Chamfered Primer Flash Holes

In ".224 Valkyrie Precision Problems" (August 2019) author Joe Carlos discusses the titular cartridge in detail. This prompted me to expound on the effects of the inner chamfered or beveled primer flash hole. In the text he mentioned he had not personally conducted any testing related to flash hole tools. He further stated that Sinclair International makes a tool that inserts through the case neck to chamfer the inside of the flash hole. Precision shooters claim this creates a more even powder ignition.

Some claim this tool results in more consistent velocities. All this strongly suggests further investigation in this area may be helpful to the shooter. Perhaps my more than 60 years of experience working with all aspects of the modern day cartridge may shed some light on this not-oft-discussed subject as well.

While I have been unable to solicit any reliable outside information on the subject of inner primer hole chamfering, the following research is largely that which I have previously worked with over an extended period of time, and now, more recently. Primer Function The primer consists of a relatively thin wall (0.015") brass cup containing an explosive material held by a paper disk and brass anvil. The firing pin compresses the primer composition between the anvil and the cup, causing it to explode.

Hot gases pass through the primer vents that are about 0.078" in diameter and 0.070" deep to ignite the powder. These primer hole dimensions pretty much hold true for all cartridges, large or small, including those termed magnums. These holes are close to the size of a number 47 drill bit, which is the same as a 5/64 SAE bit—both with a diameter of 0.078".

Shock wave energy ignition time, the interval between the firing pin striking the primer and when pressure of hot gases is high enough to start the bullet moving from the case mouth, is only about 0.0002 of a second. The reasoning on precisely how the bullet is released from the case following ignition varies somewhat. There is evidence to support that bullet release from the case neck takes place as hot gas pressure passes between the bullet and the case mouth, thereby releasing its hold on the bullet.

A small puff of gas at the muzzle immediately ahead of the bullet bears this out. Gas actually passes the slower moving bullet before its full diameter obturates the bore leade or throat. Bullets are sometimes seated beyond the case neck, directing pressure to the sides of the bullet equal to that at the base of the bullet. Still, gas escapes past the bullet, sometimes unevenly, causing bullet tilt. Overly firm bullet seating tension can also play apart, causing muzzle velocity to vary by as much as 60

The cartridge is placed on a square surface directly below a drill bit entering the case mouth. Precise chamfer depth is controlled by a graduated stop built into the milling machine to the nearest thousandth of an inch. Here it is set at 0.020 inch, the depth used by the author in chamfering flash holes. feet per second with a noticeable effect on accuracy.

Chamfering Inner Flash Holes The precise phenomenon surrounding cartridge ignition has been debated by shooters and even 7 drill bit passed through the case mouth to the sectioned case head in the chamfering process. As described in the article, a secondary stop is used at the case mouth attached directly to the drill bit.

If such a case stop is used, cases would all need to be trimmed to the exact same length for uniform chamfering. 020" chamfer depth using 59 degree No. 7 drill bit. ballistic experts since powder and primers have been invented, but there is no great mystery here. As a primer explodes, shock-wave energy is forced into the powder column through the primer flash hole. This small flash hole is quite uniform, about 0.075" diameter, thus directing the blast largely up the center of the powder column.

The energy from primer ignition tends to distribute or compress the loose or compacted powder to the front of the cartridge case, immediately followed by partial powder ignition. Peak pressure is achieved quite early in the movement of the bullet into the barrel.

However, a varying amount of the propellant bolus may follow the bullet down the barrel largely dependent upon cartridge configuration and volume, as well as the kind of powder and primer used and the bullet seating depth and seating tension within the case and clearly the position of the powder in the case. An inner beveled flash hole tends to allow the primer shock waves to be diverted peripherally and burn the powder more uniformly from the base of the powder column.

The angle and depth of the inner primer flash hole chamfer or bevel is important and here I will cover some of the work I have done in this area and the subsequent results. A standard SAE drill bit has a 59 degree bilateral cutting angle. I use a 3/16" or number 7 drill bit to form the chamfer inside the cartridge case, preferably anew one. This can beanew or fired case with the primer removed. Never attempt to chamfer a case that has alive primer in it.

