Before we can look for maximum, we should not only define but also understand it. There are two ways to define the concept. The first is purely scientific. Our Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) sets standards for maximum pressure (sometimes arbitrarily) for each cartridge. What the pressure actually is depends on the individual cartridge. Different units can be used to measure this pressure, ranging from LUP (lead units of pressure), CUP (copper units of pressure) or psi (pounds per square inch). I won’t attempt to fully describe the minutiae of each, for it is a very complex system. Basically the lead and copper units measure the amount a little cylinder made of those materials is compressed when the load is fired. The psi measurement is done electronically. Lead is used for lower pressures, while copper functions at high pressures and psi works for all cartridges.

To give you a hint of the complexity, if for example, a .30-06 is measured with copper and psi - simultaneously for the same cartridge - the numbers and accepted standards are different, even though the pressure is the same. Explaining this fully takes something akin to a Ph.D. The important point here is that when you see vastly different pressure numbers, for the same cartridge, especially from older to newer manuals, this is the reason. Pressure, represented in electronic psi, is often a higher number than with other systems.

The other more broad, less scientific and maybe more reasonable definition of maximum is a load that is safe and reliable, one that won’t hurt you or your gun. One that won’t hurt your gun, that is, while extracting all possible performance.

Maximum pressures as used by SAAMI vary dramatically, depending on the type of cartridge. The numbers you see in manuals represent average pressure, while SAAMI sets limits for maximum average and maximum individual shot in the string. Beginning at the low end of the spectrum, we have shotshell pressures. They vary from the lowest maximum of 11,000 psi (electronic) for the 10 gauge through 13,500 psi for the .410, through the 12 gauge 3 1/2-inch at 14,000 psi. Yes, that is correct. Those who pick the maximum number choose different ones for different gauges and chamber lengths.

As we move up the pressure scale, we find the various pistol and revolver cartridges. The lowest pressure that comes to mind is for the .45 Colt, limited to 15,000 psi, followed by rounds like the .45 ACP and .38 Special that operate at just less than 20,000 psi, or CUP. (The two scales are almost identical at these levels.) The top of the normal handgun list would be the .44 Magnum with its pressure limited to 40,000 CUP and apparently 35,000 psi from Alliant. Rounds like the .454 and .480 go even higher, right into the realm of highest rifle pressures.

When we look at rifles we see the lowest pressure for the .45-70 in “Trapdoor” rifles at under 28,000 CUP, while loads for the same cartridge rated for Ruger No. 1 rifles work at 50,000 CUP. When we reach the high end, we find cartridges like the .300 Winchester Magnum with a limit of 55,000 CUP or 62,000 psi.

Please understand that none of the numbers above are designed to represent absolute rules nor are they certainly the precise SAAMI numbers. They are being used to illustrate a very important concept - within the spectrum of firearms there is an extreme range of maximum pressure. What that means to us reloaders is many of the concepts intended to allow us to: see, smell or touch maximum pressure simply do not work. Actually, only the very highest limits can be reliably discerned “at home.” That is, we can make reasonably reliable judgments about the highest rifle pressures, but for others we are dependent on published data. We will look at rifle pressure judgment in a moment, but now let’s spend a little time understanding the variety and variations in the data that is available.

The most complex reloading also has the lowest pressure limits. Shotshell loads are wonderfully complicated and sophisticated. As a base line, there is nothing, absolutely nothing, in the way of visible pressure signs that will keep you out of trouble. If you see anything like sticky extraction, primers actually flattened by pressure or other accepted pressure indicators, you are literally miles above maximum. Put another way, if you see excess pressure in a shotshell load, be very glad you are not looking at pieces of your gun instead. The only way to create safe, reliable shotshell loads is to absolutely follow the printed recipes.

Why? Because, even apparently insignificant changes in shotshells, ones that may make little or no difference in metallic rounds, can send pressure into relative orbit. To illustrate this we take an example from the Hodgdon Shotshell Manual. They loaded a specific recipe, basically a 1 1/8-ounce trap load, in five different hull brands. A Federal Gold Medal produced 1,200 fps and 10,400 psi. This is a perfectly normal, safe load. Then the same ingredients were loaded in a Remington Premier hull. The velocity fell to 1,186 fps - telling us all is well, except the pressure increased by almost 50 percent to 14,800 psi. Another recipe might have just the opposite effect. Simply changing wads or primers can create the same havoc. Of course, changing shot material from lead to steel is almost guaranteed to rip your gun apart.

