You certainly don’t want primers sticking above the case head. CCI states that high primers are the most common cause of mis fires. For a primer to reliably ignite, the anvil have to be rested on a solid surface, and the gap between cup and anvil peak has to be properly set. You set that gap by setting the anvil in the pocket and pushing the primer cup (and thus primer cake) towards the anvil peak. Too much gap and the primer is too insensitive to ignite, too little gap, you are likely to crack the primer cake and have a misfire.
However, given a shallow pocket, or if you stuck a spacer in the bottom of your primer pocket, then you have the conditions for a high primer and a solidly rested anvil.
It is my recollection that Frankford Arsenal set their primer seating depth to 0.0 to -0.005”. I want a little more depth to the primer, just in case something is on the bolt face, so something around -0.002” to - 0.005” is better.
Even though I think I am rationale, it turns out, I am like all the other irrational humans on this world, easily swayed by emotion and stupid thoughts. Decades after having two out of battery slamfires with Garands, which I know were caused only by sensitive primers, still, I thought that maybe reaming pockets deeper was a good safety measure. This is foolish as primers only have to be below the case head and the best reloading preventative measure is to use the least sensitive primers in Garand type mechanisms, which I do, and these primers are CCI #34’s or Tula7.62 mil spec primers.
So, after reaming these pockets extra deep, I took that ammunition to a 1000 yard match and shot it in my match M70. I got lots of misfires: click and no bang. I had at least 20 misfires, and the ones that went off, well I am going to blame my poor groups on poor ignition. Much easier to blame the ammunition than a lack of shooting skills and wind reading ability, which of course, are faults I won’t admit having.
So, here is one of those cases that did not go bang when hit by the firing pin of my M70. Looks like a good hit. But it was not, the primer was too deep, and it was already a rather insensitive primer to begin with.
After the match I decided to measure the depth of the primers below the case head.
Primer seating depth on CCI #34 primers in LC79 308 cases that misfired in M70
0.004
0.004
0.004
0.005
0.005
0.005
0.006
0.006
0.006
0.008
0.008
0.008
0.009
0.009
0.009
0.009
0.01
0.011
0.012
avg depth 0.007
N = 19
Next local 100 yard reduced match, I decided to shoot that ammunition in my M1a. The basic Garand mechanism has a very powerful ignition system and plenty of firing pin protrusion. The picture of ammunition in this box, the tall cases are some of the pre struck M70 ammunition, the short cases are pre struck M70 cases that went bang in the M1a chamber. At least I did not have to pull the cases down, and that M1a had more than enough energy to make these pre dimpled primers ignite.
While I still ream pockets, I no longer ream to the crazy depths that I used to.
I recently sent my Garand bolt to Roland Beaver and he installed a firing pin spring that will positively retard firing pin rebound. This, combined with my mil spec primers, should provide the most protection against slamfires, in this mechanism, caused by firing pin kinetic energy.
Roland charged me $25.00 total, I shipped it on a Monday, received the bolt back next Monday.
This has not eliminated firing pin indentations in my Garand. As a worse case test, I placed these rounds in the chamber, pointed the muzzle at the ground, and tripped the bolt release. Even with a firing pin spring, these primers were dented.
Keep cleaning as you are and you will be fine.