Forming the chamfer is an easy procedure but there must be precise depth control of the drill bit as the chamfer is formed. Here’s how I do it: The cartridge case is placed upright, regardless of caliber, on a solid block of metal squarely below a chucked drill bit. There must be a graduated positive stop on the milling machine or drill press graduated in thousands of an inch as a means of pre-

010" through 0.025" in 0.010" increments. A standard 59 degree cutting angle was used in forming optimal flash hole chamfer depth at 0.020" after hole contact. 6×44 SW- NEJ (top row) display the same degree of primer flattening with near full loads. Following a wide number of loads tested with many different cartridges, inner primer flash hole chamfering at 0.020" depth showed no increase in pressure signs over non-chamfered cases. cisely controlling cutting depth.

I would discourage the use of a hand drill to chamfer inner primer flash holes. In so doing you are left without precise depth control which is vital. A stop could be used attached to the drill bit in a hand drill but this may lack precision too, or the means of depth control could become altered, particularly 25-06 Remington cartridges display near quarter-minute accuracy while using both chamfered (left) and non-chamfered (right) flash holes.

Load used was 75 grain Hornady V-Max bullet, 48.5 grains IMR 4064 powder at 3440 fps chamfered and 3450 fps non-chamfered. All pressure signs were perfectly normal and uniform in these tests. A Model 700 Remington rifle with 26" Sendero barrel and 4-20X Sightron scope were used for these tests. 224 Valkyrie were the most revealing with chamfered and non-chamfered cases showing the accuracy performance between the two, shot on successive days.

As explained in the article, I used Federal cases with the drill bit set at a 0.020" flash hole chamfer depth. if case length varies. Both a chuck stop and a drill collar can be used together as a means of controlling chamfer depth where cases are at uniform length. Following extensive tests and evaluation I arrived at a good balance in flash hole chamfering and accuracy by advancing a 59 degree drill 0.020" following drill bit contact with the inner primer flash hole.

Flash hole diameters are close to 0.075" which finds the point of the drill bit flutes a few thousandths into the flash hole before the controlled 0.020" advance. A measurement across the perimeter of the chamfered flash hole will be very close to 0.150". This allows the widened primer flash to span a greater surface at the powder column base while still maintaining a solid flash hole web.

At the outset of the inner primer flash hole experiment, my primary objectives were to determine comparisons in grouping accuracy and bullet velocity and/or changes in the condition of the fired primers in both chamfered and non-chamfered cases. The recent outcome, as in past tests, proved most interesting with a number of different cartridges and loads.

For example, I recently conducted tests on the .224 Valkyrie, the cartridge critiqued by Joe Carlos in his August 2019 article. .224 Valkyrie Chamfering Tests While conducting tests on one of three chamberings for the .224 Valkyrie, I used Federal cases with the drill bit set at a 0.020" flash hole chamfer depth. Five-shot, 100-yard groups were shot with each bullet on successive days with flash holes

I designed the 5.6×44 SW-NEJ about 17 years ago. It is essentially a .223 Remington with virtually no taper and near square shoulder. This series of groups, shot on 10-8-19 and 11-9-19 with the author’s 5.6×44 NEJ-SW, clearly show the chamfered flash hole advantage. Results are with a Pac-Nor 1-8 twist barrel on a Model 112 Savage rifle and 73 grain Berger bullets. Non-chamfered 24 grain Varget gave a 0.520" average. Chamfered 24 grain Varget yielded a 0.449" average.

On 11-9-19 non-chamfered groups with Berger 73 grain bullets ahead of 24.5 grains 8208 powder grouped at 0.350" and 0.330". Chamfered flash hole groups with the same components grouped at 0.140" and 0.155". 224 Valkyrie. As velocities were increased, these proof loads served in closely comparing possible elevated pressure differences between chamfered and non-chamfered cases. No significant pressure differences were noted as a result of these carefully monitored tests.

Accuracy came out ahead on most of the chamfered loads, even during the proof fire process, as detailed in the article. both chamfered and un-chamfered. A model 40X Remington rifle was used with a 1:7 twist, Criterion 26" barrel (CriterionBarrels.com, and a Sightron (Sightron USA. com, S-TAC 4-20 scope with 1/4 minute dot—a setup capable of extreme accuracy. There were chamfered flash holes on the four upper groups, with the four lower groups non-chamfered.

