Since 1742 an ancient device called the ballistic pendulum has been used to find the recoil level. This shoot-from type of ballistic pendulum involves free hanging the gun from wires and firing it in mid-air. The distance the gun raises is a measure of what is called free recoil energy. The shoot-at type of ballistic pendulum is used to determine the energy of a projectile. With some minor variations and the addition of modern instrumentation, we’ve been using these methods for the last three centuries.

Controllability evaluation is more challenging. In cases where everyone fires the same weapon and ammunition and tries to evaluate controllability, a great inconsistency between shooters becomes evident. Even the same shooter does not repeat the same controllability performance. Most controllability evaluations involve little more than asking shooters how quickly and accurately they felt they could get off that second or third shot. Inconsistent results are a huge frustration to the military as controllability is important to keeping a burst of full automatic fire on target.

A few years ago, the U.S. Army’s Program Manager for Small Arms saw the need for advancement in technology and awarded a study contract to Knight’s Armament Company, Titusville, Florida. Project goals included the improvement of recoil measurement techniques and a better metric for controllability.

The engineers at Knight’s began their study with a thorough review of every recoil study report available. They found that almost all the reports had the same theme. A gun follows Newton’s 3rd law of physics: “For every action, there is an equal and opposite reaction.” The force that pushes the bullet and gun gas through the barrel and out the muzzle is equal to the recoil force of the gun. The rearward velocity of the firearm and its weight are multiplied together in a formula that gives the recoil energy. It’s hard to figure out how fast the firearm is recoiling especially if there is any device on the muzzle that diverts the gun gas from going straight ahead. Muzzle brakes and even flash suppressors turn the gas to give a forward force on the weapon that slows its recoil velocity. This is why researchers generally take the easy way out and find the energy with the
ballistic pendulum.

Knight’s engineers noted that recoil studies for the military almost always focused on the shooter. Repeated input of high levels of energy into the shoulder causes bruising and very high recoil energy can cause damage to the eye. The U.S. military measures the free recoil energy of every shoulder fired weapon it fields; classifying each into categories that limit how many rounds per day can be fired. Their table shows that if a gun develops less than 15 ft-lbs, (20 Joules) of energy, unlimited firing is permitted. The M4 and M16 fit this category. The highest level on the table is 60 ft-lbs (81 Joules), above which no shoulder firing is permitted. Knight’s testing found that a typical 3½ inch 12 gauge magnum shotgun develops 59 ft-lbs of energy which is alarmingly close to the military’s maximum.

While the energy method might be useful for making decisions about how many rounds per day are appropriate, its value is limited when studying recoil. The level of free recoil energy doesn’t tell anything about how much recoil force goes slamming into the shoulder. Here’s an example with results that may surprise you. Suppose one gun has a constant 300 pound recoil force and pushes against your shoulder for 1 inch of travel. In this case, recoil energy is calculated by a simple multiplication to give 300 inch-pounds of energy. Now take a second gun that pushes with a constant load of 100 pounds over 4 inches of rearward travel. The second gun has 400 inch-pounds of energy. It’s hard to appreciate that the gun with the lower force has significantly higher free recoil energy, but it’s true. This is what is so perplexing about the study of recoil. The energy method only tells part of the recoil story and that’s why the Army supported Knight’s investigation.

At the beginning of their study, Knight’s engineers instrumented both guns and shooters with the latest accelerometers, force gages and other measurement devices. Data recovered from the tests with the new instrumentation was good and certainly usable, but not remarkably better than what had been found previously with older test equipment. Their worst surprise came when they had shooters fire at full auto and filmed the target using high speed video looking for a pattern to shot placement. They were frustrated by the inconsistencies between shooters. The project results to that point were very disappointing, showing no promise to advance the technology in recoil measurement and controllability.

One of the engineers found an old Government report that talked about replacing the human shooter with a mechanical device that mimicked the shooter’s motion during firing. The metal body parts were to be connected with springs and dampers (shock absorbers) having the same characteristics of muscle and bones. Army researchers inserted a sketch of the concept in the report, but never built it. Knight’s engineers liked the idea and took it to a higher level. They also modeled the human vibrational characteristics in order to pick the right springs and dampers and then built a mechanical device with the same characteristics. This required the use of a sophisticated analytical method called modal analysis.