So, there is one and only one way to get a good maximum shotshell load: follow the recipes in reputable loading manuals! This is not bad news because the data for shotshells is far more complete, much more detailed than we see for any metallic shell. If you search a little you can find a shotshell load for virtually any component and application that can be safely used.

As we move forward into the various handgun cartridges we are confronted by limitations similar to those from shotshells. Once again there is very little we can see or measure that can give us a reliable indication of maximum pressure. At the lower pressure levels, say those under 30,000 CUP, almost nothing visual happens to the case or primer; and like shotshells, if you have tough extraction or primers that look bad, you are far beyond safety.

As we move into the high-pressure cartridges like the .44 Magnum, or beyond to the .454, that operates in the pressure realm of magnum rifles, we find a double-edged sword, the classic good news and bad news. The good news is that a revolver and modern revolver brass handle high pressure more gracefully than any other firearm. Pressures that will begin to stick in a bolt action or loosen primer pockets in a .300 Weatherby case are likely to behave as if they were perfectly normal in a fine revolver. Behave, that is, until the pieces of hot steel begin to fly. Thus, the bad news is that there is very little to tell us we are in trouble!

If you have a double-action revolver, say a .357 or .44 Magnum, you may have to push hard on the ejector rod as you pass maximum and move into the danger levels. The thinner cups on the pistol primers may also begin to flatten considerably more than a normal or factory load. Handguns do not exhibit the wild tendencies of shotguns, but once again we are essentially limited to using the data in the manuals. Normally, you can safely substitute a cast lead bullet for a jacketed one of the same weight, because the lead bullet usually generates less pressure.

As we move into the realm of high pressure rifle cartridges, we at last have enough room, under the ceiling of maximum pressure, to allow us to make intelligent judgments about the safety of our loads. Now we have several “seeable” and “feelable” pressure indicators. The most commonly used is a visual impression of primers. Unfortunately it is also the most unreliable.

Flattened and cratered primers can be due to excess pressure. But, slightly excess headspace is often the cause of flat primers. They back out of the pocket on ignition, then when the powder lights it slams the case head, with protruding primer, back against the breech face. The predictable result is a very flat-looking primer. Cratered primers are usually caused by an ill fit between the firing pin nose and its hole or by a weak firing pin spring. However, both of these visual effects can be used as warning signs. The way they are useful is if they are not present with factory or starting loads and then they become progressively more apparent as the load increases.

A more reliable indicator is extraction force or bolt lift. Once again this is relative. To best use extraction force as an indication of trouble, you must begin with loads that provide little, or no, resistance to lift and extraction. Then, as you work up, if you feel some increasing force needed to lift the bolt or worse need to tug or bang on the bolt after it is up, to pull the case out, pressure is surely too high.

Again there are reasons for false indications. Once I was working with a very high-performance rifle. All was fine, then things changed. After I turned the bolt up, the cases almost refused to budge from the chamber, with the same loads that had worked perfectly only hours earlier. No, the chamber was not bulged. A new case worked perfectly, but after it had been fired it would stick on the second loading. When I reached the hair-pulling state of confusion, I saw it. As I sized the cases, a tiny crack appeared in the lower inch of the full-length die. The die had cracked and was not resizing the bottom of the case. Each time it was a force fit in the chamber. A new die cured the problem.

Bolt lift can also lie. If the bolt face is not square with the chamber, or it is rough, you will feel artificially high resistance. However, it will be high for even factory ammunition.

There is one reasonably reliable way to judge near-maximum and over-maximum pressure levels. This is case head measurement. There are a few rules you must follow to get reliable results and some variables you must understand.

First, you must have a high-quality, extremely accurate micrometer. No vernier caliper has the potential. While the best digital vernier calipers may read to .0001 inch, their stated accuracy is only .001 inch. To correctly monitor pressure we must be accurate to less than one half of the vernier’s potential. The best tool for the job is a digital micrometer and even better is one called a “blade” micrometer. The blade micrometer has thin blade anvils instead of the normal round ones. It makes it much easier to measure the thin, solid section on the cases. A fine, standard digital mic costs between $100 and $200, while a blade mic is much more.

The measuring device must be accurate in “tenths” (one ten-thousandth of an inch). The reason for this is that .0005 inch, or one-half of one-thousandth of an inch, increase in case head diameter indicates extreme maximum, almost dangerous pressure. Also, understand that you must measure the solid brass head, not the expansion ring. Your measurement must be made just in front of the rim or extractor cut and on the back one-third of a belt. Finally, you must measure each individual case before and after each firing, and you must measure the same place on each case.

