The damaged Krag Jorgenson. Pic: NRA

Krag-Jorgensen breech explosion: Cause could not be established

The NRA investigation into a breech explosion of a Krag Jorgensen rifle which left two people needing surgery has concluded that the cause of the rifle’s failure could not be “conclusively determined”.

A heavily redacted copy of the report, dated 7th April 2015, was published on the NRA website last week. The bowdlerised document revealed that the accident happened on a Saturday in “spring 2014”, although a range safety notice from the MoD banning the use of Krag Jorgensen rifles was issued on 21st May 2014 by the Defence Infrastructure Organisation’s Technical Advisory Section.

UK Shooting News speculates that the ban would have been issued within a week or two of the breech explosion.

Catastrophic failure and immediate surgery

The report stated that the accident happened “when Mr D, a member of [Rifle Club B], was firing his Krag-Jorgensen rifle from the standing position in the right-hand side of the No1 fire pit using home loaded 6.5×55 ammunition.” It described how, on Mr D’s fourth shot, the cartridge case ruptured and “released hot high pressure propellant gases into the rifle action, leading to catastrophic failure of the receiver forward ring.”

A description of the full damage caused to the rifle followed.

The threaded portion of the receiver broke into at least 3 pieces and detached completely from the rest of the action, the extractor claw and a portion of the extractor arm detached, the entire left side of the action above the magazine access port distorted and failed at the rear of the magazine port. The barrel was driven forward several inches clear of the action, and the woodwork shattered around the receiver area. Metallic fragments and wood splinters were ejected at high speed in several directions. Not all parts of the firearm have been recovered.

Mr D, along with the shooter sharing the fire pit with him and an observer standing behind and to the right, was injured. All three were evacuated to hospital; Mr D and the observer both needed immediate surgery.

Ammunition was “grossly deformed”

While the redaction of the report removed all references to the qualifications held by those who carried out the investigation – and even the name of the “scientific institute” which undertook the detailed analysis of the destroyed rifle – some of the technical analysis of Mr D’s home loaded ammunition survived. Regrettably, “process detail”, which appears to be a description of the method used to determine the volume of powder recovered from the unfired rounds, was redacted.

The powder removed from 10 cartridges weighed 340.6gr and had a volume of 22ml. Useful case capacity for a 6.5×55 case is quoted as 3.68ml [in Richard Lee’s Modern Reloading, 2nd edition, p.264], thus the mean proportion of case volume taken by the propellant in the 10 cases dismantled was 0.60. It has been observed [Lee, Modern Reloading, p.35] that too little powder of a slow burning variety can in rare cases cause excessive pressure. The powder used in this accident is noted by D, in the lid of the ammunition box, to be Accurate 2520, which is a relatively slow burning rifle powder.

Continuing, the ammunition analysis discussed how excess pressure may have arisen.

The mechanism by which excess pressure might arise from a reduced charge is not technically understood. One of the conditions quoted is that the reduced charge must be 25% to 33% under normal. Since a normal load is typically about 80% of case volume, the mean load provided here (60/80 = .75) is on the limit that writers suggest is necessary for this condition to be of significance. Given that this is a mean, it is likely that about half the loads were smaller than the mean, taking the volumetric proportion into the critical range.

Critically, the ammunition analysis identified that several unfired cartridges had “grossly deformed necks and/or shoulders”, which the author stated were “indicative of improper setting or operation of the equipment at one or more stages of the reloading process.”

krag jorgenson 6.5x55 roundFurther detail was so heavily redacted by the NRA so as to provide little useful information, other than to note something about “case sizing” and “cartridge case length” (the presumably precise reference being made having been redacted) as “significant contributing factors” in the accident.

There was no suggestion that the type of powder used was a contributory factor to the accident.

Receiver ring screw thread had been damaged

A formal technical analysis of the rifle was carried out by an unidentified “scientific institute”, by a person identified only as “Dr G”. His qualifications and experience were redacted from the NRA report.

Mr D, the rifle’s owner, stated that he bought it from its original Norwegian owner in the 1960s. The rifle started its life as a standard military configuration Krag Jorgensen, built in 1897 at Steyr.

It was rebarrelled in the 1950s to turn it into a biathlon rifle. Dr G noted that while the Kongsberg match barrel used in that conversion was produced by a reputable factory, the actual work carried out at that time was “not always done by professional gunsmiths to high standards, which led to a series of catastrophic failures in the 1950s”.

The technical analysis noted that the bolt’s serial number did not match the receiver, with Dr G writing: “It is reported that the bolt handles from Steyr and Kongsberg manufactured actions were not readily interchangeable.” He did note that both bolt and receiver appeared to have been made at Steyr.

The damaged Krag Jorgenson. Pic: NRA

The damaged Krag Jorgenson. Pic: NRA. Click to enlarge

Examination of the front face of the left side of the receiver ring showed “strike marks visible on the first screw thread”, which “extended onto the top surface of the second thread producing coherent striations”. Dr G wrote:

These strike marks appeared to have been created by a flat faced object with straight sides, such as a screw driver tip or punch, and due to the coherence of the striations between screw threads they could not have been deposited when a barrel was fitted into the receiver ring.

