Variables, Tolerances and Economics
All too often, someone buys a firearm because someone else already has that precise make and model, and it shoots really, really well. How well? An article in a recent hunting magazine recounted how 20 years ago, a 1-inch group with a factory rifle and ammo at 100 yards were something to brag about. Today, a .5-inch group is expected. But why can two of us go to a gun shop, buy two identical guns with consecutive serial numbers, and yet “his” shoots better than “mine”—even when he shoots it? Welcome to the complex world of Variables, Tolerances, and Economics!
To simplify matters as much as possible, I will base this discussion on bolt-action rifles. The problem is compounded with semiautomatic firearms and goes up by orders of magnitude in the full-auto world. Along the way, we will ignore all the variables the shooter introduces—build, age, height, weight, musculature, vision, etc., etc.—otherwise this will become book length.
We will start with a modern designed, bolt-action rifle in the current popular round, 6.5 Creedmoor. You and I go to our local supplier and each buy a brand new rifle by a major maker—unfired and fresh in the boxes. We both buy the same rifle scopes and have the shop gunsmith mount them. Then we buy the same ammunition from a major maker, from the same lot. We both clean the bores and lube the bolts with the same products and head for the range.
Consider the variables just in the barrel of our two rifles. The Sporting Arms and Ammunition Makers Institute (SAAMI) have specifications for the maximum and minimum chamber size. Why? Because there are upper and lower tolerances for the size of factory ammunition, and every chamber has to accept every round. Even though our rifles have consecutive numbers, our chambers may differ. YOUR rifle was the last one made with a chambering reamer before it was replaced, so you have the smallest allowable chamber. MY rifle chamber was the first one made with the new reamer, thus it is of the largest allowable size—as the reamers wear, the chambers get smaller.
This variation is true for every possible element. Considering just our barrels, they can differ in the precise mixture of the steel; every portion of the heat treating; the actual diameter of the barrel blank; diameter of the hole for the rifling; depth of the grooves; radius of the muzzle crown; size of the chamber; length of the lead (the unrifled portion of the bore from the mouth of the chamber to the start of the rifling); the slope of the lead; the degree of the polishing of the interior and exterior; and more.
The widespread use of Computer Numerical Controlled (CNC) machines rather than manually operated lathes has closed up all of the tolerances, but there are still tolerances—there must be, as we shall explore.
Once the barrel is completed, it must be installed in the receiver. Various makers have different ways of making the installation, but there will still be some minute variations in the compression of the barrel to obtain the right fit. In other words, the machine (if a machine is used) with be set for “X” number of pounds of torque—plus or minus “Y”—the tolerance. In a worst-case situation, a friend bought an expensive rifle from a maker with a sterling reputation for quality. He happily received the rifle and proceeded to mount his chosen scope. Then he opened his safe to store it until the next morning, when he would try it out. When he picked it up by the barrel to put it in his safe, the barrel rotated freely in the receiver! He was NOT a happy camper, and it is likely some language not allowed in this magazine was used. He had to remove the scope and bases and ship the rifle back to the maker who made good on it. Obviously, sales of this maker’s guns in the local area dropped to zero—a matter we will touch on under Economics.
Consider our two rifles—yours the last assembled before lunch, mine just after. Tired from a bit of low blood sugar, the assembler uses the properly set torque wrench to tighten YOUR barrel, then breaks for lunch. Right after lunch, invigorated with a proper blood sugar level, he uses the same torque wrench and the same setting, but with a little extra vigor, to add the barrel to MY receiver. There is a tolerance level in torque wrenches as well, so MY barrel has a bit more tension applied than YOURS—both within tolerance but still different.
Consider all the variables possible in the manufacture of the receiver, bolt, each part of the trigger assembly and stock—even a polymer stock with aluminum bedding blocks! And all of this does not include those in the scope bases, rings and the scope itself! Every single part of every assembly has numerous variables, and they can stack up.
Tolerances and Tolerance Stack
As you see, YOUR rifle may be on the small end of every critical variable, while MY rifle has an unfortunate combination of very small or very large parts. Tolerance stack is the term used when you manufacture an item where, like MY rifle, every part is on one or the other extreme of the allowable tolerance, high or low. It is possible—rare, but possible—for every part in my trigger group to be on the small end of the tolerances. As a result, I pull the trigger and … nothing happens, not even a click. On the other extreme, I get every trigger part with the largest allowable size. I pull the trigger, and … nothing happens. The “tolerance stack” of all the small parts means that someplace, two parts that need to touch, don’t. With the biggest parts, there is “interference fit”—the parts are all so big that parts that should move, don’t.
Normally, the precision of those CNC machines keeps every part within tolerance. But if Karma chooses you or me, well, too bad, so sad. And with my luck, it would happen to me.
Tolerance is necessary—just imagine if you had a bolt in your rifle that measured precisely the same as the receiver. An invisible speck of dust or a minute bit of lube, and you can’t close the bolt! The U.S. military recognized this, as noted in Lt. Col. (USMC) Chinn’s The Machine Gun, Volume 1 (of 5), published in 1951. On page 589, quoting a letter from the Army Chief of Ordnance on 20mm gun development, item (e): “The accumulated tolerances in the manufacture of the weapon are too great to give uniformly efficient operation of these guns.”
