Finding Jam - I thought I knew how

Les in Wyoming

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Oct 10, 2020
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Glenrock, Wyoming
OK, the way I used to do it is by resizing a cartridge and barely seating a bullet. Then slowly closing the bolt and locking down. This pushes the bullet to "Jam." Then I got a fancy cartridge seating depth gage. I have had a horrible time finding a seating depth for Nosler bullets. None were accurate. Then I compared my old "jam" method and found there to be a big difference in COAL measurement. As much as .032. Now I am confused. But wait, there's more. I was trying to find jam with Berger bullets. They are also different from the cartridge seating depth gauge, but unlike the Noslers, they vary among themselves. Doing 7 empty cases with long seated bullets, the Bergers vary as much as .050. The Noslers vary only .005 and I got a couple that were the same.

I measure with CBTO gauge and COAL with calipers. Every one of those bullets has marks showing they engaged the lands. How in the world can you know which is right? What am I doing wrong? I sure would appreciate input. Thanks.
 

North Idaho Hunter

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Soft seat jam has been more consistent for me when I seat the bullet out just enough to engage rifling.

I’ve found that if you seat the bullet too far out initially then close the bolt you can get a wide variance in jam measurements -

I believe the big differences in measurements your getting is from leaving the bullet seated too long for the test......try seating the bullets deeper beforehand ——seat them out just enough so the bullet doesn’t have to meet as much resistance to “jam”

as for getting a wide variance from jam measurements vs. comparing it to the Hornady tool-I’m not surprised at all.

With the comparator tool your measuring where the bullet just kisses the rifling vs. the actual jam.

Hopefully that’s not too confusing 😆
 
Last edited:

QuietTexan

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Nov 16, 2020
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Texas
If you want to use a bolt-close method, then you'll want to check the runout. This would be the biggest concern because you're sticking a bullet into the lands that has a good bit of resistance to it sliding back into the case because you're using a sized case. If the bullet goes into the throat out of true, you'll never get a repeatable measurement because it can hang up differently each time. The ejector pushing the round to one side as the bullet goes into the throat, so it's common to take that out, or tape it down somehow so it's not in play when taking the measurements

If you don't want to strip the bolt and check for runout on your bullets (rolling them on the table would probably work here), try cutting the neck on a case. As thin as you can using a thin Dremel wheel, from the end of the neck straight down through the shoulder. Deburr it well inside and out with a small file. The cut neck will have less resistance and the bullet shouldn't have as much runout once you pull it out of the chamber, plus you can index the case the same each time, and scribe a line on the bullet to start at the same seating depth each time. If you get better repeatability great, if not it's just one case you cut up.
 

Les in Wyoming

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Joined
Oct 10, 2020
Messages
118
Location
Glenrock, Wyoming
Soft seat jam has been more consistent for me when I seat the bullet out just enough to engage rifling.

I’ve found that if you seat the bullet too far out initially then close the bolt you can get a wide variance in jam measurements -

I believe the big differences in measurements your getting is from leaving the bullet seated too long for the test......try seating the bullets deeper beforehand ——seat them out just enough so the bullet doesn’t have to meet as much resistance to “jam”

as for getting a wide variance from jam measurements vs. comparing it to the Hornady tool-I’m not surprised at all.

With the comparator tool your measuring where the bullet just kisses the rifling vs. the actual jam.

Hopefully that’s not too confusing 😆
Thanks for the tip. I measure coal and cbto. I didnt think of getting the bullet close.
 

Les in Wyoming

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Joined
Oct 10, 2020
Messages
118
Location
Glenrock, Wyoming
If you want to use a bolt-close method, then you'll want to check the runout. This would be the biggest concern because you're sticking a bullet into the lands that has a good bit of resistance to it sliding back into the case because you're using a sized case. If the bullet goes into the throat out of true, you'll never get a repeatable measurement because it can hang up differently each time. The ejector pushing the round to one side as the bullet goes into the throat, so it's common to take that out, or tape it down somehow so it's not in play when taking the measurements

If you don't want to strip the bolt and check for runout on your bullets (rolling them on the table would probably work here), try cutting the neck on a case. As thin as you can using a thin Dremel wheel, from the end of the neck straight down through the shoulder. Deburr it well inside and out with a small file. The cut neck will have less resistance and the bullet shouldn't have as much runout once you pull it out of the chamber, plus you can index the case the same each time, and scribe a line on the bullet to start at the same seating depth each time. If you get better repeatability great, if not it's just one case you cut up.
This is good stuff. Will give this a try. Thanks
 

orkan

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OK, the way I used to do it is by resizing a cartridge and barely seating a bullet. Then slowly closing the bolt and locking down. This pushes the bullet to "Jam." Then I got a fancy cartridge seating depth gage. I have had a horrible time finding a seating depth for Nosler bullets. None were accurate. Then I compared my old "jam" method and found there to be a big difference in COAL measurement. As much as .032. Now I am confused. But wait, there's more. I was trying to find jam with Berger bullets. They are also different from the cartridge seating depth gauge, but unlike the Noslers, they vary among themselves. Doing 7 empty cases with long seated bullets, the Bergers vary as much as .050. The Noslers vary only .005 and I got a couple that were the same.

I measure with CBTO gauge and COAL with calipers. Every one of those bullets has marks showing they engaged the lands. How in the world can you know which is right? What am I doing wrong? I sure would appreciate input. Thanks.

This may help:

 

User4302021

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Mar 17, 2018
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Cut a slot in the neck of a sacrificial case from mouth to neck/shoulder junction with a dremel tool. Then seat a bullet like you were doing. Measure off the ogive (CBTO) instead of the tip.
 

matt_3479

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Jan 31, 2010
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1,334
Location
Southern Ontario
I’ve been using a hornady oal length gauge and tool a few years and found a few inconsistencies. I switched to the method shown above from Greg with primal rights (which I believe is the same as Alex wheeler if I recall). I have since decided to try Erik Cortina’s method for my latest load development and the next one to come and so far it’s proving to be very accurate loads and I like the idea of his method but less focused on the actual length it self which was an odd concept lol
 

Les in Wyoming

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Joined
Oct 10, 2020
Messages
118
Location
Glenrock, Wyoming
I used Eric Cortina's method, which is what I always used to do. He said it doesn't matter if you get different readings, but if I have a large difference from different shells, how do I know if I my load is going to be .010 off jam or .020 into the rifling?

I used 3 different de-primed cartridges and they were all different. So I did them over and over again. It looks like each cartridge comes out the same each time - but each are different. Hmmmm. So I checked cartridge length. Each are different. In my pea brain, it seems to me it would not matter what the cartridge length is, because we are going for base of cartridge to ogive. That figure should be the same and the bullet jump should therefore be the same when you measure the OAL. Am I right?

It looks like my fired cases measure differently. I have a Lee case hand sizer. It sticks into the primer hole as a pilot and you spin it which cuts to size. But it looks like the length is different.

So, it could be that I am going to reveal some ignorance here, but I am here to learn. Does the actual case length make a difference in how deep the bullet sits from the lands when you measure CBTO?

Secondly, does cartridge length make a difference in accuracy if they are all seated the same CBTO? If so, what would the tolerance be? within .001? .002? or .009? etc. I will appreciate any input. Thanks
 

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