Reloading Basics - Under Pressure

When you are reloading, it is always important to start by following published reloading data from a reliable source (not loads suggested by unknown sources on the internet) and match your components to those listed in the data, particularly the type of powder.
The pressure figures included in reloading data are produced in test barrels, in laboratory conditions, and are an indication of the pressures generated in those specific conditions by those loads, but not in all guns and not in all conditions. You should use caution and gradually work below the approved maximum loads to create the best load for your specific gun and watch out for signs of high pressure such as heavy recoil, flattened or blackened primers etc.
The maximum pressure listed in the reloading data is not an indicator of the best, most accurate, or most consistent load for your specific rifle, and the optimum load is most often found well below the maximum load. This is why they always publish a starting load, to give an initial powder charge to start your testing with.
As an alternative to checking for visual and felt signs of high pressure, there is another more ‘scientific’ way to check for pressure signs, which is well worth trying.

The basic idea
When ammunition is fired, the case will expand in all directions, and it will momentarily lengthen inside the chamber until it is stopped by the bolt face at the rear and the shoulder of the chamber at the front. The brass will then spring back down in size as soon as the chamber pressure drops, and it will normally be easy to extract. If fired cases are hard to extract, then this is a clear sign of excessive chamber pressure. The brass will not have fully shrunk back to its previously unfired size, which is why it requires resizing, and the amount that it remains stretched by is a good indicator of the amount of pressure that it has been subjected to.

Starting afresh
For this to work properly, start with new unfired cases, as used cases may be partially and inconsistently hardened by previous firings and could therefore react to pressures differently. The cases need to all be from the same batch, and they should be inspected for defects, measured, and trimmed to length.

Initial measurements
The cases must be the exact same length, and for testing to be optimised, you need a calliper or a micrometre that is as accurate as possible, preferably measuring in ten thousandths (0.0001”) of an inch. For the best results you should mark your cases somewhere on the case head, and this is where measurements should be taken, both before and after firing. Measure the case length as accurately as you can and record the results. You can number each case being tested and record the exact length of each one, so your results can be case-specific and even more accurate. When rounds are loaded into the rifle, the mark on the case should be placed at 12 o’clock in the chamber each time, to again optimise the consistency of results.

After firing
The fired cases should be remeasured as accurately as possible in the same place as indicated by the mark on the case head. By deducting the unfired length of the case from the post-firing length, you can calculate the amount of expansion, and if you have numbered your cases, you can calculate this for each case rather than an average for the whole batch. It is normal for there to be an increase in the length of each case, but it is the amount of that expansion that is important.

Results
Although the results are subjective and can vary between case brands etc., there are generally ‘normal’ amounts of expansion associated with maximum pressures. Lower-pressure cartridges, like the .30-30 Winchester, will usually show case length expansions in the region of 0.0003” to 0.0004”. A modern cartridge like .223 Remington might generate case length expansions of 0.0004” to 0.0005”, while .308 Winchester and .270 Winchester will yield 0.0005” to 0.0006” at maximum pressure. Finally, a magnum calibre, such as the .300 Winchester Magnum, will display in the region of 0.0005” to 0.0006” at maximum pressures. It is hard to measure such precise changes in length and you may have to interpolate dimensions, depending on the accuracy of the tool you are using, to assess the change. The important thing is that if your case lengths have increased significantly, the cause is more than likely excessive chamber pressure.

Other checks
The results of the case expansion measurements should be considered in conjunction with the other signs of pressure, which should always be monitored and checked. These signs may include very flattened primers, slightly cratered primers, or even blown primers, ejector marks on the case head, and stiff or difficult case extraction after firing. All these signs indicate high pressure, and loads should be reduced until these signs disappear.

Conclusion
Very high pressures are of course dangerous and can result in damage to the gun and injury to the shooter, or even bystanders, but they also have other implications. When cases expand in the chamber, the amount that they initially stretch, the forces they are subject to, and the amount that they remain lengthened by after firing, all contribute to case wear and case life expectancy. Higher pressures will exert more wear and tear on the brass, and you will get fewer uses out of each piece. If you keep your chamber pressures low and monitor signs of pressure, your cases will last longer, meaning you will not have to spend money replacing them so often.
When reloading, always start with the minimum load from a reliable source, and gradually work up while monitoring for pressure signs. Remember, maximum loads and pressures do not mean maximum accuracy or consistency.