Moreover, the damage to the threads was significant. Close inspection revealed that “the first thread from the front [had] been partially removed at some time, reducing its thickness by approximately half,” with “numerous cut marks visible on subsequent thread walls, which may have been as a result of attempts to alter the thread width or depth”.

In addition, on the exterior surface of the left side and lower receiver ring sections were “a series of uniform impressed marks”. Dr G speculated that these “may have been the result of this area being struck by another object or due to compression by another surface, such as being clamped in a vice or similar holding device.” He wrote that the marks’ presence “may indicate… [that] the work may have been undertaken by an inexperienced or amateur gunsmith”.

Detailed examination of the screw thread area near where the receiver fractured revealed “secondary cracking of the screw threads immediately below the fracture surface”, with Dr G further noting that “the smaller of the cracks appears to be emanating from a notch cut into the wall of the thread”.

Mr D stated that no work, other than routine maintenance, had been carried out on the rifle since he bought it in the 1960s. Dr G stated that no definite cause of failure could be established.

Key findings

The NRA report’s conclusions were:

  • The most likely cause of the failure was the rupture of the cartridge case of a round of homeloaded ammunition when fired
  • The rupture most likely occurred because of some combination of excessive pressure, weakening of the case and unsatisfactory fit of the case in the firearm, although it is not possible to determine which of these factors were present and in what proportion they contributed.
  • Excessive pressure could have been generated by a known but rare and improperly understood phenomenon associated with the propellant charge being significantly smaller in volume than the cartridge case.
  • Following the case rupture, the firearm may have failed because of resulting excessive load, or because of a latent defect reducing its ability to resist a load that would otherwise have been inconsequential.

Three of the seven conclusions were redacted.

The NRA recommends that all Krag-Jorgensen owners have their rifles inspected by a competent gunsmith before using them under NRA rules.

UKSN comment

While the report itself is of interest – revealing what appears to be a rifle with inherent if insignificant flaws, caused by what seems to be a part-botched rebarrelling job carried out 65 years ago, and hand loaded ammunition which certainly doesn’t look right – the level of redaction removes crucial detail; in particular, almost half of the report’s conclusions.

While UKSN defers to the technical experts’ views on the quantity and type of powder used by Mr D, surely, given the bulge in the unfired rounds’ necks, the remaining ammunition should have been test fired under laboratory conditions in a chamber cut to the dimensions of the damaged barrel (as near as can be determined) to conclusively determine its average working pressure? Again, UKSN’s “formal technical qualifications” start and end with a GCSE in physics, but it would seem wise to determine empirically whether the deformed ammunition was developing higher pressures which led to the failure of the receiver. Weighing the powder is all well and good provided the cases are of standard dimensions – which, as the report states, they are not.

Certainly your correspondent experienced a nasty shock with a hangfire some weeks ago, which appeared to have been caused by a single strand of flannelette material finding its way onto a cartridge case – which dented around that strand when fired. If one simple strand of cloth can do that, perhaps the bulged cartridge case necks of Mr D’s 6.5×55 reloads deserve further testing.

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2 thoughts on “Krag-Jorgensen breech explosion: Cause could not be established

  1. Fuzzbean

    For all its faults, the Krag-Jorgensen action buttons up the cartridge head in the chamber a lot tighter than most actions, including even the famous M1898 Mauser with its “inner ring” breech.

    Looking at the remains of the gun, this is the type of failure I’d expect to see from a barrel full of grease, a cleaning rod left fully in the barrel, or a second bullet lodged immediately ahead of the chambered cartridge… such as might happen if one forgot the powder charge in the previous cartridge that was fired. Probably the cleaning rod scenario would leave noticeable marks in the barrel bore, and the grease would be ruled out if previous shots had truly been fired, but the squib load situation seems quite likely to me. Obstructions farther down the bore typically cause bulging or bursting at the point where the bullet encounters the object, while leaving the breech undamaged; obstructions nearly touching the chambered cartridge frequently leave the barrel undamaged while wrecking the breeching. The reduced charge of slow powder might produce the wrecked breech, but it seems relatively unlikely in a cartridge case of this modest capacity. Accidentally using too fast a powder is another possibility, if it were an extremely fast pistol or shotgun powder. But then the other rounds would most likely contain the same powder.

    The cartridge in the gauge appears like maybe it was sized in some type of 6mm cartridge sizing die, and then a boat-tail 6.5mm bullet forced into the undersized neck. This is certainly not a sign to give me confidence in the shooter/reloader. I’d like to know if proper 6.5×55 brass was used, or were the cases formed from one of the many more common rounds which have undersized case heads.

    It seems a real shame to me that the Krag-Jorgensen action, even one poorly gunsmithed, is taking blame for something which was certainly caused by shooter error. Things like this do not just happen… after holding together for 60 years, something specific caused that rifle to come apart now.

    Like

    Reply

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