(Continued from Part 1.)
Pressure Limits, PSI, CIP, and CUP
How many PSI are in a CUP? And to continue the absurdity, how many CIP’s of PSI are in a CUP? Could we be more baffled and confused? Yes! And this is all the comedic relief we can expect. To include CIP into our calculus is unnecessary, yet it is mentioned only to note that it is a competing metric used in European manuals and can be a source of additional confusion. Some of the false assumptions about the pressure limits of the Swedish Mauser may lie in the inaccurate translation between the European measurement of pressures expressed as C.I.P., and North American means, methods, and metrics to measure pressure that are expressed in terms of C.U.P., or P.S.I.
All three are different metrics that represent different values and none are directly correlated with each other. There is no direct conversion formula for one to the other. Yet they can be loosely or indirectly, or weakly correlated to provide a rough conversion that is essentially an educated guess. As each rifle is an individual, so the actual maximum pressure for a rifle can be higher or low than the maximum working pressure any particular reloading manual states. There can be significant differences in the test barrels and other factors that influence the results that are recorded in reloading manuals that further add to the disparity of test results and real-world application, including the individual powders used that behave differently, especially at top pressures. I would not use a faster burning powder than IMR3031 in an antique rifle.
Turn of the century Swedish Mauser M96 rifles with proper headspace may tolerate a tad more pressure when using slow-burning powders such as H4831, IMR7828, RL22, RL23, RL26, MRP, and WXR that produce top velocities in their 28.5-inch barrel. The pressure spikes of the very slow burning powder might occur at the furthermost end of the barrel and be a tad more gentle, and a muted percussion on the bolt face reduces the impact upon the bolt lugs. Yet it is not good a operating procedure to push the limits on a regular basis and split hairs if we might split the receiver one day, because of a poorly chosen standard practice is misapplied.
Discretion is the better part of valor and there is no prize for the stupid. It is better to be wise and not surprised. When developing maximum loads, it is my experience that 46,000 CUP is the most sensible maximum pressure for small ring Mausers, including the Swedish Mauser. As a turn of the century Swedish Mauser sets the bar high in terms of metallurgy and craftsmanship, all other small rings Mausers might be assumed to be of slightly lesser quality. Therefore, 46,000 CUP is the safe ceiling.
The exception to the rule of thumb that 46,000 CUP limit is the 1891 Mausers that should not in my opinion exceed 44,500 CUP (approximately 47,000 to 49,000 PSI). This pressure limit is my self-imposed limit that is based upon my personal research into the matter. I am not a professional and therefore it is better to be exceedingly conservative. I will further explain why I have formed this opinion.
Reloading Manuals as a Guide
This admonishment is worthy of reiteration ad nauseam when working with antique rifles in general, and in particular when developing a load for a custom-built rifle that uses an antique action. I find that there is little discussion about this topic because of the possible litigation and the complexities of the topic. It is a complicated issue where the nuances can be difficult to convey. Because we are the edge of a collapse of this country, it is important to be well informed so that we are well armed with all the possible implements of a defense, in particular, anything that launches a projectile.
Sadly, I often see data in various manuals over the past 30-plus years of analyzing reloading data that I must question and verify. That is the attention to detail we should strive for. We should apply the cold analytic eye of an accountant to all load data, lest the balance be incorrect. Meticulous attention to detail is a must. A conservative attitude toward reloading is a must as well, as there can be human and mechanical input that create variations and errors in any data set.
As an example of the human element, It is all too easy to transpose data and mix up numbers and the terms and use CUP instead of PSI, or perhaps the barrel used in the testing happens to be ‘fast’ and not an accurate representative sample that can or should be applied to most other rifles. Always measure twice and verify all data by cross-referencing from one to three additional data sources–or even more, if available.
Ask if the manual repeats early data sets without testing. This happens often if the cartridge has been in use for decades, if not a century. And other anomalies can occur during a century of use. For example, in my humble opinion, .300 Savage data should be based upon a pressure limit of 47,000 psi in deference to the early Savage 99 rifles, yet somehow over the decades a higher pressure standard of 46,000 CUP (51,000 psi) was accepted and became the industry standard. This is an example of the particularities buried by time.
Reloading data from any source can only be used a guide, and should not be considered an absolute. There are simply too many variables. Therefore, always rely on well-founded standard procedures of load development and as a good standard procedure defined in reloading manual, and reduce all listed maximum charges by 10 to 20 percent at a minimum, and even further if using certain powders and load data intended for cast lead bullets in some circumstances. We would then increase the powder charge incrementally and increase the pressure and velocity whilst carefully examining the spent brass cases for signs of excessive pressure and other abnormalities. Yet first, we need to vet the data before embarking on load development.
