Handloader’s Guide To
Understanding Chamber Pressure M.L. McPherson
of decisions. Among these is the choice to either ignore chamber pressure, and assume that they can simply abide by modern published data (precisely as published, in every aspect), or try to understand pressure and look at various indicators that act as proxies for actual chamber pressure measurement, in order to (perhaps) produce safer and potentially superior handloads. Often, target shooters and varmint hunters face the reality that the load needed is either not available from any com- mercial source or such a factory load is unaffordable. The former handloading approach
A
(abide by published data) might seem safest but that approach is not necessar- ily adequate to actually assure safety. Even when one follows a recipe ex- actly and chooses less than a maximum charge (or even the suggested starting charge!), it is still possible to generate a load that is patently unsafe when used in some particular gun (or for other reasons). This does happen and I have personal experience to prove this point. BACKGROUND
As one example, in 1974, I was
helping a young man learn how to hand- load. He had a bolt-action rifl e, cham- bered in 243 Winchester. He also had several boxes of cases that he had fi red in that rifl e; these came from 80-grain Remington factory loads. At that time, I did not know that
the 243 was notorious among ballisti- cians for generating unexpected and un- explainable pressure spikes. I did know that the 243 was reasonably similar to the 244, with which I had considerable experience and with which I had never had a problem. I had 85-grain Sierra HPBT and
100-grain SBT bullets. I also had surplus H4831 and CCI-200 primers. He bought a set of RCBS dies and we were all set to handload some 243 ammo for testing in his rifl e.
ll metallic cartridge hand- loaders face an entire series
Striker Size and Striker-to-Bolt Fit and Headspace Matter - Another example of how deceptive primer appearance can be. At center, load was near maximum that this factory-stock rifl e would tolerate before primer blanking (piercing) occurred when using this otherwise desirable but unusually soft primer; right, after reducing fi ring pin diameter and bushing bolt face (to create a good fi t between striker tip and fi ring pin hole in bolt), same load generated perfectly normal primer appearance without cratering (incipient blanking) but primer was fl atter; left, same load with new case that had signifi cant working headspace, showed unusual fl attening but no blanking. Serious primer fl attening of left example might lead some to suspect excessive pressure but, in this instance, primer appearance is merely a refl ection of what happens with a normal load when using an unusually soft primer in a case that has signifi cant working headspace.
I chose the middle charge listed in
a then-current and reputable handload- ing manual and seated the bullets at the recommended overall length. Consider- ing that the charge we used for each bullet weight should have generated similar velocity to what factory loads with similar bullets actually produced and that H4831 is a fi ne choice for 243 loads, by all rights, our reloads should have produced perfectly normal 243 pressures and should, therefore, have been perfectly safe. We went to the range and set up
targets at 100 yards, to record accuracy. Then we set up my chronograph, to record velocity. That chronograph was built from
a kit and used printed-circuit paper screens to record time-of-fl ight. A bullet crossing the 2-foot distance from start- screen to stop-screen would break the printed circuit of the start-screen and start a clock. Breaking the printed circuit of the stop-screen stopped that clock. Given time of fl ight and distance, veloc- ity calculation is a simple math problem.
Flat Primers Do Not Always Indi- cate Excessive Pressure - Result of combination of relatively soft primer and dramatically excessive headspace; despite monumental primer flattening, pressure was unlikely to have exceeded 50,000 psi. Note relatively modest engravement of bolt face features into primer cup, lack of incipient cratering around relatively large primer indentation, and complete lack of case head marking.
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