AR-15 Chamber Selection, Part One

If you want to start a good fight, get a barrel maker using button rifling in the same room as another manufacturer employing cut rifling. Perhaps the second bloodiest brew-ha would be to ask two armorers which is the best AR-15 chamber. If one of those gunsmiths happens to be a military builder and the other caters to civilians, the cuts and bruises will likely be worse.

Since I have about a decade working for a big military shooting team and an equal amount of time catering to civilians, I figure I’ll be more than capable of ruffling a bunch of feathers in both camps. This discussion will be limited to chambering for the original .223 Remington/5.56mm cartridge that most shooters are still using. The number of possible AR-15 chambers is nearly infinite.

There are reamer companies out there that will custom build a chamber reamer to your specs leaving the possibilities almost as endless as the human imagination. Therefore, we will try to confine our discussion to the more common chambers in general use today. The first questions to ask yourself or your customer are, "What is the finished gun going to be used for? What bullets will be fired in it?" Most folks will be using ammo set to magazine length which makes the chamber selection process much more limited.

I cater to National Match Course Service Rifle competitors—the Camp Perry crowd. Most of them actually shoot two different bullets. Their match starts at 200 and 300 yards shooting magazine-length 75 or 77 grain bullets in standing and rapid fire strings and they finish all the way back to 600 yards where most switch to more wind resistant and heavier 80s or 90s. Since 600 yard shooting is a slow fire stage, shooters usually single feed their ammo and seat the bullets out well beyond fitting in a magazine.

Chambers for these competitors are normally a compromise between what works best for the shorter ammo verses the long stuff and there is a fair amount of disagreement between gun builders on the specifics of those compromises. VLD bullets, bullets with thin jackets, military ammo, and brass and propellant preferences can also come into play in selecting a optimal chamber for any AR project. Specifics of the rifling will also need to be considered in some cases.

Before proceeding we need to include a few user friendly definitions. First is Overall Length, the distance from the rim of the cartridge to the tip of the bullet with the bullet seated to where it is just contacting the lands. This is measured with an Overall Length Gauge, such as the Stoney Point unit now owned by Hornady, consisting of a hollow rod with an insert and a dummy cartridge casing screwing onto the front.

Some gunsmiths and reloaders make their own chamber-specific dummy casing using fire-formed brass. The convention is to take this measurement with an 80 grain Sierra Matchking bullet, even if shooting another bullet, to get a common reference when communicating with other shooters. If one person takes his reading with a Berger 90 grain boat tail and someone else measures with a 55 grain FMJ there is no common reference because the bullets differ so much in shape, specifically, differing ogive profiles.

Outside of talking to others, there are many applications where you will be using the actual bullet you are firing. Looking at a typical Wylde chamber schematic you’ll note there is no overall length annotated, largely because every time the rifle is fired this gets longer as the throat erodes forward (and gets rougher) due to the hot gasses eating away at the metal in that part of the barrel. It is mostly this throat erosion that wears out a gun barrel with the rest of the rifling changing very little.

The part of the chamber the body of the case occupies doesn’t vary much between common reamers with the exception of the NATO chamber spec, which is just plain fat almost everywhere to keep functioning in dirty battlefield conditions. The front parts of reamers vary quite a bit, however, so we will spend most of our time studying those dimensions. We will start at the neck and work forward. The diameter of the neck is important for two reasons. The first is function.

Get the neck too tight and you will likely induce jams. The second is accuracy. Get the neck too loose and group sizes can get fat and fliers can be more common. There are variables that you and your reamer maker need to take into consideration. What is the rifle going to be used for? If you are magazine feeding some or all of the time you don’t want a tight neck.

However, if you are building a gun just for long range belly shooting from 600 to 1000 yards and single feeding long ammo with heavy bullets, there is benefit by shrinking the neck diameter 0.003" or so. Some advanced handloaders outside neck turn their casings. This shaves brass off about

25") ammo, such as the Black Hills 77 on the bottom. Using the same gun, they shoot 600 and 1000 yard lines with long-seated, heavier bullets like the behemoth Berger 90 grain on top. 50% of the surface of the outside of the case necks. With regard to chambers and reamers, outside neck turning makes the case necks skinnier and a tighter neck dimension in the chamber is helpful. However, if the shooter is mixing ammo for Across The Course shooting the chamber neck needs to match the fattest ammo used.

Magazine fed, magazine length ammo for rapid fire without turned necks require room in the chamber or risk the possibility of feeding malfunctions. Brand of brass is another consideration as it varies in thickness between brands. And brass thickness isn’t entirely consistent from lot to lot even from a single manufacturer. I tried— and soundly rejected—some Lapua .223 brass a couple of years ago, having used their cases in other cartridges successfully.

That particular lot was significantly heavier than Lake City and Winchester military brass and the measured case walls were thicker. To no surprise, the thicker walls shrunk internal case capacity and pressures jumped noticeably. I ended up adjusting powder charges down a half grain to keep primers from blowing out. The neck diameters were fatter with seated bullets as well.

While I achieved reliable feeding in my Wylde chambers, some reamer makers might have recommended going a little fatter in neck diameter as insurance for 100% reliable function. Referring to the reamer drawing, note the area just in front of the case mouth labeled "lead," often spelled "leade" or called freebore. Confused yet? With so many spellings and labeling it’s no wonder people scratch their heads. Leade is an area with no rifling or lands.

It is normally the same as the bullet diameter, with 0.2240" being the most common groove diameter in AR-15 barrels. The leade is sized to help keep the bullet from tipping to the side on its way forward but without offering resistance. Much like headspace, we don’t normally mess with making any changes to this dimension and the reamer manufacturers will just usually use the 0.2240" size automatically unless specifying a reamer for NATO chambers.

