Pushing the Limit
January 04, 2011
Hornady's Dave Emary often reminds me that if you record velocities that exceed the reloading manual's numbers by 100 or even 50 fps, you're almost surely exceeding safe pressures.
The author's recent work with wildcat cartridges such as the .280 Ackley Improved
demonstrates that learning is a continuous and sometimes humbling process.
Although I've been reloading for probably longer than the average age of a typical U.S. citizen, I still occasionally forget. A recent incident clearly illustrates the point.
In my column for the May issue, I mentioned that barrel maker E. R. Shaw has started producing complete rifles. I ordered one chambered in .280 Ackley but didn't know at the time that Nosler was about to obtain SAAMI recognition of the popular wildcat cartridge that is based on the .280 Remington. For those who may not know, rifles chambered for Ackley Improved rounds share a quite practical trait, which is you can safely fire the parent factory cartridge in an Ackley Improved chamber.
I ordered samples of two Federal .280 Remington loadings and a couple of boxes of Nosler Custom Trophy Grade .280 Ackley Improved ammo. The Federal loads featured 140-grain Sierra and Nosler spitzer bullets, and the Nosler rounds were topped with 140-grain AccuBonds. After checking the rifle's action screws, mounting a scope, and giving it a thorough cleaning, I planned a session at the range.
I usually prefer longer barrels on my rifles in order to achieve top-end velocities. The Mk VII's 26-inch tube promised to deliver just that. During the first shooting session, I shot 10 rounds each of both Federal factory loads to fireform some brass for reloading. The Oehler M35P chronograph recorded velocities for both Federal loads at around 2,950 fps, which is pretty good considering the case volumes expand by five or six percent when each round is fired.
I also shot a couple of three-round groups with the factory Nosler ammo. Although the box advertised 3,200 fps, the Shaw rifle delivered 3,150 fps, which is close enough to reasonable expectations. All the groups ranged from 0.75 to about 1.50 inches, and I returned home feeling pretty good about the initial results.
After performing the usual research to select suitable handload recipes, I determined that ample charges of Reloder 22 were appropriate to launch the two bullets I chose for my first handloads. Trying to ensure the maximum initial case volume (and hopefully minimize peak pressures), I seated the bullets to the longest overall length possible. Speer's 145-grain Grand Slams just touched the lands. As recommended by Barnes, the solid-copper 140-grain Triple-Shocks were positioned so the ogive was 0.05 inch off the rifling.
Ruptured primers and velocities that are too good to be true clearly indicate that your handloads exceed safe pressures. It's time to stop the foolishness and back off!
I returned to the range and set up my chronograph and targets. I fired a few more groups with the factory rounds, and the chronograph recorded values close to the previous results. Everything seemed in order, so I switched to the handloads. The first batch of five Grand Slam loadings performed just fine. The average velocity and standard deviation (SD) were 3,152 and 12 fps, respectively. Empty cases extracted easily, and the fired primers looked just like the Nosler factory loads.
The next five-round batch with a half-grain more powder was also fired without mishap. Average velocity increased to 3,188 fps, and the SD remained exactly the same. These fired cases also appeared to be okay, so I pulled the targets, measured the groups, and recorded the velocity data.
The 140-grain TSX test loads were next on the agenda.
Although I knew the Barnes monolithic bullets probably wouldn't respond like the conventional cup-and-core-construction Speer bullets, I rationalized that the longer, solid-copper grooved bullets could be loaded over the same ample charge weights of Reloder 22. Even when I adjusted the overall length shorter for the TSX bullets to provide the recommended ogive-to-rifling gap and slightly compressed the powder charge, I didn't anticipate any problems.
I fired the first round and looked at the printout. The reading was 3,275 fps. Wow! That's when I lost my sense of reason and concluded I must have stumbled onto the mother load. I fired the four remaining rounds and punched the summary button. Average velocity measured 3,287 fps, and the SD was less than 20.
Things were looking good, right?
I reset the chronograph, chambered the first round of the next batch--with a half-grain more propellant--and touched it off. The bolt opened a bit stiffer and, although there was no smoke, the case head around the primer was sooty. When I noticed the chronograph reading of 3,367 fps, I finally regained my senses, packed everything up, and returned home.
Once again, I've seen the consequences of pushing the limit. Depending on case volume, bullet weight, and maximum allowable pressures, a specific cartridge can only achieve a relatively certain maximum velocity. Of course, barrel length is also a factor. But when those first, apparently safe handload velocities were 10 percent greater than the factory rounds (even considering they were fireforming), I should have realized I was already way into the danger zone.
As ST's ballistics editor Allan Jones responded when I related this experience to him, "There's no such thing as a free lunch."