To get accurate results you should measure over a specific place on the headstamp. For instance, I use the WW on a Winchester case, aligning that stamp with the anvil on the micrometer. Next, I “zero” the micrometer on each case and repeat the measurement at least three times before I fire the round. That is, set the mic on the case and zero, then open and reset. You must be getting a reading each time that is on zero or not more than plus or minus .00005 inch away from zero. This number is the limit of the micrometer’s scale. It is important to understand that this kind of precision measurement takes practice and a certain skill level. You must be able to get repeatable measurements, or your test results can be invalid and dangerous.

Opinions differ about the kind of case that yields accurate results. Some sources suggest it is only good to use an unfired case. I do not agree, because often a factory load will move the case head considerably. In my opinion, once the case has been fired, it settles down and becomes a reliable indicator.

If you are working with cartridges with a high maximum pressure, those with 50,000 CUP or higher, case head expansion of less than .0005 inch usually means safe pressure. My most important premise is that cartridge brass varies, and that cartridge brass is the weak link in the system. If that brass is not flowing, if the velocity is not higher than recommended in the manuals with the same powder and bullet weight, your load is probably okay. Soft brass may flow at lower pressures and velocities than indicated in the load tables. This means that a maximum load, in those cases, is lower than it would be in a stronger case. If the load is moving the brass more than the indicated .0005 inch, regardless of other factors, the load is too hot.

Before we move on to indications of dangerous pressure, ones that figuratively or literally slap you in the face, there is one that is an intermediate indicator. This one may indicate you are over the top, or very near it. Loose primer pockets are telling a story. By loose, I mean when you seat a new primer little or no resistance is felt. In worst cases you can actually push the primer in with your thumb.

If you begin with a case that has a tight primer pocket, fire it and find the pocket is loose from one shot, the pressure is very high. This kind of pressure will probably show up between .001 and .003 inch head expansion on the micrometer - or, from two to six times too much. This load almost “blew” the primer, almost put you in big trouble. A more subtle kind of pocket expansion may occur after several firings. That is, the pocket grows progressively larger, a little bit at a time, shot after shot. This is probably not dangerous. It simply means you are right on the edge.

If I am loading for a high intensity rifle cartridge and the pockets are okay for three shots or more, I am comfortable with the load. If you have a case, or batch of cases, that prime very easily, it is time for them to join the junk pile. Their useful life is behind them.

There are other indicators of extreme pressure, dangerous pressure, that must not be ignored. These show up without a micrometer. If you see a shiny place appear on the case head where the extractor cut, or ejector pinhole, is in the bolt face, you are nudging serious trouble.

If the primer leaks, indicated by soot on the case head around the edge of the primer, the load is shouting at you to back off. A blown primer, where you get a big gas leak and the primer is out of, or falls out of, the fired case, is kissing the grim reaper. When you blow a primer and you and your rifle are still blood free and in one piece, be sure to thank the necessary gods and the gun’s maker.

He was about to take a friend antelope hunting, so he simply instructed the would-be antelope hunter to assemble some ammunition with the known load. Our expert stopped by the loading bench to be sure all components, weights and bullets were correct. They were. Then he sent the hunter to the bench to zero the new, out-of-the-box Weatherby Accu Mark .257 Weatherby. There were three shots, then the shooter reappeared with the three fired cases and a very quizzical look on his face.

Every primer pocket was black and huge. The shooter instinctively knew something was wrong but had such deep and implicit trust in the expert that he fired two more shots, simply believing the expert could not be wrong. Well, I was wrong, dangerously wrong. Aside from being wonderfully strong, that particular Weatherby had a tight barrel and throat that pitched the pressure into orbit. No, all maximum loads are not safe in every gun - just like the manuals tell us!

Beyond normal caution and consideration for variations in factory arms, there is the question of cartridges/calibers for which there is no “certified” loading data. If you are an advanced reloader and are working with a new or wildcat cartridge, one without precise data from a pressure gun, you become solely responsible for generating safe loads. With the micrometer and other indicators you should be able to arrive at a safe, sane maximum load for your rifle.

With the understanding of some of the pressure indicators available to us, it is time to address the question/dilemma of why loading data varies so much. That is, if you consult loading manuals A, B and C for the same cartridge, using the same bullet weight and the same powder, you will probably find the maximum load varies several grains.

