Eugene Stoner is widely known as the designer of “America’s Rifle,” the AR-15/M16/M4, and its original parent, the AR-10. He also created the U.S. Survival AR-7 .22 rifle, which has passed through many hands and is still in production. This is the takedown rifle that fits in its own stock. Then there was his aluminum receiver, extremely lightweight 12-gauge shotgun which was produced for several years but soon left the marketplace. And since there was an AR-7, AR-10 and AR-15, there must have been AR-1s,-2s, and so on, and SAR has covered these numerous times.

While the AR-15 guns were originally chambered in .223/5.56, the AR-10, the original design, was chambered in .308/7.62 NATO. Now the design is available in a raft of calibers, from .22 Long Rifle to .50. It also serves as the base for bolt-action guns all the way up to .50 BMG.

The direct impingement gas system dates to 1900 France, but it was the combination of the gas system, the intelligent use of plastics and aluminum and, most important, the ergonomics, which revolutionized small arms design. The author recently discovered that Stoner’s talents were not limited to rifles. He is credited as the designer of a 25mm autocannon!

The Project

In 1964, Stoner was a consultant to Thompson-Ramo-Wooldridge (TRW), which was working on a 25mm cannon. TRW has a long and storied history. It was started in 1901, after developing a method for making bolts by electrically welding the heads on. TRW moved from that to making parts for cars, then complete aircraft engines, later rocket engines, semi-conductors, satellites and ultimately spacecraft! They evolved into one of the first and largest “aerospace” companies, until they were taken over in 2002. But they also were involved in firearms development and production.

In 1961, TRW received a contract to build 100,000 M14 rifles and another contract the next year for 219,691 rifles. They eventually made 4,874 National Match rifles, which are held to a high standard for accuracy and speak to the quality of the company’s manufacturing abilities.

It was in this time frame—1964 to be exact—that a new project came to TRW, the Vehicle Rapid Fire Weapons System, which became known as the Bushmaster cannon. It was a tight competition, including multiple candidates each from Philco-Ford, General Electric and Oerlikon. The prototypes included a Vulcan type rotary gun, a revolver (single barrel, rotating chambers), traditional single-barrel guns utilizing several different operating systems, even a two-barreled system. The TRW candidate was called the “TRW-6425.”

The competition was somewhat open-ended, as no round was specified, only a result. The intention was to create a vehicle-mounted fully automatic cannon, capable of defeating a lightly armored vehicle at a distance. Guns of 20mm, 25mm and 30mm calibers were contenders. There was already an “interim” gun, and the performance of the new gun had to be better.

Being mounted on a vehicle offers some advantages and some constraints. The gun would have both electrical and hydraulic power available, and weight would be less of a problem than say, in an aircraft. But if you have ever been in an armored vehicle turret, space is very limited, a heavy gun system reduces the ability of the vehicle to carry men and ammo, and the gases from firing must be exhausted from the turret. In the early tanks, crewmen were incapacitated by the fumes from their guns.

The TRW entry prototype only took 22 months from contract to first firing in 1965. Great Britain and France, among other countries, also tested the gun. Various tests and studies went on until 1973, but work at TRW had stopped after Philco-Ford acquired the rights to the design in 1969. Only six of the guns were built. Renamed the PFB-25 (and Oerlikon’s related KB series of cannon), the ammunition was developed by Oerlikon.

The TRW-6425

The maximum rate of fire was 550 rounds per minute, with lower rates available, including semiauto fire. It is fed from a belt of linked rounds. Locking of the bolt to the barrel extension was done via the multi-lug system, cammed by a bolt carrier similar to the familiar type in the AR series guns. But there the similarity ends, as the 25mm was recoil-operated. It uses the short-recoil action, where the barrel recoils less than the length of a loaded cartridge. A hydraulic accelerator moves the bolt carrier assembly at 1.5 times the speed of the barrel, providing the impetus for the bolt carrier to cam the bolt out of engagement with the barrel extension. This allows for a short receiver inside the turret.

In the long-recoil system, the bolt remains locked to the barrel until the entire assembly recoils more than the length of a loaded cartridge, then the bolt is held back and the barrel moves forward, extracting the case by pulling the barrel off of it. This requires a very long receiver.

Testing

In short, the TRW-6425 was a failure. There were many malfunctions and lots of part failures. Of course, the idea of testing is to find any weaknesses and see if they can be remedied. Some issues were in fact resolved during testing, and the Ordnance Corps found the gun had military potential but needed further development. Of course, Stoner worked for TRW, and there is no record of him working for Philco-Ford.

It is possible that Philco-Ford was “hedging its bet” by submitting two different guns using two different operating systems into the competition. Certainly having Stoner‘s name associated with the gun would not have hurt.

