[ What is Headspace? ]
Hi guys, thanks for tuning in to another "Ask Ian" Q&A on Forgotten Weapons.
I'm Ian, and our question today comes from LongBeef on Patreon,
and he asks, "What exactly is headspace? And how important is headspace
on old military surplus guns as opposed to more modern guns?"
So we'll dig into this in a couple of parts.
First, what is headspace? Basically I would say headspace
is a measurement of the amount of free play that a cartridge has in a chamber.
So when you're looking at the chamber of a firearm
it has to be slightly larger than the actual cartridge case that you're going to use in it
because you need some clearance for the cartridge to actually
be able to get in and get out of the chamber.
The reason that brass is used for cartridge cases is because
it's really good at expanding under pressure to seal against a steel chamber,
and then shrinking back down slightly after the pressure
has dissipated so that it can be extracted back out.
As a result you don't have to try and make a perfect seal
between the brass cartridge case and the steel chamber.
Make the chamber a little bit bigger, the ... case will seal itself when the cartridge fires.
Now that of course brings up a question of how much play do you want?
Because too much is a problem for a couple of potential reasons, too little is also a problem.
So that's where we get into measuring, defining and checking headspace.
So before we go further let's go over to the table where I've got a sketch pad.
That'll be an easier way for me to explain this.
Alright, let's start with an example of good and proper headspace.
So this is my fantastically skilled artist's rendering
of a barrel with the chamber, and then barrel and rifling.
And our bolt, we have locking lugs, and the darkened part here is the locking shoulder.
This is whatever the locking lugs are locking against.
And so when you chamber a cartridge it's going to sit right there in the chamber.
The back of the cartridge is flush against the bolt face.
And there is going to be a little bit of gap here, and that's OK,
that's there so that if my cartridge is slightly too large it'll still fit. Everything has tolerances.
And with a small little gap like this,
there's not enough brass exposed to create a pressure problem.
So there's no chance that the brass is going to blow out through this little tiny gap.
In theory the cartridge will actually expand to fill that little gap, and that's also fine.
As we've said, that's one of the really nice properties of the brass
is that it will expand like that under pressure without breaking.
Now let's consider insufficient headspace, what if you don't have any gap?
Or alternatively, you can get the same effect by having the chamber cut too short.
So ... if we're talking about headspace as the free area in the chamber,
that can be affected by how deep the chamber is cut.
How deep can the cartridge go in before the shoulder of the cartridge
hits the shoulder of the chamber and stops.
It can also be affected by the position of the locking lugs and the locking shoulders.
If we were to cut these locking shoulders back a little bit,
then the bolt doesn't come as far forward when it's locked,
and then you have more of a gap between the bolt and the barrel.
Now in this case, we have no gap at all, which means when I ram a cartridge into the chamber
I'm more than likely going to actually jam it a little too far up into the rifling.
And that's going to do a couple of things. It's potentially going to like crimp
the ... neck of the case into the beginning of the rifling, the lead or the throat.
And that's going to reduce the overall volume of the cartridge inside,
which means when it ignites you're going to have higher pressure.
Probably not high enough pressure to cause a significant problem,
we design guns with safety factors for that sort of thing.
But higher pressure is a problem over time.
Higher pressure will also impact our accuracy.
If we are jamming the bullet too far onto the lands and grooves,
we may not be doing it consistently and that's going to impact accuracy.
So in those ways, headspace that is too short can cause problems.
Oh, also if we're jamming the neck into the rifling that's going to expand
and it's more likely to get stuck and cause problems extracting the case after it fires.
So you can actually get like failure to extract malfunctions as a result of insufficient headspace.
But usually insufficient headspace isn't the one that gives us exciting malfunctions,
that would be ... excess headspace.
So here you can see I've drawn this with a lot more gap
between my chamber and my bolt face.
And ... this can't really happen just from the chamber being too deep,
because the ... cartridge can only go so far in before the bolt stops it.
This typically happens when the locking recesses are too far back.
And what will go on is when you chamber the round,
you're either pushing it forward manually on a bolt action,
or the bolt is being pushed by springs,
and so it will chamber all the way in. And the bolt will actually come up
basically flush with the barrel face.
However, when you fire the cartridge, the pressure in here is going to push the cartridge back,
which will cause the bolt to open up and slam back against these locking lugs.
So what we've got going on is essentially this.
This is going to be the condition until you fire,
with an open space here between the locking lugs and the locking shoulder.
When you do fire then you get this situation.
And there are a couple of different things that can happen as a result.
One of them is, well, the brass is going to stretch back,
if it stretches too much you get a weak point in here and, kaboom.