To understand modal analysis, you must first accept that all bodies vibrate at their natural frequency. For example, a guitar string vibrates at a natural frequency when plucked. It is also true that most bodies – guitar strings included – have more than one natural frequency, and these can occur simultaneously. The lowest natural frequency is called the first mode of vibration, followed by the second mode, etc. Each mode is at a higher frequency than the preceding one, and each has its own shape. For all bodies, there is also a natural tendency to stop the vibration called damping. Some bodies, like the Tacoma Narrows Bridge built in 1940, didn’t have enough damping and destructed when excited at its natural frequency (YouTube shows a fascinating video of the Tacoma Narrows Bridge failure.) In contrast, there is so much damping in the human body that vibration dies out quickly. To find the natural frequencies and mode shapes, engineers input different levels of vibration into a mock up weapon being held by a shooter. Each shooter was fitted with instrumentation to study the body’s response to each level of vibration. In this way, they found the vibrational modes of what the military describes as their smallest, average, and largest size shooter. Using this information, the WRSS was built to have the same characteristics.

To be able to measure controllability, Knight’s put angular measurement devices on the WRSS in order to determine the up and down movement of the end of the barrel (pitch) as well as the side-to-side motion (yaw). The WRSS precisely tracks the point of aim during and after the firing event. For hunters this information is critical for the follow-on shot. For the military this is important for controlling bursts of automatic firing, and essential to the design of muzzle devices. A precise measurement system is invaluable in the development of devices designed to reduce muzzle motion during shooting. Why? Simply because unless there are huge performance differences in these devices, even an expert shooter can’t detect changes in performance.

Besides controllability measurements, the new shooting fixture records the force on the shooter’s shoulder, the acceleration levels (g-loads) at the buttstock and on the barrel. The new WRSS has other benefits as well. Using the acceleration data, the WRSS has already been useful in solving problems with failures in gun mounted optics and other electromechanical devices. A data plot called a Shock Response Spectrum (SRS) has been used to study how many g’s the shooter, gun, and mounted accessories must endure at various frequencies. (Remember that at 1 “g” a 10 pound body weighs 10 pounds, but when subjected to 10 g’s, that same body weighs 100 pounds.) These g-levels are important to shooter reaction and more important in the development of relatively fragile accessories like scopes, laser pointers, and night vision.

Using the WRSS fixture and SRS data plots, Knight’s engineers determined the cause of a puzzling failure of a night vision scope. The scope was tested on one gun and determined to be capable of withstanding the high shock environment, yet failed when fired from a differently designed weapon of the same caliber and weight. Why the night vision scopes failed on the second gun, but held up well on the first gun, became immediately evident on the SRS data. The SRS curve of the two guns was almost a perfect match at low frequencies, but at high frequencies where electrical equipment is susceptible to failures, the second gun showed that much higher forces were being experienced.

The US Army intends to use the WRSS in its testing laboratories, and the design has been turned over to a not-for-profit organization called the Institute of Military Technology (IMT). IMT will offer the WRSS to weapons manufacturers, government laboratories and testing facilities worldwide. Commercial firearms manufacturers may also procure the WRSS from IMT for their use.

By the summer of 1918 he was nearing his 65th birthday and might have been thinking about retirement, or at least slowing down.  John had enjoyed an illustrious and profitable career in designing and building of firearms, and now faced a request that would be difficult to turn down.  Fighter pilot and son of ex-president Theodore Roosevelt had just been shot down over France in a dogfight with an armored German aircraft.  This tragedy further proved we were being outgunned in the air war.  General Pershing, commander of the American Forces, had sent a message from England asking for a new .50 caliber round and two new machine guns; one for aircraft and the other for ground application.  This was a time of war and American patriot, John M. Browning, would answer the call with his best effort.

For the operating system, Browning selected his famous short recoil cycle, deriving drive power from a recoiling barrel rather than porting the barrel for gun gas.  This was a logical choice as the test gun from his earlier design, the .30 caliber Model 1917, fired over 39,000 rounds without a stoppage in acceptance testing.  These new designs would be essentially scale-ups of the Model 1917.

Browning met most of his design objectives but there were two design challenges that he could not achieve.  With the machine tools available at that time, the dimensions that established the location of the bolt face and the depth of the chamber could not be held tightly enough to control the fit of the cartridge in the chamber.  This important dimension, known as headspace, can cause problems when out of specification.  Depending on tolerances, the round could be too tight in the chamber, and the gun wouldn’t shoot at all.  At the other extreme, the round was too loose in the chamber which resulted in a stoppage at best; or a ruptured cartridge at worst.  The ruptured cartridge presented a serious danger to the shooter, spewing brass shards at high velocity out the bottom of the receiver.  Another dimension that couldn’t be held close enough governed when the firing pin would fall – a dimension that later became known as timing.

