Tolerance in Reloading

  1. Tolerance in Reloading (Introduction),
  2. Tolerance in Powder Throw,
  3. Tolerance in COL and Crimp,
  4. Tolerance in Bullet Weight, and
  5. Tolerance in Brass.


The difference between practical engineering and pure science is tolerance.

E = mc^2 is pure science. It’s exact. You take this many kilograms of mass, multiply that number by the velocity of light in meters per second squared and, voila, you will get this many joules of energy. Period, end of story.

Practical engineers, however, would add something to that equation because they know that if they try to convert several kilograms to energy all at once, it doesn’t happen “all at once”.

Engineers would know that as soon as some of that mass is converted, the resultant energy will start propagating outward. That shock wave of energy will then push some of the otherwise fissionable material away. That material won’t be converted. It is now out of the reaction. With fission devices, that leftover material is part of what we call the fallout and, in a so-called “dirty” atomic bomb, that’s what some terrorists want, fallout, lots of fallout.

When some of the material is not converted to energy, engineers might say it had a degree of “inefficiency”. And to express just how inefficient the reaction might be, engineers would add another term to the physics formula. In words, they might say that the yield of some atomic device might be “about X kilotons” and it is the word “about” that concerns us in these articles.

That’s the tolerance.

Let’s turn our attention now to the reloading of ammunition. If you are a Bullseye shooter, one of your firearms will be in 45 caliber and while that could be a revolver, in fact most Bullseye shooters use a 1911. (I have two but I’ll bet you have more!)

When reloading for my 45 ACP wad gun, I set the crimp about 0.469 inches*****. I then spot check the ammunition as it comes out of the reloading press to be sure it stays at about 0.469 inches.

But how much is that “about”? That’s the topic for this series of articles.

The tolerance. How close is close enough?

In this introduction to the series, we need to set out some basic terms and notations that will be used in all the remaining parts.

The most important of these will be, of course, the tolerance for a particular measurement whether it’s the length of a finished cartridge or the amount of powder therein or any other measurement we care to take.

Tolerance is usually written after the main number and is most commonly expressed as “±” (plus or minus) some amount. For example, I would say the crimp I use is 0.469" ±0.002". If I then make a round and measure a crimp anywhere from 0.467" to 0.471", then that would be a good crimp; it is within the allowed tolerance.

But some tolerances are written a different way.

Other times, the measurement and tolerance might be written as a minimum followed by a “+” and the tolerance, or as a maximum followed by “-” and a tolerance.

This second type of notation is found, among other places, in engineering drawings when the engineer wants to tell us that something must be at least “yeah big”, and it’s OK if it’s “this much bigger”, but no more than that. Or vice versa if he/she wants to tell us a minimum dimension.

For example, in the SAAMI specification for ammunition*****, the width of the bullet for 45 ACP ammunition is 0.4520-.0030 for a jacketed bullet. That is, its maximum diameter is 0.4520" but the bullet is still “within spec” if it is not less than 3/1000’s of an inch smaller than the maximum.

When you are reloading, how exact do you want your COL to be?

Not finding that number anywhere in the literature, I decided to ask the experts.

In 2006, I surveyed the general Bullseye community through the then very active Bullseye-L mailing list. (Sadly, that list is now defunct and there is no longer one place to find [almost] everyone.)

In 2006, the majority of Bullseye shooters who responded to my request agreed that:

  • powder throw should be ±0.1 grain of the desired weight;
  • COL tolerance is ±0.005";
  • crimp tolerance should be within ±0.002"; and
  • bullet weight tolerance is, depending on who you talk to, anywhere from ±0.5 down to ±0.05 grain (that’s a 10:1 range of opinion – more on this later).

Those numbers – those tolerances – are what the practitioners of the sport recommend. And in almost every case, these are tighter – have a lower tolerance – than commercial ammunition.

Tony Brong has a couple of excellent articles on the care and feeding of electronic scales: click for Part I and again for Part II.

In the next part we look at the powder throw and attempt to answer the question of how accurate can we be in measuring miniscule amounts of propellant, and how accurate is “good enough”?

(Click here for part two.)

Note: The ANSI/SAAMI Z299.3-1993 standard says the crimp on 45 ACP should be 0.473" but my wad gun doesn’t like it that big. My tighter (by 0.004") crimp is at the limit permitted in the standard but when my tolerance of ±0.002" is factored in, some of my ammo would be considered to be “out of spec”.

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