Ultimately, the Hughes 25mm M242 chain gun was selected for the Bradley Infantry Fighting vehicle. It weighs 230 pounds and can fire 100 or 200 rounds per minute. The same gun in 30mm was adopted as the M230E1 for use in attack helicopters. Both guns are externally powered electrically and operate using an “endless chain” (allegedly designed from a bicycle chain), hence the name. The use of electric power means a shorter receiver inside the turret and less problems with the gasses produced entering the confined space of the turret. It also means a failure to fire (dud round) does NOT stop operation of the gun. The dud round is ejected along with the empty cases, and the cannon continues to fire.

Had TRW kept the rights to the project, Stoner may have perfected it, but history is full of “could haves,” “would haves,” and “should haves.” Eugene Stoner will always be secure in his place among gun designers for his work on “America’s Rifle,” the AR series. The TRW-6425 gets placed with other designs by Stoner—and every other inventor—as one that just missed the niche. Sources: The Machine Gun, Volume V, by George M. Chinn; U.S. Rifle M14—from John Garand to the M21, by R. Blake Stevens.

Gatling-type weapons utilize external power, usually electric motors or hydraulic systems, to rotate a cluster of barrels into firing position. While this can promote greater reliability and high rates of fire, the penalties tend to be an increase in complexity and maintenance difficulties, a high system weight and bulk, plus a greater cost.

In contrast, Hughes engineers offered their single barrel, rotating bolt, electrically driven “Chain Gun,” so called because of its motorcycle type sprocket and chain operating mechanism. Not unlike an automobile engine’s timing chain, the Hughes system provides unparalleled precision, smoothness and positive action to all phases of the cycle of operation.

Anyone who has been frustrated by the process of separately adjusting headspace and timing on a Browning .50 cal should appreciate how simple and “idiot proof” the Chain Gun is. Each movement from feed to ejection takes place at exactly the right time with no fuss and bother.

Use of a single barrel means lighter weight, relatively few moving parts, and consistent accuracy. Unlike Gatling system revolving cannon, the Chain Gun’s single fixed barrel is not subject to boresight deviations inherent in multi-barrel clusters, nor to the “lawn sprinkler” effect of tangential velocity as experienced from Gatling pipes spinning at high rate of fire.

A logical alternative to self-cycling designs is provided by the Bradley’s electrical system as a power source. Operated by a small 1.5 horsepower motor, the M242 runs quite smoothly with little recoil-induced vibration. In the unlikely emergency of a total electrical system failure, a hand crank can cycle the gun for a last-ditch shot or clearing.

Damping of the short recoil stroke of the barrel is efficiently accomplished with a sealed unit containing ordinary coil springs.

Dual Feed Not Double

Dual feed – its capability to instantly switch over between two types of ammunition – is one of the more useful characteristics of the Chain Gun design. This means that a gunner who is expecting to engage an armored vehicle can immediately change to a high explosive if an appropriate target presents itself.

The Chain Gun’s cycle of operation begins and ends with the bolt fully rearward and no round in the chamber. This eliminates “cookoff,” the possibility of accidental explosion of a round left in an overheated chamber. Similarly, “double feed” – attempt to feed a round when another is still in the chamber – is blocked by a sensing rotor that instantly tops the feed system if a round fails to extract. In addition to avoiding ugly malfunctions, this eliminates the possibility of catastrophic in- turret explosion from jamming the nose fuze on an HE round being fed.

Its deceptively simple mechanism is all the more remarkable for its extraordinary reliability. Not only do published figures cite better than 10,000 mean rounds between stoppage, the gun can also be cleared with speed and safety.

Not surprisingly, the Hughes CHAIN GUN overcame virtually every objectionable aspect of competitive systems and quickly took the lead in the Army’s Vehicle Rapid Fire Weapon System test program. The M242 was formally adopted on 19 February 1973 with the official nickname of BUSHMASTER, the large and deadly poisonous snake of the Central American jungles.

Ultimate Machine Gun Mount

Since the hard-hitting, multi purpose 25mm M242 version weighs over 230 lbs and has an effective range in excess of 3000 meters, the best way to maximize its lethality is to put it in what might be called the ultimate machine gun ground mount.

This is the Bradley Fighting Vehicle, a fully tracked armored weapons platform with a stabilized turret and highly sophisticated day and night sighting systems. Designed to keep up with the M1 Abrams’ main battle tank in the mechanized “AirLand Battle” once envisioned for central Europe, it clearly outclasses its rival, the Soviet BMP armored fighting vehicle.

As mounted in the Bradley, the Chain Gun has distinct advantages in mobility and crew protection, allowing the gunner to identify and destroy a wide variety of threats including bunkers, other armored vehicles, and even helicopters. And for those situations when the 25mm isn’t ideal, the Bradley turret also features both a 7.62mm M240 machine gun and a pair of TOW missiles, fully integrated with the vehicle’s fire control system.

