September 30, 2019
Got gas? Too much…too little…or, for a “modern sporting rifle,” just enough?
Managing chamber and port pressures is a major consideration when shooting a direct-impingement, gas-operated AR-style rifle. After all, it has to reliably feed and eject the cases before you can think about assessing ballistic performance, right? I’ve learned that successfully handloading for my Sun Devil SD308 AR hinges on determining just the right amount of a compatible propellant. But more on that later.
Toward the end of World War II, the U.S. military decided to replace the M1 Garand and its battle-proven 30-06 Springfield cartridge. Initially, the goal was modest: develop a shorter .30-caliber round with similar ballistics chambered in a modified M1. The new battle rifle would provide full automatic as well as semiautomatic capability with a higher-capacity, detachable box magazine instead of the not-so-user-friendly, eight-round en-bloc clip charging scheme.
Using the 300 Savage as a starting point, the new cartridge design evolved through several iterations at Frankford Arsenal. The case neck was lengthened, and the shoulder angle and body taper were reduced. Because the new round (unlike the 30-06) was being designed for (and not adapted to) an autoloading rifle, the case head was beefed up, and the rim thickness and extractor groove width were increased. And to achieve the 30-06’s ballistic performance in the smaller-capacity case, the maximum chamber pressure was increased slightly. The final 7.62x51mm cartridge configuration launched a 147- to 150-grain bullet at 2,750 fps, almost matching the .30-caliber M2 cartridge ballistics.
Springfield Armory was in charge of developing the companion selective-fire rifle. Winchester carefully followed these developments and in 1952 introduced a new commercial version of the proposed new military round. The final military configuration was not adopted by the Army for two more years, so there are both physical and ballistic differences between the 7.62x51mm and the 308 Winchester cartridges.
Eventually, in 1957, the M14 was chosen over the ArmaLite AR-10 and the FN FAL as the Army’s new battle rifle. Essentially an “improved,” next generation Garand, the M14 was a little lighter and eliminated or replaced the M1’s undesirable features. Although the AR-10 received positive reviews, it suffered an embarrassing composite barrel burst. It was obviously still a prototype design and needed further development.
Concurrent with this effort, U.S. small arms strategy was shifting once again because a much smaller, .22-caliber round chambered in an even more compact rifle was already being considered.
ArmaLite designed a scaled-down AR-15 chambered for a .22-caliber round. Again, this program proceeded so slowly that ArmaLite eventually sold the new rifle’s production rights to Colt. The rest of this story is marked with many missteps, but eventually, the military adopted the AR-15/M16 to complement (i.e., essentially replace) the short-lived M14.
ArmaLite continued to market the AR-10 to foreign governments but with little success and more mishaps. Many years later, the ArmaLite name and AR-10 trademark were sold, and until recently, the company continued to make both 5.56 and 7.62 NATO rifles. Several other firms offer AR-10-style rifles.
Handloading the 308
Last year I prepared a review on reloading the 223 Remington for a typical bolt action and an AR rifle. It was a pleasurable and rewarding effort. The CZ 527 and Sun Devil test rifles both performed quite satisfactorily. In fact, I purchased two additional upper assemblies (204 Ruger and 6.5 Grendel) and tested hundreds more factory and handloaded rounds with similarly excellent results. Based on those successful experiences, I ordered a scaled-up Sun Devil AR for this report.
I loaded several boxes of once-fired commercial and military brass with carefully assembled component combinations. Bullet weights varied from 125 to 190 grains, and propellant charge weights ranged from 40 to 49 grains. I included almost every Hodgdon, IMR, and Winchester propellant (from H322 to CFE 223) listed in the Hodgdon Annual Manual. I also compared that data with other recent loading manuals just to make sure caution prevailed with my test loads.
Within a month, I received a larger, but just as finely crafted, rifle featuring a stiff, 22-inch stainless Lothar Walther barrel fitted with a muzzle brake. I performed a quick maintenance regimen and mounted the same Weaver tactical optic I had used previously. I proceeded to the range with boxes of Federal and Hornady 308 Winchester match ammo and a couple of my handloads, intending to fire a few groups to break in and familiarize myself with the new rifle.
That first range session was frustrating! Multiple failures to feed, weak case extraction/ejection, blurred/burred headstamps, so-so accuracy, etc., suggested the AR-10’s direct-impingement gas system was, apparently, more difficult to manage than my AR-15’s. Think about it. There’s about twice as much propellant burned in the 308 compared to the 223. Balancing all of the variables (e.g., chamber pressure, port size/location, carrier/buffer weights, recoil spring rate/height) is apparently still more art (trial and error) than science (based on demonstrated analytical methods).
I eventually learned I would have to load faster burn rate propellants. The corresponding lower powder charges generate less propellant gas and ensure adequate chamber pressures while reducing port pressures.
I also swapped a couple of parts to help slow the carrier retraction. I ordered a Tubbs flat-wire recoil spring from Brownells and installed a heavier buffer assembly.
When you fire 308 handloads in a bolt rifle, all you have to worry about from a safety perspective is that the propellant gas and bullet exit the muzzle. You can be a lot more flexible as to which powder and how much you load to safely achieve the best ballistic performance. Manually turning the bolt provides immediate feedback relative to chamber pressure—i.e., if it’s too “tight” and the case head has shiny ejector marks, you’re obviously exceeding safe levels.
