A Short (Stroke) History of Tappet Operation, Part I: How It Works

Nathaniel F
by Nathaniel F

Despite being the subject of a 1952 drama film, David Marshall Williams and his tappet form of gas operation remain somewhat poorly documented at the introductory level. This post and those that follow hope to provide a starting point to readers by describing the history and operation of his very important method of powering selfloading firearms.

The key ingredient of tappet operation is a lightweight, fast-moving piston that imparts considerable momentum to the bolt carrier group. For such a small object to have enough momentum to reliably cycle a firearm operating group, it must be moving very, very fast, which in turn requires that great pressure be exerted on it. Because of this, true Williams-type tappet guns are readily recognized by the placement of their gas blocks; a standard short-stroke variety will have its gas block fairly far out on the barrel, where the pressure is lower, while a true tappet operated gun will have its gas block as close to the breech as possible to impart maximum force to the moving components, through high pressure on the tappet itself.

A simple diagram of a tappet gas block. The tappet actuator is highlighted in red, and the operating rod or bolt carrier is highlighted in blue. The very, very light tappet is the diagnostic feature of this type of gas system. Original image sourced from rkba.org, modified by the author.

Further, conventionally-operated short-stroke guns typically feature long rod-shaped pistons with their own dedicated return springs and no gas sealing rings, while true tappet guns generally have short, cylindrical pistons (the “tappet” of the name) with no return spring, and have gas sealing rings similar to those found on the bolt stem of an AR-15. The variations on these two themes are so few and far between, that they can be considered diagnostic: One might say, a weapon with a rod-type short stroke piston featuring a dedicated return spring is always a conventional short-stroke design (e.g., SVT, FAL, AR-18) and one with a short, cylindrical piston with no return spring and gas sealing rings is always a tappet design (M1 Carbine, SCAR). However, the design space of short-stroke weapons is represented by a continuum of different designs and implementations of the concept, and in theory there is no defining line between a gun operated by Williams’ invention and a “conventional” piston.

What makes the Williams-type short stroke piston significant is the mass it saves. In order for a selfloading firearm to operate reliably, a certain amount of momentum must be available for the moving parts group (that is, anything that moves through the entire stroke of operation) to both transmit force to the bolt and fire control group on the back stroke of operation, and transmit force to the ammunition as it strips rounds from the magazine, during the forward stroke of operation, both without losing velocity. While in theory a weapon’s moving parts group could be made very lightweight, and reciprocated at very high velocity to generate the necessary momentum, in practice weapon life and durability concerns prevent the operating group from moving much faster than in current designs. Because of this, for military firearms there exists a “minimum weight” for the moving parts group, and any components of the weapon that are involved in the action but which do not travel the full length of the stroke (such as a short-stroke piston) constitute additional mass over this minimum. Williams’ design minimizes the mass of the piston, while also moving it rearward, allowing for a much lighter – and handier – design. Indeed, some designs of the Williams’ family, which will be covered in following articles, are among the absolute lightest weapons in their class, in no small part because of his excellent gas piston design.

Continued in Part II, Early Tappet Designs.

Nathaniel F
Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.

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