Since these weapons had to be made on existing machinery, Browning made an easy adjustment that required the operator to screw the barrel into the barrel extension, moving the barrel toward the bolt face to reach the proper headspace.  He developed a couple of simple gages that allowed the operator to adjust to the proper dimensions.

Weapon development proceeded rapidly, and then slowed after the Armistice was signed on November 11, 1918, bringing World War I to an end.  Between 1923 and the ten years that followed, both aircraft and antiaircraft models were further refined with the new designation as the M2 for the aircraft model standardized in October of 1933 and the ground gun one month later.  John M. Browning did not live to see this day, having passed away in 1926.

Only three years after standardization, the Spanish Civil War offered the M2 its first challenge.  It was the ammunition, not the gun that was in question.  The socialist Spanish Republicans supported by the Soviets and Mexico were in a civil war with the Nationalists who were supplied armament by Hitler and Mussolini.  The war was heavily covered by the press and a proving ground for new armament.  The Russian 20mm was quite effective as an aircraft weapon inspiring a reevaluation test against the M2.  With its lighter, but still quite effective ammunition, testing in 1937 determined the M2 would be retained as an aircraft weapon.  Military tacticians preferred the increased ammunition capacity of .50 caliber, even at the expense of a lightweight cartridge.  Additionally, the high rate of fire of the aircraft M2 increased hit probability over slower firing larger caliber weapons.

T176 machine gun. (Springfield Armory National Historical Site Archives)

When the U.S. entered World War II, the M2 air and ground variants were quick to demonstrate their effectiveness.  In anti-tank applications gunners would attack the tank treads whenever they encountered heavy tanks.  The aircraft model was found on almost every aircraft in both fixed and flex applications.  The Mitchell bomber, for example, carried fourteen.  Ironically, even the Japanese were using the Browning, having purchased the design before declaring war, chambering the weapon for the 12.7 Breda cartridge.

As World War II ended, the military decided to look at some other gun designs.  By now, modern manufacturing methods allowed most weapons to have fixed headspace and timing, yet there was one other major design drawback.  The M2 wasn’t well suited to use in tank turrets.  In designing a tank turret, it is important to keep access to the gun feeder from inside the turret so the gunner can load and unload from within.  The M2 has a very long receiver, particularly the distance from the front of the feed location to the aft of the weapon.

At the war’s end, tanks had proven their battlefield effectiveness, evolving from vehicles that adapted existing weapons to a point where tank designs dictated dimensional constraints for new weapons.  With a new main battle tank program on the horizon, the military had to get serious about finding a more suitable tank weapon.  In the early 1950s they funded three different gun development projects to find a replacement.  The days of the M2 appeared to be numbered.

The first of these was inspired by a large caliber German aircraft revolver cannon that looked like a promising approach so it was redesigned as a .50 caliber.  Low reliability and an excessive amount of toxic gun gas introduced into the tank turret caused the design to be dropped.  No matter.  Two other designs were in the works.  One was the T176, developed by Frigidaire and the other was the T175 by Aircraft Armament Incorporated (AAI.)  Both had short receivers, fixed headspace and timing.  Each had a new ammunition link that allowed the designers to shorten the receivers.

The receiver of the T175 was a full 7.5 inches shorter than the M2’s and that of the T176 was comparable.  The M9 rearward stripping link, used with the M2, was a metallic link that had evolved from the cloth link belts used at the beginning of the century.  Ammunition could only be drawn rearward for removal and was the principal cause for the M2’s long receiver.  Frigidaire and AAI opted to develop a link as well as a weapon in order to meet the size constraints.  Neither the T175 nor the T176 were developed in time for the Korean War so the M2HB and its aircraft variants, ANM2 and M3, was the heavy machine gun of choice, and performed admirably.

In 1958 it was time to end the dynasty of the M2.  The new M60 tank was beginning development and its designers were promised a machine gun with a short receiver.  Since the new weapon would require a new link and offer fixed headspace and timing, there would be no place for an old weapon with a World War I link?  Besides, having the same ammunition on the same battlefield with two different and incompatible ammunition links was an unthinkable logistics nightmare.

