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JustC...
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Member Posts: 168,427 ✭
I'm trying to understand what difference it makes using a fired case in your chamber when determining the OAL to from the bolt face to the lands. I know I'm probably missing something here, but it seems to me that distance would be the same whether you are using a fired case, or a resized case. I'm just trying to better understand why it makes a difference. In my mind, it seems like OAL to the lands would be OAL to the lands.?.?
Thanks for the feedback! [:)]
Thanks for the feedback! [:)]
Comments
A resized case might have the shoulder pushed back, which would change the reading. Trimmed after sizing to the same length as a fired case would be the same.
How would that change the distance from the bolt face to the lands? I can't see this distance having any variables...
clear as mud??
The difference is, that if the stoney point case (not fireformed in your own chamber) has a different casehead to shoulder (datum line) measurement than your fireformed case, then the measurement you get with the stoney point case will have an error built in to it, whereas the fireformed case does not, since it matches YOUR chamber dimensions. There is NO WAY, they can provide one standard case for a given chambering, and have it perfectly match every customer's chamber, since chamber dimensions vary quite a bit from gun-to-gun, manufacturer-to-manufacturer, and from factory chamber to match chamber. Even reamers from different companies vary from one another. When using a fireformed case from the particular chamber you are going to be loading for, you are assuring yourself an accurate measurement for THAT PARTICULAR chamber. And since the case expands during fireforming, to fill the chamber, then there is no need to worry with the bolt, since by default, the fireformed casehead is in contact with the boltface, since it has been fireformed. The formed case itself, plus the bullet, provide all the necessary measurement data.
your cartridge headspaces off of the shoulder (if it is a belted mag, it will headspace off the shoulder after fireforming and then neck sizing). If that shoulder is pushed back after sizing, then the distance from the casehead to shoulder of the case, will be altered. The fired case has expanded (with ~.002" springback) to match the chamber dimensions. Thus, you have an accurate sized case to indicate what your true chamber dimensions are. Now, when using that fired case, you are inserting the rod and setting the second "stop" as it hits the casehead of the fired case. Since that case has been fireformed to YOUR chamber, the fact that the "stop" is set when the rod hits the casehead, means that you are at the boltface, without having the bolt inserted. This is because, during fireforming, the casehead is now in contact with the bolt face, and the shoulders are in contact with the shoulder portion of the chamber. You have, for lack of better terminology, "grown" the case to fit your chamber dimensions, from bolt face to shoulder.
clear as mud??
The difference is, that if the stoney point case (not fireformed in your own chamber) has a different casehead to shoulder (datum line) measurement than your fireformed case, then the measurement you get with the stoney point case will have an error built in to it, whereas the fireformed case does not, since it matches YOUR chamber dimensions. There is NO WAY, they can provide one standard case for a given chambering, and have it perfectly match every customer's chamber, since chamber dimensions vary quite a bit from gun-to-gun, manufacturer-to-manufacturer, and from factory chamber to match chamber. Even reamers from different companies vary from one another. When using a fireformed case from the particular chamber you are going to be loading for, you are assuring yourself an accurate measurement for THAT PARTICULAR chamber. And since the case expands during fireforming, to fill the chamber, then there is no need to worry with the bolt, since by default, the fireformed casehead is in contact with the boltface, since it has been fireformed. The formed case itself, plus the bullet, provide all the necessary measurement data.
I understand what you are getting at...and thanks for the answer. If I understand you correctly (considering the fact that "factory sized brass" is smaller than just about every chamber out there), the only way a "factory sized" case versus a fire formed case would be IF the chamber and head spacing were shorter than normal. Is this correct. It will help me better understand where you are coming from. I kind of figured you were talking about headspacing...
Thanks! [:)][8D]
Now you HAVE TO remember, that the comparator that you use during the casehead to ogive measurement, ABSOLUTELY has the be the comparator you use for all future measurements when you change bullet designs for that particular rifle. A different comparator will most likely NOT be the same when you take the measurement.
I use the sinclair comparators since they will accomodate multiple bore diameters with each one.
I tried the hocus pocus stuff like sooting the bullet, splitting the neck down it's length and trying to seat a bullet by closing the bolt and then measuring OAL, etc. They ALL provide varying results when done 3-5x in a row. the sinclair tool, yields the same measurement EVERY TIME, right down to the .001"[;)] I have done it 5x over, to see for myself, and the measurement was the exact same, each time.[^]
you are correct. A case that is supplied by stoney point (which has since been purchased by sinclair) will never be long enough (from casehead to datum line) to actually match YOUR chamber, or anyone else's for that matter. Sinclair figured that out, and made their tool to compensate for that error, the only correct way you can, by designing a tool that uses brass fired in YOUR chamber.
Now you HAVE TO remember, that the comparator that you use during the casehead to ogive measurement, ABSOLUTELY has the be the comparator you use for all future measurements when you change bullet designs for that particular rifle. A different comparator will most likely NOT be the same when you take the measurement.
I use the sinclair comparators since they will accomodate multiple bore diameters with each one.
I tried the hocus pocus stuff like sooting the bullet, splitting the neck down it's length and trying to seat a bullet by closing the bolt and then measuring OAL, etc. They ALL provide varying results when done 3-5x in a row. the sinclair tool, yields the same measurement EVERY TIME, right down to the .001"[;)] I have done it 5x over, to see for myself, and the measurement was the exact same, each time.[^]
Thanks man! [:)]