Measuring Jam Length or Rifle Seating Depth?

Prerylyon

Veteran member
Apr 25, 2016
1,076
125
Cedar Rapids, IA
Wanted to reach out to the brain trust to hear how/if everyone measures a jam length COAL for a specific bullet and rifle when they start load development?

I've used 2 methods over the past year and gotten different results-which messes with my mojo. Both methods have been around for a long time.

Lately, I've used this method, which has been in the Nosler manuals nearly forever:

#1


Last year, I used a method using scotch tape and cleaning rods:

#2


Using method #2 last Sept, I consistently got 3.330" for jam length. This happens to be the SAAMI max COAL for .280 Rem, which is the caliber I'm working with. This seems almost too coincidental.

Using method #1, I get 3.250", again, very consistently.

Here a couple pics of a bullet that I used yesterday to make the 3.250" measurement, all smudged up, showing the ring where the bullet made contact:

28213


28214

What say you? Which is the correct jam length measurement?
 

Prerylyon

Veteran member
Apr 25, 2016
1,076
125
Cedar Rapids, IA
Dang. Looks like I have to buy yet another gadget. 😣

I have read/heard that this OAL gage is not accurate due to an angle required to place the caliper over the dummy round base to nose to ultimately make the COAL measurement (measurement not centered on dummy round end to end)? I also read that that Hornady has machined a flat somewhere in the gadget centered on the dummy case base to prevent this (which makes sense to me). Appreciate any comments. ☺

Why am I deep diving into this all of sudden? Last year, you might remember I posted about a velocity drop that didn't seem to make sense. If I was jamming bullets way out into the lands-and my new measurement using that shade tree mechanic smudge pot method seems to indicate that-I might have created an issue where the depth was set long and possibly creating pressue spikes or unusual pressures that might have been supportive of the velocity drop. Yeah, I got a load that shot super accurate out of all that last yr, but the OCD in me can't leave it alone...😆
 
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Micah S

Active Member
Jan 11, 2016
158
48
Sandy Oregon
Dang. Looks like I have to buy yet another gadget. 😣

I have read/heard that this OAL gage is not accurate due to an angle required to place the caliper over the dummy round base to nose to ultimately make the COAL measurement (measurement not centered on dummy round end to end)? I also read that that Hornady has machined a flat somewhere in the gadget centered on the dummy case base to prevent this (which makes sense to me). Appreciate any comments. ☺

Why am I deep diving into this all of sudden? Last year, you might remember I posted about a velocity drop that didn't seem to make sense. If I was jamming bullets way out into the lands-and my new measurement using that shade tree mechanic smudge pot method seems to indicate that-I might have created an issue where the depth was set long and possibly creating pressue spikes or unusual pressures that might have been supportive of the velocity drop. Yeah, I got a load that shot super accurate out of all that last yr, but the OCD in me can't leave it alone...😆
Purchase the attachment to measure to the ojive with the gauge. It's offset and is a better way to measure your seating depth.
 
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Triple BB

Active Member
Jun 22, 2013
282
5
Wyoming
An easy way to do it is cut two or three slits in the neck of a sized & used case. You want enough tension so the bullet slides into the neck and comes out of the chamber. Put a bullet in the case and slowly close the bolt and then remove. Measure your coal. Do it a couple times and you’ll be damn close on yer max coal. You can work backwards from there on your seating depth. Pick up a Redding competition shell holder kit. It’s the easiest way to get yer 2 thou’s headspace and yer good to go.
 
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Prerylyon

Veteran member
Apr 25, 2016
1,076
125
Cedar Rapids, IA
Update on my OCD:

So, I got a nicer set of calipers and the Sinclair comparator (not the nut) and 7mm bullet insert.

I also plan to make another set of jam COL measurements with the Hornady lock and load when I get around to borrowing it.
I very carefully did the measurments again using the 2 methods shown in my OP.

I still get 3.250" using the pinched fire formed case-smudge bullet method. Using the rod-masking tape method, I get 3.245". They are within 5 thousandths of each other.

I was beyond careful and made 5 sets of measurements.

All I can figure, is somehow I screwed up the rod-masking tape method last yr when I did it.

I know this measurement has much to do about the bullet shape as anything, but 3.240" is EXACTLY in the middle of the SAAMI spec COAL for the .280 Rem. My updated jam COL is very close to this.

I wonder if my rifle just has a short throat?

Looking a few threads down, if this all holds, I may have been seriously jamming my bullets and causing a bad pressure situation that may explain that weird velocity drop at the top end loads.😲
 
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Prerylyon

Veteran member
Apr 25, 2016
1,076
125
Cedar Rapids, IA
Yet even more OCD...😨

Awaiting access to a Hornady OAL tool, I continued this madness with 2 other bullets-2 longer and heavier critters.

Again, using the two OP jam depth methods, I was able to get within 5 thous of meas agreement following the same procedure with these bullets-agreement between the meas methods.

Nosler 150 LRAB
3.460"

Speer 150 GS
3.350"

These two are shaped differently than the Speer 145 BTSP and are heavier. I feel a little better now seeing these numbers that something is probably not wrong with my rifle. The jam depth is correlating more to the different bullets than as if there is something else limiting it-like a short throat or some kind of chamber limitation less than SAAMI specs. Both results are longer than SAAMI max of 3.330" I would guess a true chamber measurement would need to made to the head space datum, anyway.
 
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