The chamfered flash hole groups edged out the non-chamfered in each case. Velocity remained quite uniform with the .224 Valkyrie and other cartridges tested chamfered versus non-chamfered. This is likely to vary with cartridges and load, at least to some degree. Sierra (SierraBullets.com, 77 grain Tipped Match King and 73 grain Berger (BergerBullets.com, bullets were used in these tests. Following the .224 Valkyrie, I conducted similar tests on a cartridge that I designed 17 years ago—the 5.6×44 SW-NEJ.

This is essentially the .223 Remington with virtually no taper and near square shoulder. I used a special reamer made by Clymer Tool ( ClymerTool.com, A standard .223 cartridge can be fireformed in this chamber as it has the same headspace. It is a very efficient cartridge capable of extreme accuracy. As the photos show, tests were conducted on both chamfered and non-chamfered cases—those shown with a 73 grain Berger boat tail target bullets and three different powders: Hodgdon Varget, Norma 202, and IMR 8208 XBR.

Results were impressive. I used a Savage Model 112 Target rifle with Pac-Nor (Pac-Nor. com, 28-inch Super Match 1:8 twist barrel and Sightron 4-20X S-TAC scope with 1/4 minute dot on a Talley (Talley Rings. com, Picatinny Rail Mount—a combination capable of extraordinary accuracy. A look at the targets dated 10- 08-19 show an accuracy average of chamfered cases at 0.449" and 0.520" for the non-chamfered.

Velocity average for the chamfered cases was 2745 fps compared to 2762 fps non-chamfered, averaging out the two powders. The targets dated 11-9-19 using IMR 8208 powder and 73 grain Berger bullets are most interesting. Using the same rifle, the non-chamfered groups measured 0.350" and 0.330" while the groups with chamfered flash holes measured 0.140" and 0.155".

The trend toward improved grouping with properly chamfered cases appears to be real enough, but it requires a highly accurate rifle and well thought out loads to prove it. From the outset, working with the chamfered cartridges beyond that of merely cleaning up the flash hole, I was constantly vigilant of untoward signs of pressure. It would appear that an internally chamfered flash hole would tend to funnel internal pressure retrograde as the primer shock wave (Continued on page 22)

energy is reversed by the reversal in chamber pressure. While conducting a great number of carefully controlled tests on chamfered and non-chamfered cartridges capable of generating relatively high chamber pressures, pressure signs remained essentially normal and quite unchanged between the two. This included many different chamberings and manufacture while closely observing and noting all the typical signs of elevated chamber pressure.

While accuracy was my primary objective in these flash hole chamfering tests, I did shoot a representative number of proof fire loads with various cartridges. These included my standard 0.020" depth flash hole chamfering while approaching and sometimes exceeding maximum loads listed in reloading manuals. The chamfered and non-chamfered proof loads remained quite close together with emphasis on the typical pressure signals. A good example is the proof load series on the .224 Valkyrie.

Here I used a premium 1:7 twist Criterion 26" medium heavy barrel on a Model 40X Remington rifle with a Sightron 6-24X scope, Hornady 75 grain BTHP Match bullets, along with Varget powder and Remington Bench Rest primers. Target grouping was also recorded. I referred to the Hodgdon 2019 Reloading Manual as a guide for load selection, which listed the Hornady 75 grain bullet and Hodgdon Varget powder. The targets will show the loads used relative to velocity and grouping… there were no compromises.

While closely observing pressure signs, I exceeded the Hodgdon maximum of 25.5 grains of Varget at 2853 fps by 1.5 grains. Here, 26 grains produced 2940 fps and 27 grain produced 3051 fps. Visible primer extrusion appeared around the firing pin at 26 grains which increased slightly at 27. This was very similar when comparing the chamfered cases to those nonchamfered. As the targets show, the chamfered loads proved more accurate despite the conditions.

When asked if inner flash hole chamfering is beneficial or recommended for all handloaders, I would have serious reservations. Hunters, aside from those shooting at extreme ranges, may derive little benefit. Benchrest shooters or long range target shooters do stand to gain by it, as would varmint hunters shooting at extended ranges. If you have a really accurate rifle capable of sub half-minute accuracy you would stand to benefit the most by properly controlled inner flash hole chamfering.

It could mean the difference between winning and also-ran in a Benchrest match. I would hesitate to encourage those lesser experienced handloaders, or those lacking at least some machining ability, from delving right into inner flash hole chamfering. For those active and knowledgeable in precision shooting and handloading, the benefits appear almost certain. AG