There are a multitude of reasons. If we use the most professional laboratories as a base line, we begin to get an idea of the complexity of ballistics. They use “pressure” guns, generally a universal receiver that accepts large diameter barrels. These barrels are made by a few highly qualified barrel makers to very strict tolerances of bore, groove and chamber. The numbers are specified by SAAMI. So, each lab has barrels that are made the “same,” as close as machining tolerances will allow. They then “calibrate” that barrel, using a very special lot of “reference” ammunition. This ammunition is loaded in one lot, tested and stored in controlled conditions. Thus, each barrel receives a correction factor that “zeroes” the barrel relative to the reference ammunition. Now, they can begin shooting with unknown ammunition.

For the moment we assume that if each lab fired the same ammunition in their barrel, that is same powder, primer and bullet, they would see the same pressure. This is essentially true. However, if they change primers or, even more significant, powder lots, differences can occur.

Now, the big variable enters the picture. Bullets are very different, different that is from brand to brand within the same weight. A soft bullet with a short bearing surface will usually show less pressure (and often velocity) than a “hard,” long bullet like the Barnes X-Bullet. Further, pressure is influenced by seating depth of that individual bullet.

Then there is another grand variable - each company has different views of what “maximum” pressure should be. Some, quite frankly, are more afraid of their lawyers than others and print “softer” loads. As the wild cards get wilder, some makers of loading manuals do not use  sophisticated pressure guns. Some use strain gauges, a test that is scientifically marginal at very best. Without SAAMI-spec barrels, reference ammunition and correction factors all bets are off. And, in the not too distant past, complete loading manuals were published, by big companies, without any formal pressure testing equipment at all.

Last, but not least, there is the transition from lead and copper crushers to electronic piezo (psi) testing equipment. As mentioned earlier this new equipment is like viewing pressure through a very different window.

Then, it is fair to ask, who can we believe? In a way we can bring this whole conversation full circle. Who can we believe? Well, actually, all of them and none of them, at the same time. Once we see the variables, under very controlled scientific conditions, imagine what goes on by the time the lead, primers and powders reach us. Few pay attention to primer lots and not many more notice powder lots. Then, powder gains or loses moisture, throwing its actual energy content per weight out of kilter.

Next, enter our very random barrels and chambers. The good news here is that normally a production barrel and chamber will be “bigger” than the minimum SAAMI barrel used by the laboratories. Thus, our guns will usually generate lower pressure (with the same ammunition) than the test barrel. We can normally use most published (from the major manuals) data. To do this, you must use the same bullet, powder and primer. Then, you can apply the tests I have offered to be sure you are okay.

Always follow the advice given with the individual loading data regarding starting loads and work-up procedure. Last, but not least, you be the final safeguard and responsible party. Watch the signs, double check and listen to your rifle.

As a conclusion, I thought it would be fun to look at some loading data, going back as far as we can, while still having similar powders, to see just how much, or little, data has changed over almost 50 years. I began with the Lyman Ideal Handbook No. 39 (1953) and ended with the latest Hodgdon, Number 27, published in 1998. The former was   probably produced without formal pressure apparatus and with the relatively crude chronographs of the day, the latter with the most sophisticated piezo-electronic pressure setup and laboratory quality chronographs.

I wanted to see cartridges that were not influenced by the military, therefore did not use the .30-06. Also, I wanted relatively high-performance rounds for rifles. The two that appear in the early Ideal data are the .270 Winchester and .300 H&H. This data predates the .300 and 7mm magnums. For the same reasons we will look at the .357 Magnum revolver round also. I selected the load that gave the highest velocity for any given bullet weight, comparing the same powders, old and new, and then picked the highest velocity load available today, with any powder.

As we look at the numbers in the table, a few thoughts surface. First, contrary to popular belief the data is not being “softened” in modern times. Within these examples and many others, manuals today, with the sophisticated equipment, are giving us just as much as did the old Ideal. Further, with the improvement in powders, translating into slower burning rates, the good old days of high velocity are really right now. Way back when, when 4350 was the “slow” powder, even the midcapacity rounds like the .270 WCF wanted slower fuel. Now with the advent of H-4831, IMR-7828 and Alliant Reloder 22, pressure/velocity relationships are better than ever.

Of course, we should give credit where due, to the grand masters who created the old manuals. By and large they did so “by the seat of their pants” without great laboratory equipment. Instead they used perhaps a micrometer, skill, experience, common sense and good judgment - tools that are becoming very rare today. That the computer era equals the old days is not remarkable; that the old days equaled the computer certainly is worthy of our respect.