The very high maximum peak pressure of the cartridge explodes out.
And the specific result of that can depend on how the gun is built.
Modern guns generally are designed to redirect gas from this sort of worst case scenario
in a way that it's not going to injure the shooter.
Fairly old guns do that too. There are elements of like the Mauser 98,
in fact elements of the Mauser design process from 1889 through 1898,
a lot of the iterative changes in the design
were in fact ... safety features to improve reaction to cases like this.
But ... that's a catastrophic case, there are less catastrophic cases.
And I actually have a good example of one.
I have a 6.5 Arisaka firearm and it has excess headspace.
... When I got it I went and tried shooting it.
And you can see here on this case that I fired through it
the case is actually bulged at the back, and that's because it did exactly this.
The case stretched back open, and that is
where it was actually free, unsupported by the chamber, and it bulged open.
Nothing catastrophic happened here, but one of the other rounds fired did this.
And what happened here was the neck of the cartridge ... has stretched out
and is tight against the neck of the chamber.
The back end of the cartridge stretched out backwards, and the stretching was enough
that the case actually sheared in half right at the shoulder.
That's ... a separated shoulder.
You will also sometimes get case head separations.
Case head separation is what leads to this, and that's where
you can see this shiny distinct line.
The ... head of the case just rips off.
Both of these become obnoxious malfunctions to clear
because like in this case the neck of the cartridge is still jammed up inside the chamber.
And so you'll find out about this typically when you load the next cartridge
and it gets stuck in the neck of the previous one.
So it only cycles in like that far, instead of properly chambering.
... There are broken case extractors that are typical equipment,
in particular for military machine guns, to fix case head separations.
To get the ... leftover bit of cartridge case out of the chamber.
Anyway, that is a first-hand example of what can happen from excess headspace.
Now let's consider how headspace specifically applies
to various different types of guns that you might run into yourself.
So to start with old mil surp. Usually mil surp guns are just fine.
When they left the factory the headspace would have been great.
However over the last 50 years, 100 years, 150 years in some cases,
these guns have gone through ... potentially a lot of firing, a lot of wear,
and sometimes they will have had replacement parts.
It's not uncommon with a lot of military surplus stuff to find it with mismatched bolts.
Meaning that the bolt that's in the rifle now is not the bolt that was in the rifle originally.
Now our manufacturing methods are generally pretty good,
and they have been pretty good for a pretty long time.
And typically one of the contingencies of modern,
well even 100 year old, firearm manufacturing is that the parts ought to be interchangeable.
You should be able to take a bolt from one rifle and swap it into a different rifle
and have it fit and tolerance properly. And that includes proper head spacing.
So usually it's fine, but it's not always fine.
And the standard liability society rule of thumb is if you're getting a surplus firearm
and you're going to shoot it a lot, you should have ... its headspace checked.
Either by yourself, it's not a complicated process, or get a gunsmith to do it.
A gunsmith theoretically, hopefully, already has the gauges,
so then you don't have to pay to buy the gauges yourself.
Now, how often have I done that?
Mmm, I can't say I've taken my own advice very well on this.
I generally haven't actually gone out and headspaced the firearms that I'm using,
and I've generally not had problems with them.
However like this guy for example, this is an Enfield that was re-armouried by the Turks,
and re-chambered into 8mm Mauser with a new barrel, but still has an Enfield bolt.
That thing? If I were going to go out and shoot that thing I would absolutely
get its headspace checked, because who knows what the deal is on that thing?
And the same exists for guns that have had a lot of wear.
If in doubt, ... the safe choice is get the headspace checked.
Now, ... this is something that ... is much more often important I suppose
is with more modern guns, specifically people who are making guns from parts kits.
If you are doing that it's not just a matter of,
"OK, the gun's headspace here was great when it left the factory, is it still OK?"
Now, if you're the one who's actually ... setting the barrel in the trunnion or the receiver,
you are the one who is responsible for making sure that the headspace is correct to begin with.
And if you set it wrong, you will have some of the problems that we discussed earlier.
So in this case, when I fitted the barrel into that trunnion,
I had a set of headspace gauges and we set the barrel.
Thought we had it about right, check the headspace, made sure,
then drilled, pinned and fixed the barrel.
And if there was a problem with the headspace you adjust the barrel in or out,
or adjust the chamber in (you can't really make the chamber shorter once you've already cut it).
But you set the headspace correctly.
So if you are engaged in project building like that,
you absolutely need to get yourself a set of headspace gauges,
understand how to use them, and you will be using them
when you are building something from parts.
So ... I think that pretty much covers both parts of the question.
Hopefully, this was useful and informative. Thank you guys very much for watching.
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