Hornady OAL Gauge

Eeegadd

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May 12, 2021
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Co
Hi all, I'm looking for guidance on the correct usage of the Hornady tool after using it to load some rounds recently that ended up with flattened primers:

I used the OAL gauge with a generic 6.5 CM modified case to measure the CBTO in my Savage 12 LRP rifle. I inserted the body into the chamber until it rested against the shoulder and then pushed in the plunger stopping at the first sign of resistance. The CBTO measured 2.0000" on average using my mitutoyo digital caliper. On my 5th measurement (using the same bullet) I wiggled the plunger a little after initial resistance and the bullet slid in an additional 0.25" before stopping again. I did not force the plunger, just rotated it and the bullet slid in further without any pressure. I repeated this another several times with the same results. The CBTO measurements afyer wiggling were an astonishingly consistent 2.2500."

I loaded 100 rounds using new Lapua LR brass, Berger 140 hybrid target bullets, H4350 powder, and CCI BR-2 primers seated at 2.2300" (0.020 less than measured CBTO) and cycled a number of the rounds without feeling any resistance or seeing markings on the bullets. I measure COAL before and after cycling and it did not change.

I took the rounds to the range and had very good accuracy with reasonably low SD measurements (8.6 fps at 41.5 gn and 9.0 fps at 42.5 gr of H4350). The issue was that all of the primers were flattened which makes me wonder if I had seated into the lands of the rifling using the 2.2300 CBTO length?

Should I have used 2.000" CBTO as my seating reference? COAL for my loaded rounds was 2.9375" on average using the 2.2300" CBTO seatinng depth.

The online Hodgden reloading resource lists 40 gr of H4350 as max with a Hornady 140 gr bullet, but Hornady ammo boxes indicate (or at least they used to indicate) that their 140 gr A-Max rounds were loaded with 41.5 gr of H4350. I had heard that earflier versions of the Savage 12 LRP CM rifles had tight chambers resulting in popped primers, but my gun is new production so I wouldn't suspect that this is the problem either.

Thanks for your thoughts.
I had something similar happen to me but it wasn't .250 it was .100 deeper. I measured again and found the true dimension was .100 shorter. As you know, start seating depth .020 short of that for the satterlee powder ladder test. My guess is that you were into the lands causing the pressure.
 

jdyoung

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Mar 1, 2020
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hybridspecies wrote:

"I wiggled the plunger a little after initial resistance and the bullet slid in an additional 0.25" before stopping again. I did not force the plunger, just rotated it and the bullet slid in further without any pressure. I repeated this another several times with the same results."
I would want to know why after wiggling the plunger the bullet slid in further to the lands ! Is there a buildup or slight obstruction that when the bullet is wiggled it moves past ? Maybe viewing with a borescope will provide an answer and after a good cleaning the wiggling affect will be eliminated.
 

WiscGunner

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Aug 2, 2016
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I had similar issues with my Hornady COAL tool. Be sure to test it outs of tge rifle for tight spots in the plunger. It is just molded plastic. Sand down any tight spots and be careful not to tighten the thumb screw too much as it deforms the plasic as well. Once youhave the tool operating smoothly, recheck your measurements to verify.
 

hybridspecies

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Jul 9, 2021
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I don't have a borescope, but the possibility of an obstruction or machining marks crossed my mind also. I gave the bore a thorough cleaning on Saturday and am hoping that the 101 rounds I put through it on Thursday smoothed things out enough that so when I use the tool to remeasure seating depth again I won't get the same resistance. If not I'll call a plumber and use his scope haha.

The tool itself seemed to operate smoothly outside the chamber, but I will definitely check for tight spots and burrs. Great suggestion - thanks.
 

QuietTexan

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Nov 16, 2020
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I would call that last push what really gets you to the lands - I've always thought it is something like a minor concentricity issue in the jacket of the bullet hanging up. I use the same bullet over the life of the barrel to measure - eventually there's no more hang up, and there are marks on the jacket of the bullet. I jam the bullet into the lands with the tool, and get repeatable measurements that vary by less than 0.001".

I think that finds a more reliable "hard stop" of what the maximum length is, and ensures I'm only working one way away from that point.
 

hybridspecies

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I just read an article explaining that lubrication in the chamber can result in flattened primers (and higher pressure) by reducing the friction between the case and chamber wall during firing, resulting in the case head being pushed back against the bolt face harder than it should. I left in some Boretech Eliminator when I cleaned it previously and forgot to swab clean before taking the rifle to the range so that may be playing a role. Just need to eliminate the OAL gauge (seating depth) as the source of the problem and then I can concentrate on the other factors. Thanks all.
 

QuietTexan

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I guess that sounds logical. Did you have any hard bolt lift to go with the flattened primers? I forgot to dry out the chamber on a 243 Win and with a factory round I almost had to find a hammer to beat the bolt open, that's what I normally view as a sign of a wet chamber.
 

hybridspecies

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The gun is new and bolt operation is the opposite of smooth, very disappointing. I did notice increased hard lift as the powder charge of the rounds increased, but bolt operation was rough even with the lower powdered rounds. Do you think the lubrication would have burned off after intial firing or would it persist to 101 rounds? Just trying to eliminate variables at this point for the next round of testing.
 

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