Colt Salvo Squeeze Bore

In 1961 Russell Robinson, designer of among other things the Robinson Model 11, patented a multiple projectile cartridge system to be used in conjunction with a ‘squeeze bore’. The Salvo Squeeze Bore concept (not to be confused with the US Army’s Project SALVO) sees the barrel taper towards the muzzle the theory being that this will separate the projectiles so each leaves the muzzle separately. The concept was developed as a force multiplier capable of vastly increasing an individual’s firepower. When tested in an M2 Browning with a cyclic rate of ~500 rpm this saw ~2,500 projectiles fired per minute. 

The patent wasn’t granted until 1969 during which time Robinson and Colt had developed the concept in a number of calibers including .50/.30 fired from the M2 Browning, 7.62/.220 and 9mm/.30. Additionally the concept was also tested using .45ACP and 5.56mm. The calibre designations reflect both the integrated dimension (.50) and the calibre of the individual projectiles (/.30). The projectiles were adhered together with a wax material and some were encased in early polymers to enable reliable feeding. All firearms firing the salvo ammunition had the front portion of the barrel removed and replaced with a smooth bore tapered section (see image #5).

Various SSB projectiles and plastic jackets (source)

The US military’s interest in the system saw it become part of Project Agile in the early 1960s where the Advanced Research Projects Agency oversaw the development. Project Agile’s aim was to coordinate research and engineering support for various projects which might aid military and paramilitary forces engaged in or threatened by conflict in remote areas of the world (i.e. Vietnam).

The aim of the .50/.30 squeeze bore project was to convert the M2 heavy machine gun (designated an anti-materiel weapon) and give it an anti-personnel role by increasing its firepower and beaten zone of fire. In order to convert the gun only a squeeze bore barrel and the SSB ammunition was needed. Early testing was accomplished by taking standard barrels and cutting them down and threading them for a 19inch long tapered smooth bore section. The work was carried out by Robinson’s own company Robinson Improved Conventional
Arms. This system was tested by the  the US Army Limited War Laboratory with preliminary reports describing the concept as sound with effective penetration out to 200 meters. However, problems with the wax bonding of the projectiles and concerns over fouling and barrel wear were raised. It was field tested aboard US Navy brown water patrol boats in Vietnam with the aim of suppressing targets in thick foliage along river banks.

Agile also oversaw a project developing a projectile described as ‘strip bullets’ which also acted as a force multiplier by increasing firepower in a shotgun-esque manner by using a this was also tested in weapons ranging from .30 to .50 calibre much like the SSB system. This system was described as: 

‘a number of short lengths of lead wire pressed into a die to form a core of conventional bullet size and shape and then clad with a thin copper foil. When the strip bullet is fired through a bore, the centrifugal force imparted by the rifling causes the strips to separate as they leave the barrel. This produces a shotgun effect and increases the hit probability.‘

However, by 1964 the strip bullet project had been abandoned as it required further development and other systems of improving the kill capability of small arms appeared to offer greater probability of success. 

In 1968, Colt released a technical report based on the US military testing of the .50/.30 M2 on river patrol craft (image #1 show the dual M2 mount used). Colt’s report also acted as a reply to the Limited War Laboratory ’s 1963 Test and Evaluation report which noted concerns about smoothbore barrel wear and lifespans. The Limited War Laboratory report had recommended that while promising the SSB ammunition used by the guns was not sufficiently reliable to be recommended for immediate field use. Colt’s 1968 report countered this claiming that improvements had been made to both the ammunition construction and the barrel design and profile, boasting that effective range had been increased from 200m to ~1,000m. The report also included details of penetration tests and projectile dispersion patterns.

Diagram from Colt’s 1968 technical report showing the various M2 barrel configurations available (source)

Colt claimed that two SSB barrels and 5,000 round of ammunition would cost approximately $10,000 - the argument being that this is less than the cost of a new weapon system. With production underway Colt believed that their .50 calibre SSB rounds could be produced for the same bulk cost of standard M2 .50 armour piercing ammunition. Despite this the US military appear to have lost interest in the concept sometime between the late 1960s and early 1970s. American involvement in the war in Vietnam began winding down during the early 1970s and it is probable that the need for the weapon system was waning. 

However, direct military interest in the concept survived until at least 1965 in the form of pistol-calibre experiments using a .45/9mm cartridge in tests at the Frankford Arsenal with an M3 submachine gun. These tests found that the SSB concept had promise but there were again concerns about barrel wear and fouling as the test ammunition’s projectiles were made from cast steel.

Above: 1969 patent drawing for the .50/30 calibre SSB round with 5 projectiles (source)
Below: .50/.30 Colt Salvo Squeeze Bore Cartridge (source)

The squeeze bore concept was also considered for use in a variant of Colt’s abortive Model 1971 which would have offered a double stack 9mm pistol with a 15-round magazine which when using Robinson’s triplex salvo ammunition (see image #3)would have increased the pistols firepower to 45 projectiles.

A technical report (on the Model 1971) from Colt in March 1971, describes how “the average person does not have the ability, especially under stress, to reliably obtain hits with a pistol except at the shortest ranges” and that the SSB ammunition simulated a shotgun effect which would increase hit probability while retaining the ballistic efficiency of pistol ammunition. While in theory more ballistically stable than say shotgun pellets the rounds do not have the same point of impact Colt’s testing using a rest mounted Uzi showed that at 50 feet the 9mm/.30 SSB rounds had an extreme spread of some 6.3 inches and a mean radius of 1.7 inches while at a much longer range (47 yards) the mean radius opens to 8.8 inches while the extreme spread increases greatly to 42 inches.

It appears that there was some Israeli interest and involvement with the SSB system as Israeli headstamped cases exist. Much like the Model 1971, the SSB project seems to have been quietly shelved making it an interesting side note in the evolution of ammunition.  

Sources:

Image One Source

Image Two Source

Image Three Source

Image Four Source

Image Five Source

Colt Salvo Squeeze Bore (SSB) (source)

Colt’s Model 1971 Double Action Pistol, Technical Report #850-2112, March 1971

Calibre .50/.30 Salvo-Squeezebore for Riverine Warfare - Colt, 1968 (source)

Sidearm and Shotgun Ammunition for COIN & RAC (Project Agile) Report #BAT-171-24, Aug. 1965 (source)

Advanced Research Projects Agency, Project Agile, Semi-annual Report, July-December 1963, (source)

Advanced Research Projects Agency, Project Agile, Quarterly Report April-June 1963, (source)

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