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Stacking Tolerances: How Little Gaps Lead to Big Performance Problems

We’re getting philosophical for a bit. I’m filing this under the “Mindset” category because it has broad implications across many areas of life beyond just shooting, competition, or defense. In fact, I think this is one of the biggest cultural issues I see among gun owners in general. In light of the recent discussion about standards here on this site, I thought this was a relevant topic to discuss.

I first came across the term Tolerance Stacking with engineering and systems design. In short, it refers to the overall reliability of a system based on the combined deviations of its individual components.

Let’s say I have Widget X and Widget Y, and they fit together. The specs for Widget X’s attachment say it needs to be a specific size with +/- some amount. The same applies to Widget Y. Well, tolerance stacking occurs when Widget X might be “within spec,” but on the large side and Widget Y is just a little on the small side. Together, they might actually not work that well together.

Here’s another example: cars.

Modified Jeep
Modified Jeep, fun but pushing tolerances

People love to modify Jeeps. However, one of the tradeoffs within the modding community is that every mod detracts from the overall reliability of the vehicle. When engineered, all the individual components of the Jeep work together within some small margin of error. As you add more aftermarket items, often beyond the original engineering parameters, you introduce stress.

And it’s with stress that we see issues.

Tolerance Stacking applies to firearms as well, with each individual part contributing some measure of uncertainty to the whole. That’s why I always suggest buying from companies with strong reputations for quality control.

Applying Tolerance Stacking to Shooting

John Buol Jr. over at The Firearms User Network recently put up an article on this very subject. He quotes Chuck Pressberg, a former 75th Regiment SGM and owner of PressCheck Consulting.

Combat shooting is a complex math game where you are stacking tolerances of maximum spreads of human, weapon, and ammo in real time against the acceptable impact zone, what’s in front and beyond it and usually while both you and the impact zone, as well as potential intermediate barriers are all in movement.

– Chuck Pressberg

Think of your performance in any shooting event, defensive or otherwise, as a system. This system has many components:

  • Your weapon and it’s reliability and accuracy
  • Your ammunition and it’s capabilities in accuracy or terminal effect
  • Yourself, and your shooting abilities in the moment

A lot of people, too many if we’re being honest, focus entirely too much on the first two bullet points at the expense of the last.

Shooting Performance Degradation

For a long time now, there’s been a statistic floating around gun boards that your shooting performance decreases by 50% while under stress. And by stress I specifically mean high-adrenaline defensive situations.

Competition still induces stress because of timers and scores, but it’s not quite the same.

I have no idea where this 50% number came from, so I can’t validate it. But that’s not the point, it’s a good illustrative number.

Another example is a study undertaken at a Canadian university and hosted at the National Institute of Health. The study compared Elite and Rookie Law Enforcement Officers on a series of shooting tests under stress.

These are some key points from the abstract:

  • During simulated lethal force encounters, the Elite officers shot much more accurately than the rookie officers (74.60% to R 53.80)
  • The Elite officers made much better decisions under stress, with 61.5% of the rookie officers firing at an assailant only carrying a cell phone. Only 18.5% of the elite officers did the same.
  • Elite officers had much better reaction times under stress, successfully drawing and firing their weapons much more quickly
  • Rookie officers needed to look for their own weapon 84% of the time, causing to lose sight of the assailant

Another study in the Journal of Strength and Conditioning Research involved a pistol match. They monitored the heart rate variability of competitors and compared it to performance. The competitors with higher variability, meaning less stress, performed better. How do you improve that kind of stress response? By practicing.

Relating this to Baselines

So what does this all lead up to?

Your success in any shooting endeavor, be it a competition against a timer and squad or a defensive shootout, comes down to your skill. Mindset is absolutely a factor here, but mindset alone does not overcome a lack of capability.

A lot of folks think they will “rise to the occasion,” but that’s false. Instead, people fall back to the baseline proficiency.

As harsh as Ash Hess’s words were in the discussion on shooting standards, I see where they’re coming from.

• 0-25 4-inch 1 sec or less splits standing (US Army zeroing ONLY 4 MOA for zeroing prone)
• 25-50 4 inch 2-3 second splits standing
• 50-100 4 MOA standing or Kneeling
• 100-199- 4MOA Standing supported
• 200-250- 4MOA Kneeling supported
• 250-500 4MOA Prone   

When the pressure mounts, you’re going to make sacrifices. That might be your sight picture, it might be the “perfect” position, your trigger control, or any number of other things. But something’s going to give, maybe several things.

This is where why your standards are important. If those “plus or minus” points on your personal shooting standards all start trending in the wrong direction at once, will you still make that shot?

Wrapping Up

Don’t aim to be the exception.

Gaining mastery of your fundamentals is your paramount goal. Mastery of the fundamentals directly translates into better and more consistent performance during the “fun” stuff.

Fundamentals, however, aren’t as sexy and therefore don’t get their due.

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Matt is the primary author and owner of The Everyday Marksman. He's a former military officer turned professional tech sector trainer. He's a lifelong learner, passionate outdoorsman, and steadfast supporter of firearms culture.

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Mark C
Mark C

All of this is verbatim what I have been told and trained to. Setting aside cool guy stuff for the sake if it, I’ve never seen a stock weapon from a decent company fail in the 400 hours of training I’ve done…put in the coated BCG? Failure. Didnt use screw in pins on your awesome trigger? Failure. Cool guy muzzle break that pisses everyone off around you, especially in the shoot house, fly off cause you don’t know how to put a crush washer on? Failure. I did see a $1200 Sig 1911 fail because of Remington ammo and wet… Read more »

Mark C
Mark C
Replying to  The Marksman

That’s probably the story of the ages. Some of those guys, two as a matter of fact were at the battle of Mogadishu, Blackhawk Down. One Delta and the other a Ranger, later a Green Beret and boy do they have some stories of things going sideways… One guy used a Cold Steel Trail Master on a fella who tried to throw a grenade in the hummer….because that’s when chest rigs for pistols started coming out and he left his sidearm on his waist and couldnt draw it in time. But he could draw his knife… Gear, S.O.P’s, tactics, strategy,… Read more »

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