Comparing Load Data, A Critical Analysis
Unfortunately, I see inconsistency in Hodgdon’s data in the .308 Service Rifle data and question the accuracy of the pressures stated for some of the powders listed. But it is one of the best guides we have available for this article. In this article, I will also rely upon and refer to Lyman’s 44th Edition which states higher velocities for some of the powders listed than is actually produced in most .300 Savage rifles in Lyman’s 44th Edition’s .300 Savage section.
To involve data from additional manuals in this discussion would be helpful, yet an it would result in having too many balls in the air for this discussion. A list of free-to-download reloading manuals are available in the last installment of this article. I have examined several vintages manuals in depth. The modern manuals appear at this time to be adequate sources for the purposes of this article.
Hodgdon’s Reloading Center presents .300 Savage data that is incomplete. This might be a quiet admission that they question the usual sources and have yet to perform their own testing to certify the historic and currently available data. I might concur if that was indeed their concern. In contrast, I suspect that the test data in Lyman’s 44th Edition Manual was produced by a .300 Savage test barrel that was an abnormally ‘fast’ barrel as one example that should cause reloader to pause. It does seem however that this Lyman manual is not regurgitating early load data, but has actually developed their own data set and this is a big plus. It is a good source of modern data points.
However, Lyman’s velocity recorded using Varget is well beyond the velocities recorded for powders that are slightly faster or slower in burn rate, namely IMR 4064 and IMR 4320. A ‘fast’ barrel not only may produce higher velocities than most barrels, but possibly lower pressures given a particular charge weight of any powder used in the ‘test’ barrel. That subsequently and naturally encourages the reloader to chase that abnormal performance level. It is best, statistically speaking, to throw out data that could be abnormally high. Both manuals offer essential pressure data in either CUP or PSI.
But as one would compare and contrast data between both of these or other manuals, I find the same charge weights from one listing to be higher than the other listing and the pressures listed from either manual can be as varied as their pressures listed alongside the particular charge weight. Differences in charge weights recorded as the maximum safe working load can be as much as 2 grains higher or lower. This is a very significant difference — especially if we are working with antique actions.
This is a significant disparity to note and mitigate given the metallurgy and the advanced age and history of the actions involved that may have in the last century been used to fire countless thousands of rounds. Differences in pressures can also be as much as 2,000 CUP, or there might not be any pressure listed. This is important so that there can be variances in load data that might result in an unexpectedly high-pressure condition as we approach the maximum load.
When working near the pressure limits of any action I will compare at least two sets of data from different manuals that provide pressures in the same metric (either CUP or PSI), or both or none at all, and accept the manual that records the lowest charge weight that it lists as a maximum charge. I will ignore the pressure and the velocities that are listed and err on the side of caution. With exceptional examples aside, regardless of the exterior appearance, as a policy, whether or not the rifle would be used for hunting or long-range targets, it is better to use the accuracy node that occurs nearer to the starting charge than to test the rifle to its limits.
In addition, it would be important to keep in mind that if the action has been re-barreled or is wearing a well-worn barrel that the action has been exposed to a great deal of stress and the metal of the action possibly work hardened in the process. If so, the action would have presumably or possibly been used to fire at least 10 to 20 thousand rounds or more than is needed to wear out its original military barrel. If we can positively determine that it wears the original barrel, and that the rifle and barrel is in near overall ‘minty’ or excellent original condition, it might tolerate for the remaining service life higher pressures, or provide a long service life that a military organization would require of a rifle.
If, however, at any time the shooter might during the service life of the rifle experience a heavier than normal bolt lift sensation while shooting ammunition that previously was determined to be safe, do not fire this rifle again without having a gunsmith inspect it first. If the rifle is not a collector’s piece, I would ask the gunsmith to drill a hole in the bolt located in approximately the same place where there is gas relief hole in the M96 bolts. That will reduce the amount of hot gases that could be blown back and into the eyes of the shooter should a primer or a brass case fail.
These early actions are not designed to avert hot and high-pressure cases and brass debris in the event of a blown primer or worse, such as a ruptured case. This is another reason and hopefully motivation to reload cartridges for these actions with lower pressure loads and to use certain powders, slower burning and temperature-stable powders that help reduce the risk. As yet another measure to reduce the odds of case rupture, we can fit the brass tightly to the chamber.
(To be continued, in Part 3.)