Just for a point of reference, I pulled the 168-grain bullets from four different factory loads and weighed the powder charges. They ranged from 40.8 grains (stick) to 43.0 grains (spherical). And I eventually pulled a bullet from a sample of all the factory loads I tested to measure the propellant weight.
I retrieved my Powley Computer and, after determining the case capacities of the various brands of brass to be loaded, calculated the “preferred” propellant and charge weights. As you can see in the case weight and capacity chart, IMR 4064 was the universal selection for 165- to 168-grain bullets.
After firing nearly 1,000 rounds of factory ammo and handloads, I’ve learned a lot. Coincidentally, the last group I fired, comprising 43.5 grains of IMR 4064 behind 168-grain Sierra HPBT bullets loaded in Lapua brass, functioned perfectly and put four of five shots into one hole. Of course, I couldn’t keep the last round in the first four-round cluster.
Tips & Techniques
As I noted earlier, the 7.62 NATO and 308 Winchester cartridge/chamber specs differ a bit. The most significant difference is that the 7.62 NATO case headspace is nominally a few thousandths greater than the 308’s. In addition, the commercial round is also typically loaded to higher chamber pressures than the military version. But, of course, the careful and competent handloader can (and must) adjust the tooling and component selections to accommodate these circumstances.
Let’s start with the most important factor: selecting propellants. My experience indicated I should stay within the range of IMR 3031 (fastest) and CFE 223 (slowest) when loading for my 308 Sun Devil rifle.
Also, I won’t push for maximum velocities—especially with the “slower” burn rate propellants. I found loading 100 to 150 fps under maximum provides satisfactory accuracy and ensures reliable operation. An autoloader has to run before it can shoot straight, right?
The next most important factor is properly preparing your brass. As soon as I experienced functioning problems, I ordered an RCBS Precision Mic and a 308 Winchester case gauge from Dillon to better understand and verify proper headspace. Maintaining headspace is always important, but it’s critical in an autoloading rifle. Unlike in a bolt action, chambering and extracting the cartridge/case is not augmented by muscle power. For reliable operation, the cartridge must readily “fit” the chamber and, when fired, must provide internal ballistics compatible with the rifle’s self-loading operation.
Not to worry: You can precisely adjust the full-length sizer die to ensure the resized case shoulder position is not too long or too short. The case heads of commercial 308 factory loads I checked always fit the gauge just below flush of the max reference surface. However, cases fired in the Sun Devil rifle always felt a bit above flush, i.e., the fired case shoulder had been fireformed to the 7.62 NATO chamber’s slightly greater headspace.
I carefully adjusted my RCBS full-length sizer die so that it would resize all of the neck but just “kiss” the case shoulder and set it back no more than a couple thousandths of an inch. Then I trimmed the test case to minimum length and dropped it into the chamber. After releasing the bolt/carrier assembly, I simply grasped the charging handle to see if the bolt could be easily pulled rearward to extract the empty case.
If not, I carefully cleared the jam (I learned the GI method from my gunsmith last year while testing the AR-15) and readjusted the sizer die down no more than an eighth of a turn. I resized a different case, and that time the shoulder was set back just enough to readily chamber and extract.
I learned after a subsequent range session that I must take the resizing process one step further. My initial handloads assembled with “heavy” cases, e.g., Federal and military brass, exhibited approximately 75 percent failure to fully chamber when fed from the magazine. I had to lock the carrier back, drop a round into the chamber, and release the carrier before I could fire each shot. Ammo assembled with lighter cases with the same sizer die adjustment and topped with the same bullets fed almost 100 percent from the magazine. I can only assume the material “spring back” properties varied enough to cause the problem. Obviously, I’d attempted to avoid excessive headspace too diligently.
So now I take the trial case mentioned above, seat a bullet to make up a full-scale “dummy” round, pop it into the magazine, and let ’er strip! If it can be extracted easily by drawing back the charging handle, I resize the rest of the batch. If not, I readjust the die as I noted and repeat the process with another case. When I reloaded all of those initial test loads, resizing the cases a bit more eliminated the problem.
Remember, the sizer die may be adjusted to set the shoulder back even further. However, that will surely create excessive headspace, which may cause a misfire from a weak primer strike. At best, the excess gap between the case and chamber shoulder will allow the case to stretch when fired and may cause the body just ahead of the case head to crack when the round is discharged or being resized. There are already plenty of hot gases flowing into the AR’s receiver under normal conditions. No need to add to the chaos, right?
Okay…you’ve selected a few promising propellants to try, and you’ve prepared batches of cases to load. There’s another detail to evaluate before you proceed. Another chart lists various brands or types of brass with their average case weight and case capacity.
The two samples of military cases I used are quite a bit heavier than most of the commercial ones. Since the external case dimensions are essentially the same, they must have thicker case walls. That means the internal case capacity is reduced, so less propellant is required to generate the same chamber pressure as a little more of the same powder loaded in the commercial cases. As the chart indicates, this general rule of thumb is not always true. Note the Federal and Lapua match case weights; they are almost as heavy as the military samples. I also discovered that Winchester cases from a box of match factory loads weighed 11 grains more than typical Winchester brass.
To determine case capacity, I weigh a clean, fired case and fill it with water to the bottom of the neck. I weigh it again and subtract the empty weight. Of course, this can be a bit messy, and you must make sure there are no air bubbles inside the case or drops of water on the outside of the case.