Primers and Pressures
Date: Feb 13 2005
Many handloaders think a primer is a primer, or differentiate only between “standard” and “magnum” primers. But primer choice can make a big difference in load performance—and a REALLY big difference in safety.
This has been known among shotgun handloaders for years, but too many rifle handloaders rarely consider the side-effects of primers. Various experiments (including some I’ve performed myself) have shown that the choice of rifle primer can change the pressure of the same load over 12,000 pounds per square inch (psi). This means a load that produces a very safe 58,000 psi with one primer can produce an unsafe 70,000 psi with another—and often there’s no way for the home handloader to tell the difference.
According to handloading lore, a basement loader should be able to tell when pressures get too high by such signs as hard bolt lift, or marks from the ejector slot of the bolt face on the head of the brass. But such “pressure signs” (in actuality signs of excessive pressure) were relied on long before present-day piezo-electric testing equipment became standard. Older methods of pressure gauging were relatively inaccurate, older rifles weren’t as precisely made as today’s, and some older brass not as hard. Recent testing has shown that pressure signs such as hard bolt lift and, in particular, brass flow into the ejector slot indicate pressures ABOVE 70,000 psi. Modern bolt-actioned rifles can withstand pressures of 65,000 or less almost indefinitely, but if subjected to 70,000+ psi loads continually they will eventually fail—the reason even the hottest American magnum rounds have a maximum factory pressure rating of 65,000 psi.
If you’re lucky, continued firing of over-warm handloads will result only in certain parts stretching, rather than breaking. Headspace, for instance, might grow because the bolt lugs are stressed. If you’re unlucky, the rifle’s action can come apart all at once. This happened a few years ago to a friend who was fond of loading rounds beyond normal velocities—and hence pressures. He got lucky. The right side of the action blew, sending the heavy scope over his left shoulder and into the side of a building over 30 feet away—instead of into his face. HE OBSERVED NO “PRESSURE SIGNS” BEFORE THE RIFLE CAME APART but had been shooting hot loads in the same varmint rifle for over a year.