Weird chamber damage on P72 Hornet

Discussion in 'Sako Rimfires and Small Action Rifles' started by icebear, Mar 19, 2021.

  1. icebear

    icebear Sako-addicted

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    Looks pretty flat to me. I've seen worse, but there's definitely some pressure at work here. The cases in the photo are from both the P72 and a CZ527. I have no idea which case came from which gun. Click on the photo for a more detailed look.
    Hornady Primers.JPG

     

  2. ricksengines

    ricksengines Sako-addicted

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    Its kinds hard to see but I think I'm seeing some signs of minor cratering as well. I would need to use a loop to confirm but it sure looks like its there.

    rick
     
  3. stonecreek

    stonecreek SCC Secretary Forum Owner SCC Board Member

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    Flattened primers are more often the result of slightly loose headspace (which is very common in Hornets) than a result of high pressure. If you'll look at almost any fired, low-pressure .30-30 fired in a lever action rifle you'll see flattened primers every time.

    Here's what happens in sequence to flatten a primer without excessive pressure being involved:

    (1) When the firing pin strikes the primer the entire round is driven forward until the rim, shoulder, belt, or whatever portion of the case the round headspaces on meets with resistance from the corresponding part of the chamber.

    (2) The primer then ignites the powder, pressure is quickly generated, and the case walls hug the chamber walls very tightly.

    (3) The primer, due to internal case pressure, is forced part way out of the primer pocket until it rests firmly against the bolt face.

    (4) Then the unsupported head of the case is forced back against the bolt face (stretching the brass at the pressure ring, which is the point just in front of the head where the brass becomes thin enough that it expands against the chamber walls while the thicker -- and harder -- case walls just in front of the case head are strong enough to resist deforming against the chamber walls.) Everybody is familiar with this type of case stretching and the thinning of the walls at the pressure ring, right?

    (5) The case head moves to the rear and, in effect, reseats the slightly unseated primer while in the process the primer has flattened and spread against the bolt face.

    This process leaves a flattened primer which looks very much like one flattened by excessive pressure. The difference is that, assuming a proper fit between the firing pin and the firing pin hole, there will be no extrusion of the primer metal into the firing pin orifice -- otherwise known as a cratered primer.

    "Reading" primers for pressure signs is a time-honored and greatly misunderstood methodology. You first have to know the relative headspace of the round being "read" to assure that the flattened primer is not a result of loose headspace, and to watch for secondary signs like cratering -- or even piercing at the firing pin strike. Also, you have to know how relatively thick or hard the primer cups are, since there is considerable difference in some brands and types. For example, the small pistol primers often used by Hornet shooters tend to be somewhat thinner and therefore flatten and even crater more easily at the same pressures than do some small rifle primers.
     
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  4. icebear

    icebear Sako-addicted

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    I'm not sure what you mean by cratering, but if you mean a slight raised ridge around the firing pin indent, it's there. Not much, but it's visible with a 5x magnifier.

    With regard to Stonecreek's comment about headspace, the Hornet headspaces on the rim, and I've noticed a lot of variation in rim thickness between different brands and even different lots of the same brand. So that could also be a factor in excessive expansion.
     
  5. tilleyman

    tilleyman Active Member

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    There is indeed HUGE variation in nominal rim thickness between Hornet case brands, here's a table I put together as part of the 17 HH research... note S&B had the thickest rims, followed by Hornady and RWS.

    SAAMI nominal cartridge rim spec is .065" -0.010" which means a rim of .055" thickness is still OK.
    The SAAMI chamber rim depth spec is 0.065" MIN and 0.072" MAX so if you by chance have a minimal spec rim in a maximum spec chamber with 0.017" difference the case will stretch excessively and head separation occur after only a few reloads.

    SAAMI 22 Hornet Cartridge Specifications.PNG

    SAAMI 22 Hornet Chamber Specifications.PNG

    17 Hornet Rim Thickness.png
     
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  6. stonecreek

    stonecreek SCC Secretary Forum Owner SCC Board Member

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    Right! And if you have as much headspace as would be left with the Sako .0581 rims then case head separation (if sized back to factory dimensions) might come even quicker (depending on thickness of the brass in the case wall).

    Lots of people think that the extremely gradual taper of the Hornet case means that it can't headspace on the shoulder but must headspace on the rim. Well, I've got news for you: If you neck size-only the Hornet case stretching will stop immediately and (assuming loads of reasonable pressure) the little cases will last for multiple firings. Of course, this most likely means that if you have more than one Hornet rifle (and what Hornet enthusiast can get by with just one Hornet?) then you'll have to keep your cases segregated and dedicated to each individual rifle.
     
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  7. kevinlg

    kevinlg Well-Known Member

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    Icebear.........

    I blew-up your pic, and any cratering is very minimal.

    I do think there is a slight deformation at the bottom of the firing pin indent.
    You might want to closely inspect the firing pin tip.....for any damage/erosion.

    BTW.....due to varying rim thicknesses, after initial firing, I usually just neck-size the cases. That is.....until they get too tight.....

    Hope this helps.
     

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