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Thread: Evaluating Pressure Signs in Reloaded Cartridge Brass

  1. #1

    Evaluating Pressure Signs in Reloaded Cartridge Brass

  2. #2
    Thanks for that.

    I've been trying a 140gr bullet in a .260 and noticed the primer begining to flatten on 39.3gr of N160. i.e. there is still a notable shoulder on the primer - not flattened into the case.
    Will try and find a pic - I am thinking it's probably a very early sign but is it something I should worry about?

    CCI large primers in Nosler custom brass...

    P.S. thanks for your guidance on the sizing bushes. Everything is working a treat now!

  3. #3
    I've posted these pictures before but perhaps they might be helpful to some trying to determine pressure from primers.

    The first picture is of some very low pressure home loads, as you can see the primers are ok but slightly backed out. I colour the primers to ID different loads:

    This next picture is of two types of factory ammo fired in the same rifle as above. It can be seen that the primers are pretty flat but there are no other signs of pressure and bolt lift was as normal:

    Lastly these are full house home loads that were fired in the same rifle as above but with neck sized brass. As can be seen the primers are OK and nowhere near as flattened as with the factory loads even though the velocity is more with the same weight bullet:

    My take on this is that the chamber in my rifle is a fraction long for the factory ammo and so on firing the primer is backing out slightly and then been flattened as the case grows in length. This is supported by the fact that, as can be seen, the primers on low pressure loads are left backed out a little because the case has not grown in length to fill the chamber. With the neck sized reloads even though the pressure is higher there is no scope for the primer to back out and so it is not damaged in the same way as with the factory load.

  4. #4
    The article stated that flattening primers is a poor indicator and you've got a handle on that, for sure, but I'm wondering about the last pic. There are drag marks on the headstamp not evident in even the factory loads which should have impressed themselves hard into the bolt face due to the relative headspace issue. Is this from friction closing the bolt on the neck sized cases? I'm guessing it is. What's your take on it? It would have been great if all three loads were fired with the same brass. Nice photos, by the way.~Muir

  5. #5
    These were shot in a Blaser R93 so the brass should not have been rotated in the rifle as per a normal turnbolt.

    I did try and work out what caused the marks on the case head and am fairly confident that I introduced them at some point in the reloading process. My feeling is that my shell holder for my Lee hand primer is a bit tight with a little burr on it somewhere and sometimes I rotate the brass when removing the brass and that this is the cause of those marks. I'm not 100% sure of this and could do some simple testing but as I'm sure this is not happening in the rifle and as it doesn't impact on functionality I'm too lazy.

    The brass in the final photo had been fired a considerable number of times (probably minimum 10 and maybe as many as 20 firings) so I suspect that I mark the head of the occasional case each time I reload but over course of a large number of loadings I eventually end up with nearly all the cases marked. I did consider that, perhaps, expansion of the head on firing might be causing it to catch up on the shell holder but I found that a similar proportion of brand new, unfired, Lapua brass also caught up on the shell holder.

    The first and last photo was taken with the same batch of brass, if not exactly the same individual cases and I suspect the amount the primers are backed out in the first photo gives some idea of the amount of headspace - I have a feeling that those cases in the first photo were on their first firing and so should be pretty much sized to standard.

  6. #6

    your primers have all backed out slightly showing you are running with excess headspace - not a lot but enough to give false primer flattening indications, to shorten case life, and to reduce accuracy.

    The answer is to back your sizer die out slightly in the press so you only just set the shoulder back from the fireformed position. You can do this by either buying a Hornady L-N-L comparator body and appropriate L-N-L headspace gauge or set thereof and which lets you measure the distance from the case-head to shoulder datum line on fired cases, then again on resized cases.

    The cheaper alternative is to use the rifle bolt and chamber as a gauge. To do this, you really do need to remove the striker / mainspring firing mechanism assembly from the bolt otherwise it introduces resistance to bolt closure. You take a fired case, back the sizer die out a full turn from its standard position touching the shellholder and resize a fired case that has been used with a full pressure load. Oh, I should have said remove the decap / expander stem too as you'll be sizing the case several times. The case shouldn't chamber as the shoulder will have moved forwards as its body diameter was sized down. So, screw the die in a quarter or eighth of a turn and resize the case again. You'll soon reach a point where the case chambers but with some resistance to final bolt closure. Then screw the die in further in really small increments until you just reach the point where the sized case chambers without any resistance at all to final bolt closure. Resize several more cases at that setting and if they all chamber freely, fix the die lock-ring there. You now have minimal headspace and you should find the primers stay level with the case-head, likely looking more rounded. As you surmise, if there is slack in the case's longitudinal fit in the chamber, the sequence is as follows:

    firing pin pushes cartridge forward until bullet hits the lands or more likely case shoulder reaches far end of the chamber, leaving a small gap betwen the case-head and bolt-face surfaces.
    primer explosion causes the primer to back out the pocket until stopped by the bolt-face.
    main charge ignition and charge burn takes place, rear end of case moves back to bolt by stretching the case-body. The primer is partly unsupported at the beginning of this process and flattens.
    as the case body stretches, the primer is forced back into the pocket but usually doesn't go in fully as the unsupported back end is now too big due to flattening.

    If it's a very mild load, or with some case and chamber tapers, the case will not stretch back to the bolt-face and the primer will be left sticking out seriously proud of the case-head.
    Last edited by Laurie; 20-06-2011 at 14:05.

  7. #7
    What happens to the hook around the rim when the cartridge is chambered?
    I have a 270 which has quite a wide extractor (approx a third of the circumference of the base) that I would expect pulls the cartridge into the bolt.

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