a consistent shooting-hand pressure harder to obtain. Thus, the general rule is the lighter the pull, the better—within reason. Hunting triggers vary from a few ounces on precision varmint guns to three or four pounds on big-game rifles that may be fired by gloved hands. In some competitive disciplines, such as certain types of NRA High Power Rifle shooting, the minimum permissible trigger weight is specified in the rules. In others, such as benchrest, there is no minimum, and one-to two-ounce triggers are common.
Pull weight depends upon several factors, such as the smoothness of the engagement surfaces or friction; the power of the trigger return spring, which is compressed when the trigger piece is pulled and resets the trigger piece when it is released; and the load or downward force on the trigger piece when the gun is cocked. With a two-lever design, in which the sear bears directly on the trigger, the spring-powered cocking piece cams the sear down forcefully, creating a considerable load on the engagement surface of the trigger piece. The power of the mainspring and the geometry of the trigger-system components set a lower limit on the weight reduction that can be achieved with such triggers, even when engagement surfaces have been stoned and the and the return spring has been lightened.
Pull weight can be substantially lightened by the addition of intermediary levers that reduce the load on the trigger piece. This principle is the basis for the three- and four-lever benchrest-type triggers that have pull weights as low as two ounces or even less.
Pull weight is not the only critical trigger characteristic. Just as important as crispness, which can be defined as the absence of creep. Creep is a slight grating movement of the trigger as it releases the sear. This is usually caused by roughness on the trigger/sear engagement surfaces, or by less-than-perfect trigger mechanism geometry. Crispness is essential to producing the surprise break that is conducive to accurate shooting. In my opinion, you’d be better off with a crisp three-pound trigger than a creepy 2½-pound pull.
Reducing creep is most often accomplished by smoothing the engagement surfaces on the sear and trigger piece, usually with a fine stone. Lubricants such as Brownells Action Lube Plus can also help reduce creep somewhat, though they are no substitute for stoning. Reducing the amount of engagement on the sear and the trigger piece can also reduce creep. In a three- or four-lever trigger, this is done between the intermediary lever and the trigger piece. However, even professional gunsmiths are usually reluctant to reduce the factory engagement, as insufficient engagement can easily lead to an unsafe condition in which the trigger may go off prematurely, or the cocking piece may not be held rearward by the sear when the bolt is worked vigorously.
Consistency—the uniformity of pull weight, crispness and other characteristics—is an important but often overlooked aspect of trigger quality. I’ve shot rifles whose pulls varied by as much as a pound from shot to shot. Even though a trigger should provide a surprise break, it must also be uniform. A trigger that is heavy, then light, or shows creep on some shots but not others, is annoying and distracting.
Another aspect of trigger quality is pre-travel. Pre-travel is the movement of the trigger piece in a two-stage trigger system that occurs before disengagement of the sear begins. Pre-travel is different from creep, which is the trigger movement that occurs during sear disengagement. Pre-travel is not much of an issue with most commercial triggers, as they are usually of single-stage design. It is an issue common to military guns, however. Some modern commercial rifles, such as the Sako TRG-22, also have two-stage triggers. On that model, the pull weight of the first stage can be adjusted.
Most target shooters also prefer a minimum amount of overtravel—the rearward movement of the trigger piece after the sear breaks. In general, the less trigger movement as the sear breaks, the greater the potential accuracy. However, a certain amount of overtravel is essential to allow the necessary clearance for the sear or intermediary lever to drop down when it is released by the trigger piece. Many triggers have a screw that allows overtravel adjustment.
Finally, some target shooters prefer a trigger piece that has a specific location, shape and angle. Some top-level target guns afford adjustment of the lateral and vertical angle and fore-and-aft position of the trigger piece.
A few guns even offer a choice of either a straight or curved trigger piece. Such triggers are seen primarily on rifles used in international-style competition.
Aftermarket triggers and accessories
Although, as noted previously, the quality of a stock factory trigger can be improved by stoning engagement surfaces and lightening springs, in many cases the safest and best way—and sometimes the only way—of getting a light, crisp trigger is to install an aftermarket unit.
Shooters have been replacing the heavy direct-pull triggers on military rifles ever since such guns flooded the surplus market in the post-war years. Timney is the largest and best-known manufacturer of aftermarket triggers for military turnbolts. Among the various manufacturers, there are models for many common bolt-action rifles used since World War I, including small- and large-ring Mausers, Enfields, and M1903 Springfields. The replacement triggers for these rifles are generally two-lever override designs that allow adjustment of pull weight and overtravel, and give pulls as low as around two pounds.
As noted earlier, most military guns have fairly long lock times, the result of the combination of heavy, slow-moving firing pins and long firing pin travels. The lock times of these guns can be speeded up somewhat with the installation of heavier mainsprings such as Blitzschnell springs and even better, speedlock kits that feature both heavier mainsprings and lighter firing pins and cocking pieces. Many speedlock kits are also designed to reduce firing pin travel as well, sometimes by as much as 50 percent.
Almost all modern commercial bolt-action rifles have override two-lever triggers which, in theory, should provide a light crisp let-off. Performance is compromised in some commercial trigger designs in which the trigger also serves as the bolt stop. While many trigger mechanisms can be adjusted to give a light, crisp pull, other designs can’t be so improved, even by a skilled gunsmith. With these triggers, as with stock military triggers, installation of an aftermarket unit is the only way to get a really good pull. Again, Timney is the most popular brand, with units for many bolt rifles affording pulls in the two- to four-pound range—perfect for hunting and many target disciplines.
Remington rifles are probably used more frequently than any other brand for building custom hunting, varmint and competition guns, due to their strong but simple actions, well-designed triggers, and round, rigid and easily-bedded receivers. Rebarreled and accurized by a professional gunsmith, these rifles can be capable of extraordinary accuracy. Accordingly, there are more aftermarket parts—and a greater variety of aftermarket target-type triggers—for Remingtons than any other make.
Lighter pulls, especially the one-to-eight-ounce pulls preferred by all benchresters and many varminters, require three- and four-lever trigger designs. Among benchresters, the premier ultralight trigger is probably the Jewell, which is made in models for many popular rifles. Pulls down to 1.5 ounces are possible with the Jewell target models.
It should be understood that many competition triggers lack safeties. Such triggers should be used only on target guns, and even then, extra care must be taken to observe proper safety procedures. Also, the proper, safe use of light-target triggers requires a certain amount of experience. While seasoned benchresters can apply a gradual, controlled pull to a ½-ounce trigger, many novice shooters accustomed to four- and five-pound pulls won’t even be able to feel a two-ounce pull, much less control it.
A final word: I’d highly recommend that you have all trigger work performed by a qualified professional gunsmith. This applies not only to delicate operations such as stoning trigger engagement surfaces, but even to installing so-called “drop-in” triggers or trigger components. While there are things the average gun owner can do to roughly test trigger safety (such as pounding the butt of an unloaded, cocked gun on the ground to see if the trigger jars off, or working the bolt vigorously to determine if the cocking piece slips off the sear), a competent riflesmith is much better able to assess trigger safety and function. You might save a few dollars trying to install a trigger unit yourself, but I doubt if it’s worth the liability you could incur if a trigger-related malfunction caused a premature discharge.