The T175 AAI machine gun was further developed, designated the M85 and put into series production at Springfield Armory.  It offered two rates of fire to meet both air and ground applications.  The M85 was used on the M60 tank, but was not popular with tankers due to its unreliability.  The M85 lacked control of the feeding rounds and problems with the rate reducer confined its use to the M60 and a few other vehicles.

Both the M2 and M85 were found in Vietnam though the tank of choice there, the old M48, still used the M2.  After Vietnam, the General Electric Armament Systems Department, developers of the modern Gatling guns, proposed a unique approach to solve the tank gun problem.  In 1978 they were awarded a design study contract for a .50 caliber externally powered weapon.  Their GE-150 could use either link interchangeably and the gun fit in both the M60 tank and the new M1 tank, then in development.  A cycling model was successfully built but a lack of funding killed the program.

Early versions of the M2 machine gun. (Springfield Armory National Historical Site Archives)

In 1979 the U.S. Army launched its own design of a .50 caliber machine gun.  The intent was to develop a .60 caliber weapon, but Picatinny Arsenal in Dover, NJ, decided they would make the first prototype in .50 caliber due to ammunition availability.  The Dover Devil, as they called it, was lightweight, gas operated, and could feed from either of two incoming ammunition belts, giving the operator a choice between ammunition types.

The design was built and tested but the U.S. military decided against funding further development since by now the M2 had proven itself and there was plenty of ammunition in the field linked with the old M9 link.  To keep the receiver short, the Dover Devil used the same link as the M85 but by now there was no longer much ammunition around that was linked that way.  The Dover Devil may not have interested the U.S. Military, but a Singapore company was interested and by its own admission copied the design and completed development.  It is now standard in the Singapore Army.

Without customer support, neither the GE-150 nor the Dover Devil proceeded past the prototype stage.  For the new M1 tank, Chrysler would be obliged to make the commander’s cupola large enough to fit the M2.

As the 20th century came to a close, the M2 was now the only adjustable headspace weapon in the inventory.  Reports of injuries from improperly headspaced weapons were on the rise.  Three companies, Saco Defense, Ramo, and FN Herstal, all produced Quick Change Barrel (QCB) conversion kits that offered fixed headspace and timing for the M2.  In 1997 the U.S. military held a QCB competition which was won by Saco Defense.  Unfortunately, funding was lost before the design could be fully evaluated and the program ended.

Even though it required headspace and timing adjustments after barrel changes, the M2HB served in Desert Storm and performed with distinction, receiving huge accolades from the troops for its accuracy, reliability, and devastating firepower.  Today in Iraq and Afghanistan the M2HB is one of, if not the top rated, machine gun in theater, yet, as expected, reports of field injuries from improper headspace adjustment continue.  Frequent troop rotation has once again brought to light that setting headspace and timing is a perishable skill.

In 2007, the military found the money required for a Quick Change Barrel Kit and held a new competition.  Saco Defense no longer existed, but won the competition again under the name of its new owner, General Dynamics (GD).  The M2 with the QCB kit will be type classified early next year as the M2A1.  The contract required that 30 kits be built and these are currently undergoing evaluation, with fielding expected for late 2010.  As it nears its 100th birthday, the M2 will finally join the ranks of modern weapons with fixed headspace and timing.  The M2 will be ready to last through another century.  But will it survive?

For a number of years, GD has had another .50 caliber weapon in development.  Borrowing design concepts from earlier work in developing a grenade machine gun, GD decided to adapt the concept to .50 caliber.  The XM806 uses new lightweight materials and a unique impulse averaging operating cycle keeping its weight to a trim 40 pounds – less than half the weight of the M2.  GD is also offering a new 14 pound tripod to accompany the XM806 that is half the weight of the standard M3 tripod.

At the end of 2009, GD will deliver twelve XM806 systems to the U.S. Government in support of a 450,000 round endurance and environmental test.  Approval to enter production is expected in September, 2010 with low rate initial production expected to begin shortly thereafter.

Soon our old M2s will be upgraded so that the headspace and timing will be fixed, thereby resolving both safety and tactical issues.  Next year, a new lightweight.50 caliber weapon, the XM806, will also be available to augment and perhaps one day replace the M2.  Both of these weapons use the same M9 ammunition link, so there will not be a battlefield logistics issue.

All the moving parts seem to work together except that the M2 receiver is still too long to fit in compact vehicle turrets and the XM806 receiver is just as long. What about the tanks?