Live Fire Gunnery Training

Small Arms Review was invited to examine the Chain Gun in action with the North Carolina National Guard’s 1st Battalion 119th Infantry Regiment (Mechanized). The Tarheel trackmen were up in my neck of the woods at Fort Pickett, Virginia’s Range 13, spending several weekends rotating Bradley crews through the demanding Bradley Table VIII live fire qualification course. This was a perfect opportunity for me to have plenty of time to observe and photograph all aspects of the training without getting too much in their way at a stressful time.

Under command of Major Greg Lusk, this is a great bunch of guys and I found them to be quite serious about maintaining high standards despite the very real problems of today’s budget- starved military. Master Sergeant Tim Davis, battalion training NCO, wisely hooked me up with a Charlie Company Bradley M2 commanded by Sergeant First Class Dannie Lewis. Along with the gunner, Sergeant James Pierce, Lewis and the rest of his crew patiently answered my endless (and often borderline stupid) questions and provided a step-by-step “show and tell” for photographs as they prepared for their turn at bat.

After a thorough before-operation check of all systems and final preparations of both the Chain Gun and the M240 coaxial machine gun, Lewis’ Bradley pulled up next to the big ugly ammo HEMMT. The Army has a standard load formula for this course so they got the prescribed mix of 25mm M910 TPDS-T plus linked 7.62mm in the usual four ball plus one tracer mix. According to Army specifications, the 25mm Target Practice Discarding Sabot- Tracer matches the velocity and trajectory of real combat ammo well beyond 2000 meters with twin advantages of lower cost and an 8000 meter maximum range that won’t exceed the impact area safety fan limits.

The gunner, Sergeant Pierce, showed me how the linked 25mm is hung in two separate compartments on opposite sides of the turret, ready for feeding as the situation dictates. A simple diagram is thoughtfully provided on each magazine to make sure the bullets are pointed in the right direction; important soldier proofing to help tired and stressed crewmen. For safety purposes the belts weren’t actually fed into the guns at this time. Instead, this final step was not performed until the vehicle was cleared to move onto the range for its trial by fire.

Riding the Range

Wishing them well, I moved up to the main range area for a quick orientation. The sand table was a great place to start, providing a miniature overview of the terrain including firing points and target types. While some might say that this gives an unfair advantage not likely to be experienced in combat, the Army contends that the job of determining baseline gunnery skills should be done in an orderly and standardized manner. Coping with surprise targets on unfamiliar terrain is a higher-level challenge, best done after the crew has proven that it has mastered the basics.

The range control tower was manned by a team of active Army Master Gunners who had been sent to Ft. Pickett to conduct the Guardsmen’s testing. These guys were making sure that each task in the course was fairly presented and impartially graded. Their work was not made easy by the somewhat poor condition of the whole radio-controlled target system. It performed erratically and sometimes just refused to work, giving the unit’s target detail plenty to do downrange with resulting delays. On a positive note, this gave me an unusual opportunity to ride with them and to get a close look at both the stationary and moving target mechanisms in the enormous impact area.

The effects of eight years of miserly military budgets can be seen in a million ways, big and small, throughout the armed forces; in this case worn-out target lifters paired with old and abused electronic controllers. While Bill and Hillary flew around like a king and queen on Air Force jets, muddy-boot soldiers make the best of a bad situation literally with duct tape, and coat hanger wire. Many have to buy batteries themselves.

In the rare situation when everything goes well, the daylight part of the table can be fired by a Bradley crew in about 45 minutes. This includes a substantial amount of time for zeroing and for getting into full chemical gear for Task 6.

The course begins with the vehicle moving up into the first of two fixed firing points identified on the accompanying map as BP2. This is used to zero both the coaxial machine gun and the Chain Gun by optically sighting clearly identified panels positioned 500 and 1100 meters distant. If the tracers hit the panels then the guns are zeroed, if not then it takes some more time and adjustments to get them working together.

When the vehicle commander and gunner are satisfied with their zero then record firing can begin, escalating in degree of difficulty. From stationary Bradley and fixed targets to shooting at trolley-borne movers while on the run, the course presents many realistic situations likely to be encountered in combat. Bare minimum qualification standards require hits in at least seven engagements including one while wearing a gas mask and two at night.

No Excuses

When the crew on the hot seat has finished its run, they download and clear their guns then dismount for an unflinchingly honest evaluation. All of the target action for each Bradley’s range time is recorded on gun camera videotape with an on-screen time code, then played back for the crew by a member of the test team like a real-world video game. Each target presentation scenario can be looked at again and again in real time or in slow motion and even stopped for a detailed analysis.

This high-teh After Action Review not only allows the Bradleymen to see what they did right or wrong, it also provides the necessary evidence to challenge any miss-scoring that might have happened in the tower such as not leaving the target up for the minimum required time. More than just a series of dry score sheets, these videos give the unit’s leadership a highly objective view of each crew’s proficiency in target identification, choice of the best weapon (main gun vs. coax) fire commands and marksmanship.