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Headspace basics
Several generations of American shooters have been convinced by bad information that something mysterious and scary called "headspace" should be checked and re-checked on almost any vintage rifle. The truth is less interesting but still worth knowing.
Stripped to its essentials, with a rimmed cartridge like the .30/40, headspace is simply the distance between the face of the bolt and the back of the barrel. It's the space where the "head" (rim) of the cartridge fits when the rifle is loaded.
Since there has to be some room to allow for varying rim thickness, the headspace is normally a bit more than necessary - giving what we call "end play" or "head clearance", a little extra space so the bolt can close easily, even on the thickest rim allowed.
When a full-power cartridge is fired, a whole string of events occurs.
1. The firing pin shoves the case forward, rim against the breech.
2. The primer detonates. If it's not heavily crimped in place, it backs out, shoving the bolt and barrel as far apart as it can.
3. The thin, forward part of the case expands to fill and grip the chamber while the bullet moves out of the case and down the barrel.
4. The solid case head can't expand and grip the chamber, so it moves rearward, re-seating the primer, stretching the case walls just forward of the head, and stopping when it hits the bolt face.
5. If (and only if) the amount of head movement exceeds the elastic limits of the case, the cartridge separates into two pieces.
New cartridge cases can normally stretch a lot before breaking. Even with a minimum rim .054" thick and maximum "field" headspace of .074", the resultant .020" end play is well within the limits of new brass and it's very unlikely a new case will separate even if the headspace is somewhat more than the field maximum (which is pretty rare, especially in Krags).
OK, but if one does separate we're in deep trouble, right? Not really. It seems the short "cup" left behind the break is pretty good at keeping most of the gas where it belongs. Here's a quote from Hatcher's Notebook -
". . . With good strong actions and moderate pressure loads, and especially if the shooting glasses are worn, this trouble is not particularly dangerous, but is annoying and inconvenient. Very little, if any, gas escapes to the rear, because the back end of the cartridge, which remains in the chamber, acts as a seal. In fact, it is just such short brass cups that the Germans have for years used as the breech seals in their heavy artillery . . .
". . . In my own experimental firing I encountered at least a couple of hundred such separations, and every one of my several thousand students had to experience and correct this situation several times and I never saw enough gas escape to hurt any one. . .
"Our riflemen are used to measuring the headspace of their rifles by thousandths, and are prone to become very much alarmed if the headspace gauges two or three thousandths of an inch more than the normal maximum. This is a good safe attitude to take, but it is something like measuring cordwood with a micrometer. . ."
If you handload for a US Krag with generous headspace, there's no need to mess with bolt swapping - changing the rifle's clearances to yield longer case life. You can control cartridge end play simply by changing technique.
When you fire a new case for the first time, use an improvised spacer ahead of the rim - anything from a precision metal washer to dental floss can work to hold the the cartridge head firmly against the bolt face and eliminate or reduce stretch even if head clearance is significant. Another way of accomplishing the same end is to use a bullet seated out far enough to jam into the lands, "headspacing" on the bullet instead of the case. Such techniques are useful only if the rifle has excess headspace. With normal headspace, initial stretch isn't enough to worry about.
After you've fire-formed your new cases they will fill the chamber fully, stopping on the shoulder just like a rimless cartridge. If you neck size, you'll have zero "headspace". If you have to full length size, adjust the die so the cases chamber with just a bit of resistance in the last few degrees of bolt rotation. Cases fitted this way and fired with moderate loads can last for dozens of loading cycles.
Several generations of American shooters have been convinced by bad information that something mysterious and scary called "headspace" should be checked and re-checked on almost any vintage rifle. The truth is less interesting but still worth knowing.
Stripped to its essentials, with a rimmed cartridge like the .30/40, headspace is simply the distance between the face of the bolt and the back of the barrel. It's the space where the "head" (rim) of the cartridge fits when the rifle is loaded.
Since there has to be some room to allow for varying rim thickness, the headspace is normally a bit more than necessary - giving what we call "end play" or "head clearance", a little extra space so the bolt can close easily, even on the thickest rim allowed.
When a full-power cartridge is fired, a whole string of events occurs.
1. The firing pin shoves the case forward, rim against the breech.
2. The primer detonates. If it's not heavily crimped in place, it backs out, shoving the bolt and barrel as far apart as it can.
3. The thin, forward part of the case expands to fill and grip the chamber while the bullet moves out of the case and down the barrel.
4. The solid case head can't expand and grip the chamber, so it moves rearward, re-seating the primer, stretching the case walls just forward of the head, and stopping when it hits the bolt face.
5. If (and only if) the amount of head movement exceeds the elastic limits of the case, the cartridge separates into two pieces.
New cartridge cases can normally stretch a lot before breaking. Even with a minimum rim .054" thick and maximum "field" headspace of .074", the resultant .020" end play is well within the limits of new brass and it's very unlikely a new case will separate even if the headspace is somewhat more than the field maximum (which is pretty rare, especially in Krags).
OK, but if one does separate we're in deep trouble, right? Not really. It seems the short "cup" left behind the break is pretty good at keeping most of the gas where it belongs. Here's a quote from Hatcher's Notebook -
". . . With good strong actions and moderate pressure loads, and especially if the shooting glasses are worn, this trouble is not particularly dangerous, but is annoying and inconvenient. Very little, if any, gas escapes to the rear, because the back end of the cartridge, which remains in the chamber, acts as a seal. In fact, it is just such short brass cups that the Germans have for years used as the breech seals in their heavy artillery . . .
". . . In my own experimental firing I encountered at least a couple of hundred such separations, and every one of my several thousand students had to experience and correct this situation several times and I never saw enough gas escape to hurt any one. . .
"Our riflemen are used to measuring the headspace of their rifles by thousandths, and are prone to become very much alarmed if the headspace gauges two or three thousandths of an inch more than the normal maximum. This is a good safe attitude to take, but it is something like measuring cordwood with a micrometer. . ."
If you handload for a US Krag with generous headspace, there's no need to mess with bolt swapping - changing the rifle's clearances to yield longer case life. You can control cartridge end play simply by changing technique.
When you fire a new case for the first time, use an improvised spacer ahead of the rim - anything from a precision metal washer to dental floss can work to hold the the cartridge head firmly against the bolt face and eliminate or reduce stretch even if head clearance is significant. Another way of accomplishing the same end is to use a bullet seated out far enough to jam into the lands, "headspacing" on the bullet instead of the case. Such techniques are useful only if the rifle has excess headspace. With normal headspace, initial stretch isn't enough to worry about.
After you've fire-formed your new cases they will fill the chamber fully, stopping on the shoulder just like a rimless cartridge. If you neck size, you'll have zero "headspace". If you have to full length size, adjust the die so the cases chamber with just a bit of resistance in the last few degrees of bolt rotation. Cases fitted this way and fired with moderate loads can last for dozens of loading cycles.
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