Over the years, I’ve had more than a few heated discussions over headspace in the 1911 and other autopistols that headspace on the case mouth…but since this is a 1911 discussion, we’ll focus on that platform.
Headspace is determined by the linear measurement from the breechface to the chamber shoulder. This is known as Static Headspace. That is, it is fixed and only changes by wear and deformation of the breechface, the upper barrel lugs, and the mating lugs in the slide.
Dynamic Headspace…or the headspace that is in play when the gun fires varies a little with each round fired, as determined by the length of the case.
So, Dynamic Headspace is defined as the difference between the static dimension and the length of the case that happens to be in the chamber.
If your static headspace dimension is set at the ordnance minimum of .898 inch, and your cartridge case is .895 in length…you have .003 inch of headspace…with that cartridge. If the length of the case on the next round is .893 inch…you have .005 inch of headspace. Simple, no?
Now to address the common misconception that headspace can be “checked” by dropping a round into the chamber and looking to see if the rear face of the rim sits flush with the face of the barrel hood. I see it all the time. I try to explain as gently as possible that it has nothing to do with determining headspace…good or bad…but I’m only able to reach a small percentage of the people who insist that a flush rim “proves” good headspace. I often meet with outright hostility during these conversations.
If you have good headspace, you can cut the barrel hood completely flush with the chamber face…and it won’t affect headspace.
You can cut the chamber too deep and create excessive headspace…then cut the barrel hood to match the rim and give the appearance of “good” headspace…and you still have excessive headspace.
While you can have a pistol in which there is good, safe headspace with the flush fit between hood and rim…it’s not an indicator of good or bad headspace. Conversely, a rim that sits above or below flush isn’t a reliable indicator of dangerous headspace. It may be just fine…or not…but the fact that the rim and hood aren’t dead flush doesn’t prove either.
You can see it sitting above flush, and still have dangerously excessive headspace. You can see it sitting below flush, and still have safe headspace.
Excessive headspace comes in two flavors. That which is potentially dangerous, and that which isn’t. It all depends on which way it came to be excessive.
Take a pistol with the ordnance minimum .898 inch dimension…ream the chamber past the ordnance maximum of .920 inch…and you technically have excessive dynamic headspace…but it’s not dangerous headspace because the case is blocked from backing out of the chamber far enough to lose head support.
Conversely, if you take the same gun and remove enough material from the front faces of the barrel lugs to create the same amount of dynamic headspace…you lose head support when the gun fires because the slide and barrel separate by that amount…even though still locked….essentially allowing the breech to partially open.
I’ve often said that I only know what I can measure…and the only way to determine headspace is to measure it. While the rim to hood relationship can alert us to a potential problem…that’s about all it does. It doesn’t tell us what the problem is. That can only be determined by further investigation. If that investigation shows that the static headspace falls within the acceptable range…then the gun is safe to fire. If not, then we have to determine why the headspace is excessive…whether its due to a chamber reamed too deep…or a problem with the upper lugs…and that problem can be due to the barrel lugs…the slide lugs…or both.
Finally, a word on headspace gauges. Ordnance gauges come in sets of GO and NO-GO. The GO gauge is .898 inch in length. If the slide closes on the GO gauge, it means that the gun will chamber a case of maximum length. Ordnance specs state that the slide must close on the GO gauge in order to be deemed serviceable.
If the slide closes on the GO gauge, we go to the NO-GO gauge with its dimension of .920 inch…the maximum allowable static headspace dimension. If the slide closes on the NO-GO, headspace is deemed excessive, and the gun requires further examination and probable repair.
The case lengths in nearly all commercially produced ammunition is shorter than the .898 inch minimum static headspace dimension. It generally runs to about .010 inch shorter, though some can be as much as .015 inch shorter…so even the most precisely fitted barrel operates with some headspace unless the builder uses a barrel with a semi-finished chamber and sets a short depth…which means that the slide won’t go to battery on the minimum gauge. I’ve seen a few bullseye pistols with the static dimension as short as .888 inch. Of course, this requires that all cases must be trimmed to length in order to chamber reliably…but most of the builders who engage in this practice allow for a little headspace for reasons of reliability when the gun starts to get dirty.