Barrel Chamber Bevel And Case Head Position

Here I will address an incident where a shooter experienced a partial case head blowout that released brass fragments into his face and right eye. At the time, the man was shooting a relatively small .224 caliber cartridge chambered in a Model 700 bolt action rifle. The load was well under maximum with no sign of excessive pressure noted on any of the previously fired cases. Cartridge headspace was normal. Extraction was normal and primer pressure signs were unremarkable.

The case separation appeared to be a fortuitous occurrence and there may not have been anything he could have done to obviate the unfortunate accident, however, upon further examination the rifle and damaged cartridge revealed the apparent causes leading to the ruptured case head. While there can be several causes of cartridge case head leak or blowout, I will isolate the more likely ones surrounding this particular incident.

Even with normal cartridge headspace, complete enclosure of the case in the chamber is not normally attained because some case head protrusion is required for normal extraction. Therefore, the case rim must protrude sufficiently for the extractor to lock onto the case rim. This is accomplished in different ways depending on bolt face and extractor design. On some firearms the case rim extends well beyond the chamber rim, exposing a greater portion of the case head.

In such a case, part of the case head is not surrounded or enclosed by the chamber wall, leaving it weaker to some degree. The farther the case head Notwithstanding, cartridge case ruptures can occur for reasons which must be identified and corrected where possible. This picture shows a case head blowout at the juncture of the chamber rim near the base of the case web. Note the deep recess for the bolt head used on the Remington Model 700 series rifle bolts.

This feature enshrouds the bolt head and chambered cartridge in a ring of steel in the event of case head rupture or separation. As seen here, cartridge case rupture can occur, causing gas leak along and beyond the bolt. protrudes, the greater the chances of case failure, particularly if higher chamber pressures exist. This, combined with a deeper bevel at the chamber mouth further increases the likelihood of case failure.

This is particularly true where an inherent defect or weakness exists in the case head or case wall, in the web portion of the case distal to the head. Of course, excessive cartridge headspace would further compound the problem. But this was not noted in the earlier incident. As I further studied the aforementioned incident of case blowout, the chain reaction essentially

Such a ring is a definite danger sign of impending case head blowout or separation. Though this circularly beveled portion of the chamber is only a few thousandths deep and little in increased diameter, this is where the case head ruptured.

While this slightly oversized bevel may not be the cause of the aforementioned case rupture, it must be viewed as a contributing factor. involved the following: A weakening in the case wall near the web, followed with excessive peripheral chamber bevel at the point of case blowout, and then the case being subjected to maximum protrusion On left, Model 700/40X with recess for bolt head. Note the slightly varying degree of chamber mouth bevel with center barrel showing slightly more than barrel at left.

Barrel on right is from a Model 112 series Savage bolt action rifle. Here the chamber is flush with the end of the barrel shank with a much greater chamber mouth bevel, yet the cartridge head can be chambered more fully. This barrel design does, however, enclose more of the case head within the actual chamber walls despite the greater amount of peripheral bevel.

Where greater strength is required, there is much that can be learned about barrel, chamber and cartridge construction that can be applied by both the gunsmith and the shooter for safer shooting. Regular monitoring of incipient pressure sign through fired case inspection is a good place to start. beyond the chamber bevel as a result of bolt face design.

Even though the case head on a Remington Model 700 style rifle is enshrouded by the recessed bolt face, it does extend out beyond the perimeter of the chamber entrance into the chamber mouth bevel. A closer look clearly showed that the partial case head blowout did take place precisely at the juncture of the chamber mouth where it is beveled. Had this case head been allowed

to enter more fully into the chamber surrounding the area where the case head had blown out, it is unlikely this particular case head would have ruptured. I cite this example as there are rifles, by design, whereby the cartridge case head has less extension beyond the chamber mouth during bolt lock up. Such a design would likely have prevented this particular case blowup. No other fired cases in the lot showed any sign of defect.

There did appear a contributing cause, as I shall point out, leading to this particular case head rupture. Chamber Bevel Close Up As the accompanying pictures clearly reveal, the blown out case head involved the juncture immediately adjacent to the bevel of the chamber. This shear area just forward of the inner case web extended 0.170" forward of the base of the cartridge where the case wall begins to taper and thin.

It was difficult to discern if this particular case would have ruptured had it not been for the 0.020" of chamber bevel added to the 0.150" of case head protrusion on this particular style rifle. While I have witnessed a good number of case head cracks and actual separations involving other rifles, this is an isolated incident where the case metal sheared precisely at the periphery of the chamber bevel. I have conducted extensive research in cartridge case head separation in the past.

One of my works appeared in the 1997 Precision Shooting Annual entitled "Case Head Condition Affects Bullet Behavior." I have also used X-ray as a means of monitoring case head condition, one work which was published in the May 1969 issue of American Rifleman. Modern rifles have gas bleed-off ports to divert gas leak. However, these were of little help in this case where the path of brass particles was forced parallel to the bolt by ensuing gases.

The brass particles actually etched the stock wood immediately rearward of the bolt shroud as well. I believe the scope ocular lens housing served in preventing further divergent gas and particles from reaching the shooter’s eye. Lessons Learned As a cartridge is loaded into a chamber there is little to guide it aside from the magazine follower, the bullet, and/or the taper of the case shoulder. In many instances cartridge case taper is minimal.

Therefore, some degree of bevel or chamfer is applied at the periphery of the chamber entrance to help prevent brass shaving or case jamming as the cartridge is chambered. This bevel or chamfer varies among manufacturers and gunsmiths and even with identical barrels of a given manufacturer. The degree of chamber rim bevel must, however, be kept to an absolute minimum. Where excessive bevel does exist, it can present a danger as was experienced here.

Being aware that such a condition can exist should be observed by the gunsmith and, if identified, be corrected. Close observance of correct cartridge headspace is also required. Conclusions As gunsmiths, we should know that excessive chamber rim bevel can present a danger and keep this to an absolute minimum. This is particularly true where the case head extends farther out into the bolt face, or where excessive cartridge headspace exists.

As remedial action, where excessive chamber mouth bevel exists, the barrel can be set back one thread and refaced before applying only minimal bevel to the chamber. Being observant of incipient case head stretching or signs of excessive case stress are also important. These can be precursors of pending problems. Beyond this, as gunsmiths, there may be little we can do to obviate problems of this nature but we must remain vigilant nevertheless. AG American Gunsmithing Association