Primers Backing Out

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  • Last Post 13 March 2018
Hoppy posted this 06 March 2018

I was testing some "de-leading" loads (COW and so forth) in a .45 Colt revolver when I encountered a problem: with some of the loads, the primers backed out just a bit--enough to make it difficult to rotate the cylinder. And it was just the primers (fresh scrapes on the primers showed that), not the whole cases. Does anybody know why this might be happening with these very light loads?

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TRKakaCatWhisperer posted this 06 March 2018

Typical for LIGHT loads.

Normal sequence:

Primer goes off, moves back.

Powder goes off, pushing case back (re-seating primer flush).

Second step doesn't go all the way with a light load.

 

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RicinYakima posted this 06 March 2018

BTW, it is an excellent way to measure headspace on your revolver.

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Ross Smith posted this 06 March 2018

What is a COW load?

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45 2.1 posted this 06 March 2018

COW = Cream of Wheat cereal. Usually a light powder charge, cream of wheat and a light wad on top.

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joeb33050 posted this 09 March 2018

BTW, it is an excellent way to measure headspace on your revolver.

 

PRIMER PROTRUSION AND HEAD CLEARANCE

 

 

 

Thanks to Kent Sakamoto at ATK/RCBS for technical assistance and providing RCBS "Precision Mic" micrometers for 308 Win and 223 Rem used in preparing this article. Mr. Sakamoto and RCBS have been very generous with equipment and advice over the past.

 

 

 

SAAMI DEFINITIONS

 

HEAD CLEARANCE

 

The distance between the head of a fully seated cartridge or shell and the face of the breech bolt when the action is in the closed position. Commonly confused with headspace.

 

HEADSPACE

 

The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.

 

 

 

Several shooters have recommended measuring primer protrusion as an estimate of head clearance. Here is a description of the method, which I do not use now.

 


1. Use fired, FL sized cases.

 

2. Measure the length of the case with a Vernier, dial or electronic caliper. 

 

3. Write down the measurement.

 

4. Start a primer into the case.

 

5. Remove the bolt.

 

6. Put the case extractor cut under the extractor-this holds the case on to the bolt.

 

7. Put the bolt in the action and close the bolt gently.

 

7. Extract the case carefully.

 

8. Measure the length of the case and protruding primer, from the primer to the case mouth, with the caliper.

 

9. Write down the measurement and subtract the first measurement from the second. The difference is the amount of primer protrusion, which is the head clearance.

 

10. This measurement, as with all measurements, is best done a number of times and the results averaged.

 

 

 

Why don't I use this method?

 

 

 

Testing showed that no matter how carefully this measurement method was used, the measurements, compared with measurements using an RCBS Precision Mic, were remarkably and unacceptably inconsistent

 

 (Here we define CASE HEADSPACE as typified by the SAAMI 308 Win cartridge dimension 1.634-.007 on the SAAMI 308 Win drawing at http://www.saami.org/pubresources/cc_drawings/Rifle/308%20Winchester.pdf  )

 

 

 

The RCBS Precision Mic measures CASE HEADSPACE as the deviation from CHAMBER HEADSPACE minimum dimension. For example, SAAMI 308 Win minimum chamber headspace dimension is 1.630". A cartridge case with the equivalent dimension of 1.632" would measure +2 using the RCBS Precision Mic. 1.625" measures -5.

 

 

 

First test  6/11/12:

 

Measure the case headspace of 30 308 Win cases with an RCBS Precision Mic. Measure the length of each case. Start a primer in the primer pocket of each case and chamber it in a M10 Savage. Measured the length of the case from primer to case mouth. Subtract one length from the other, getting the difference = primer protrusion.

 

 

 

Case                #          Primer

 

Headspace                   Protrusion

 

(.001")                         (.001")

 

                                   

 

-6                     3          3, 3, 7

 

-5                     5          2, 3, 4, 5, 5

 

-4                     19        0, 2(4), 3(4), 4(4), 5(3), 7(2), 8

 

-3                     3          2, 4, 7

 

 

 

Second test  6/16/12:

 

Same procedure as above with 12 223Rem cases.

 

 

 

Case                #          Primer

 

Headspace                   Protrusion

 

(.001")                         (.001")

 

                                   

 

 

 

-2                     10        2, 4, 4, 7(5), 8, 8

 

-1                     1          7

 

0                      1          2

 

 

 

Primer Protrusion = Chamber Headspace - Case Headspace.

 

For example, with chamber headspace of 1.632" and case headspace of 1.629"; 1.632"-1.629" = .003"; the expected primer protrusion.

 

We would expect that Primer Protrusion measurements would track Case Headspace measurements within +/-.001" or so; yet the variation is all over the map.

 

 

 

It is clear that the results of this testing show that Primer Protrusion does not vary as Case Headspace, and for my purposes this measuring method is unacceptable.

 

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joeb33050 posted this 09 March 2018

Tapping a rod in the bbl with a mallet gets the primer seated again

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Hoppy posted this 10 March 2018

This is interesting, but if the last step is moving the case to the rear (and thus re-seating the primer), presumably the case remains in that rearward position. Why does it not then bind and interfere with the cylinder's rotation? A puzzlement.

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TRKakaCatWhisperer posted this 10 March 2018

This is interesting, but if the last step is moving the case to the rear (and thus re-seating the primer), presumably the case remains in that rearward position. Why does it not then bind and interfere with the cylinder's rotation? A puzzlement.

 

There is clearance - the case is free to slide forward or back..  With the primer extended there is no clearance, hence contact and perhaps some pressure in that contact.

 

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Hoppy posted this 13 March 2018

Okay! That helps a lot. Thanks for the additional explanation.

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Ken Campbell Iowa posted this 13 March 2018

springy brass is the shooter's friend ... seals under pressure, then shrinks down and slides around in the chamber for easy extraction .. good property for sizing dies also ..

is new norma brass still a little too soft ?

ken

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BigMan54 posted this 13 March 2018

The only time I had this problem was with an old S&W 19, pinned&recessed. it did have headspace problem that was "cured" with a washer. In a full strip down I lost the washer. But stupidly I decided to fire some full power #358156GC over the old "max" charge of 2400 in some cases that were on their last max loading. I'd fired the previous 80rds in a NM BlackHawk with out a problem.

Geez I only had 20rds left in the box. Can't hurt anything right ? WRONG, first round locked up the cylinder tight as a drum.  I had to take the revolver home: LOADED WITH 5 LIVE ROUNDS ! I had to put the gun in a padded vise and WACK the cylinder with a leather mallet. Not one of my finer moments.

The primer on that fired round had backed out. The small bit of excessive headspace was enough to back that primer out of the tired primer pocket. 

Tired gun, tired brass.

Long time Caster/Reloader, Getting back into it after almost 10yrs. Life Member NRA 40+yrs, Life S.A.S.S. #375. Does this mean a description of me as a fumble-fingered knuckle-draggin' baboon. I also drool in my sleep. I firmly believe that true happiness is a warm gun.

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