Everybody knows that magnum primers are for magnum cartridges, right? But savvy hunters who anticipate shooting in very cold conditions — say, 20 degrees or colder — load their hunting rounds with magnum primers no matter the cartridge size, believing that the extra heat makes for more consistent and reliable ignition in frigid temperatures.
And what about rounds that are in between size-wise? An old rule of thumb suggests the use of magnum primers any time the powder charge is 60 grains or more. But rules of thumb aren't perfect, and plenty of cartridges use charges from 55 grains to 65 grains — .30-06, .270 Win. and .280 Rem., to name a few. It stands to reason that such cusp cartridges, as I like to call them, would have preferences.
In the past I've usually adhered to that old rule of thumb for magnum primers, but recently, I grew a skeptical bone and decided to find out just what effect the two primer types have. I primed 20 .30-06 cartridges — 10 with magnum primers and 10 with standard primers — and then charged all 20 with 60 grains of Hodgdon 4831SC powder and seated Nosler 180-grain AccuBond bullets. To broaden what was admittedly a very small test sample size, I did the same with .280 Ackley Improved cases, charged with 57.5 grains of Reloder 22 and topped with a 168-grain Nosler AccuBond Long Range bullet. In all cases, I used Federal Gold Medal match primers.
The areas I focused on were velocity, as an indicator of higher or lower pressures as influenced by primer type; extreme spread (ES), as a measure of variation; standard deviation (SD), as a measure of consistency; and, of course, accuracy. Resting the two test rifles over my Sinclair benchrest and a rear sandbag, I fired three three-shot groups through my Oehler 35P chronograph at a target at 100 yards, recording nine rounds for velocity, ES and SD and averaging the size of the resulting groups.
Both test rifles were custom hunting guns with premium barrels. The .30-06 is fitted with a 26-inch Krieger, the .280 AI with a 24-inch Lilja.
Ambient temperature during the test hovered between 70 and 80 degrees. Humidity was a negligible 16 percent, and barometric pressure (non-adjusted) was 25.81. The altitude was 5,050 feet.
I made the rookie mistake of beginning the first string through the .30-06 with a freshly cleaned barrel. Starting with the standard-primer load, the first three shots measured 2,721, 2,739 and 2,755 fps. Then the chronograph settled around 2,770 fps, and the last six shots had only 20 fps of variation. The extreme spread and standard deviation of the whole were clearly skewed — it calculated out to 62 fps ES and 16 fps SD — but in reality the load was admirably consistent. Taking the last six shots only, ES was 20 and SD six.
Accuracy was relatively good, too, averaging 1.32 inches. The rifle is capable of much better with carefully developed handloads, but for a load assembled purely for its powder charge weight — and tested out of a clean barrel without fouling shots — it did reasonably well.
The .280 AI was also very consistent with standard Large Rifle primers, averaging 2,891 fps with an ES of 26 and SD of seven, and because I started with a previously fouled barrel, there were no velocity anomalies through the shot string. The load promised good accuracy as well, with the three different groups averaging 0.88 inch.
So far so good. Clearly, standard Large Rifle primers are a sound choice for cusp cartridges, at least in summer temperatures. To my surprise, strings shot with the Large Rifle Magnum primers showed considerably more variation. Accuracy didn't degrade badly in the .30-06 — in fact accuracy averaged fractionally better — but the ES measured 68 fps and SD was somewhat poor at 18 fps.
But what surprised me most was the apparent lack of any additional pressure caused by the hotter magnum primers. I had thought that it would ignite more powder faster, causing a slightly higher pressure peak. But no. Average velocity was — in the case of the .30-06 — actually lower at 2,759 fps.
Consistency with the .280 AI likewise suffered. ES was broader (though still not bad) at 35 fps, and SD measured 12. Accuracy decreased, jumping from 0.88 inch to 1.32 inches for the average of the three groups. Also intriguing to me was that the vastly different powders (similar in burn rate but of different chemical compositions) demonstrated similar reactions. Both powders were very consistent with standard primers but rather less so with magnum primers.
Although a test comprising two different powders/loads in two different cartridges is anything but an exhaustive, all-encompassing experiment, I'm still comfortably assured that for most purposes standard primers are the better choice for standard-capacity cartridges in the .30-06-case family rather than magnum primers.
The test spanned the spectrum of such cartridges, using a near-maximum load in the classic and predictable .30-06 on one end and a near-maximum powder charge in the hot-rod .280 Ackley Improved on the other end. Burning more powder isn't really an option and thus isn't likely to change the results.
Shooting lighter bullets may allow a couple more grains to be squeezed in, but lighter bullets tend to prefer faster powders, and in my experience faster powders like standard primers. Plus, with a lighter bullet you risk having the projectile bumped forward by the small but enthusiastic detonation of the magnum primer, which can move it in inconsistent amounts each shot before the actual propellant ignites and horsepowers it down the barrel.
One element I was unable to include was the effect of cold temperatures. Another rule of thumb suggests the use of magnum primers to combat sub-freezing temps, but what effect does cold really have? Perhaps I'll load up a batch of cartridges identical to those in this test, wait for a below-zero day this winter and find out.