Here, we may see the birth of “MIL-SPEC”—military specifications.
MIL-SPEC—the Best or the Base Line?
Today, we see all kinds of guns and gun parts touted as “MIL-SPEC” or made to military specifications. From the advertising, you would think these are the absolute best, made to the highest standards. Well, perhaps not. These specifications—tolerances, if you will—are for guns that will be used—and abused—in the harshest conditions. In the last decades, they have been in the extreme heat of Iraq in the summer and the extreme cold of the mountains of Afghanistan in winter. Just consider an M4 carried all day, in the sun, on a 120-degree day in blowing dust. Now, the next gun (by serial number) is subject to days of sub-zero temps above 10,000 feet, covered in snow. Both are expected to perform as designed, when needed, by our troops. The troops clean both guns at every opportunity, but those opportunities may be few and far between. So, when the specifications are set, they are planned so both guns will work reliably and with reasonable accuracy. It is a tribute to those who write the MIL-SPECs that the guns, with exceptions, work so well under such varied conditions.
Can a manufacturer exceed the standards of MIL-SPEC? You bet! But those guns will not comply with the government contract standards. The company will not get any military contracts, because their guns are “too good.” The author’s guns, in contrast to those issued to our troops, are “babied” in comparison. After they are cleaned, they get locked in the safe. If going to the range, they get a patch down the barrel, checked for proper lube and put into a gun case. After a gentle ride in the vehicle, they get uncased on the shooting bench and are fired. Then back in the case, back in the truck, for cleaning and storage. No dust, no mud, no grit/sand/snow/ice on them! So we can buy guns that exceed MIL-SPEC and use them with confidence.
There is one other part of MIL-SPEC to consider—cost. Having worked for the government for almost 3 decades, the author understands that while “government money” comes from the taxpayers, it is still not like spending my money! Now, the author worked for an agency that made money for the government, but still, spending government money was “different” than writing a personal check.
A friend was involved in writing a MIL-SPEC for a tank cannon. He said that close tolerances on variables cost money—the closer the tolerance, the more it cost. Yes, they could write a specification with the tightest tolerances money could buy, but that would limit how many guns (cannons, in this case) the Army could buy. Even government money has its limits—they may be high limits, but they are there.
Commercially, the concern is quality control and its effect on sales. My friend with the loose barrel told all of his friends about it—two of whom were all set to buy guns from this same maker. Then they attended matches out of state, where I am sure they told others. Social scientists have worked out this effect and can predict how many people will know of this in 30, 60 and 90 days. I anticipate this maker lost thousands of dollars in sales, all because of a single incident.
Worse, this gent had spent a lot more money due to the reputation of the maker, expecting much closer tolerances. Obviously, his faith that more money equaled a “better” rifle was destroyed.
We have mentioned the two extreme weather variables, heat and cold. There is also the heat generated by firing each round. There is a complex formula for computing this, but it roughly approximates 25% of the heat generated by the combustion of the propellant and the friction of the bullet traveling through the barrel. In our example of a bolt-action rifle, rapid firing of five rounds can warm the barrel and receiver. More rounds = more heat, until we reach the very high heat generated in fully automatic firearms. I am always amused by the scenes in action movies where the hero fires several magazines or a full belt through such a gun, and then grab it by the barrel. Ah, movies are magic! In the real world, the heat of a magazine dump in a full-auto can burn your hands if you touch the barrel. Heat also causes metals to expand—which requires different tolerances. Many a machine gun has “jammed” when the heat generated by lots of ammo causes this expansion to exceed the engineered tolerances.
Cold causes shrinkage, which in extreme conditions can also cause guns to jam. Minimum SPEC parts can get cold enough to shrink just enough so they no longer function. Metals also can become brittle, which can cause breakage of critical parts.
Guns, like cars, work better with some lubrication. But the wrong lube, or too much of it, can also affect operation. I have handled guns lubed with thick greases, which over time have hardened. Even in their semi-fluid state, they can fill those vital tolerances, causing failures to fire. Consider lube getting into your firing pin “tunnel,” slowing or stopping the movement of the pin. The author has experienced this with a modern striker-fired semiauto pistol. On this handgun, the firing pin tunnel must be kept dry, or the gun becomes an oddly shaped club!
He has also tried to fire a 1911 lubed with grease for high temperatures in the winter. It was good for a laugh, as you could see the slide slowly come to the rear and then crawl forward and almost chamber the next round. After a dozen or so rounds, the pistol heated up enough for the grease to fit in the tolerances, and the gun functioned normally. By the way, this was not in exceptional cold, but temps in the low 30’s.
Can a Gun Be Too Accurate?
Indeed, they can. John Browning’s original water-cooled M1917 machine gun was so closely fitted, with such minimal tolerances, that it did not spread the shots enough—it had too little dispersion! Machine guns are “area” rather than “point” weapons. The idea is to cover an area with rounds to deny its use to the enemy. Browning was required to loosen the tolerances, so at a distance the rounds were further apart.
Ultimately, the designers must take all of the above into account, so “my” rifle will be comparable in accuracy and function to “your” rifle. When they do, firearms become marvels of modern engineering. When they do not, they become “jam-o-matics!” We should marvel that the engineers get the variables so small and the tolerances so right, so much of the time!