Tightening this diameter up too much creates function and pressure issues, and accuracy can suffer. While the diameter of the leade is normally just set at 0.2240", the length of it varies between chambers and the reamers that create them. The length of the leade conspires with the throat angle to arrive at the overall cartridge length. Moving forward in the chamber, at some point the bullet needs to contact the lands and that area on the drawing is the throat.

The throat is important, creating plenty of press and worry, and rightfully so. A bullet abruptly slamming into the lands would be damaged, so the throat is tapered in a funnel or cone shape to allow a more gradual entry into the lands. The amount of this tapering varies between chambers in general and .223 is no exception. Tapering is expressed in degrees and minutes and you would think that it would be called "throat angle" but often isn’t.

It is frequently called "leade angle." In the Wylde chamber schematic, Pacific Tool & Die expresses it as 1 degree and 15 minutes, written 1º-15’. So that everybody can be confused, however, other writers and chamber manufacturers sometimes write that as a decimal, 1.25 degrees. There are 60 minutes in a degree, so 0.25 degrees is a quarter of 60, or 15 minutes. There is no difference between 1º-15’ and 1.25 degrees.

I wish we could all agree to standardize how this is written but that isn’t likely so you’ll need to cipher the math in your head and to express what you want when custom ordering a reamer from various manufacturers. Reamer makers are used to this differing jargon so either method of stating the angle should be understood by them.

To be on the safe side, however, it is difficult for anyone to misunderstand when writing out in long hand, such as "1 degree and 15 minutes." Throat/leade angles vary in .223/5.56mm chambers. The popular Wylde chamber with a 1 degree and 15 minute angle has a very gradual taper. The more gradual the taper, the longer the throat will be. I have seen a few chamber reamers in this car-

Middle, a Hornady Overall Length Gauge for measuring overall length with an 80 grain Sierra Matchking. On bottom is a headspace gauge to check shoulder bump. The "throater" is on the left and a tight neck Wylde is on the right. Note the pen pointing to the pilot bushing. The dummy cartridge was provided to the reamer manufacturer when the order was placed. tridge with a more gradual taper at 1 degree and 13 minutes but they aren’t common.

There are a fair number of reamer designs out there, however, that go slightly steeper at 1 degree and 30 minutes and some much more abrupt in angle at a little beyond three degrees. A three degree throat will be shorter than a more gradual 1 degree and 15 minute one. How much difference do these various throat angles make in practical application?

When Bill Wylde settled on his gradual 1º-15’ angle his considerations were to provide support to guide the bullet straight into the center of the barrel while offering a minimal amount of deformation to the bullet. When I first began developing loads for the 90 grain Sierra Match King in early 2006 I was using Wylde chambers exclusively because they had been so successful with the 80 grain SMK bullets.

The jackets on those 90 SMK bullet were, unfortunately, too thin and the bullet soon earned a reputation for blow ups. In fact, Sierra credited me with the first reliable report of this problem but they were soon deluged with complaints. As a military armorer I saw it as part of my job to work with manufacturers to make products better, more reliable and more accurate and was speaking with Sierra quite frequently about this.

It was always their belief that most problems with the 90 grain jacket failures had to do with rough obduration of the base. I agreed in part with Sierra on this but felt other areas of the bullet were also weak. Building on their rough obduration theory, the amount of the bullet buried in the casing below the neck was considerable. When VLD bullets were being made with thinner jackets than today there was a notion that having the bullets extend below the neck caused rough obduration among other possible problems.

When the hot expanding gasses "smack" the base of such a bullet with no neck support the idea was that the bullet began obdurating and the back of it swelled out in an egg shape. As that swelled out part entered the barrel it had to be swaged back down, stressing the bullet quite a bit.

Since the 90 grain SMK jacket was obviously quite thin and since the bullet extended down inside my casings when seated for Wylde chambers, I decided to try a longer chamber to see if moving the bullet up in the casing to match the longer chamber would result in less deformation caused by rough obduration and lack of neck support to the sides of the bullet. I was having some barrels chambered at the time by a company that had both a Wylde reamer and another with a longer leade similar to a NATO chamber.

Using the reamer with the longer leade, we expected to lengthen the 90 grain SMK seating length as much as 0.075" before the bullet came in contact with the throat. Doing so gave better case neck support to the bottom section of the bullet and we expected to lower bullet blow ups and maybe even cure them entirely. Not! The exact opposite happened which made no sense but did fit William Bell’s prediction ("In any search for knowledge there are always unintended consequences".) Bullet blow ups went through the roof!

I talked with the folks that had done the chambering and they dug out their chamber schematic. We discovered that the throat angle was just a little steeper at 1º-30. You wouldn’t think such a tiny departure from Wylde’s would be so stressful to the bullets but there was no other explanation. The company had a "throater" reamer that only cuts at the throat and does not touch the body or the shoulder of the chamber at all.

Their throater was set up to cut a Wylde angle, so I returned the whole shipment of barrels and had them rethroat them to Wylde specs. When they came back the incidence of bullet blow ups dropped back to "normal" but some bullets still continued to blow up and that failed to completely cure that problem. The whole scenario convinced me that gradual throat angles stress bullets less. What benefit this is to more "healthy" bullets with thicker jackets, I can’t say for sure.

To clarify, this example points to evidence that the front of bullets can be stressed by more abrupt throat angles. Although the test was designed to lessen stress to the back of the bullet during obduration, it was not possible to either prove or disprove whether or not longer chambers are beneficial and this theory remains open. Next time we will look at specific chambers and their applications, discuss ordering a custom chamber reamer, and ruffle some feathers with "Encyclopedic Error." AG