Why is primer seating important?

[Plowboy85]

So can someone explain to me why primer seating and feel of seating is so important. I have heard it mentioned over the years but read no detail explanation. I do uniform flash holes and primer pockets with K&M tools. I use an RCBS hand tool and look at the seated primer afterwards to ensure what looks consistent to me.

 

[rsmithsr]

if not seated to the bottom of the pocket, the first thing the firing pin does , is move it to the bottom of the pocket. then set it off, less energy, less consistency in ignition. its the small details than make great ammo, almost anyone can make good ammo.

 

[RT2506]

I guess by feel we mean making sure that the primer has bottomed out in the primer cup and the anvil in the primer has compressed the pellet properly in the primer so it will fire. A good example of actually "feeling" the primer seat properly over just looking like the primer has seated OK by looking at the outside is back when the Wolf primers came onto the market. At first many people were talking trash about them being no good because they misfired a bunch. I also had some of them misfire but after removing them from the case and hitting them with a hammer they went bang. Turned out that the way these primers were made was that the cup was just a bit larger and harder and the anvil was set in them higher off the pellet. When seating them with my Lee hand primer where I could feel what was taking place I could feel what I had thought was the primer bottom out in the priming cup of the case. This was only the anvil hitting bottom. If I put a little more pressure on the leaver I could then feel the primer cup move only slightly and really come up solid and this allowed the anvil to compress the priming pellet properly. Once I found this I never had a single misfire again. I still have some of these primers set back for serious shooting situations because they made up some of the most accurate loads I have ever found. Wish we could get more.

 

[Plowboy85]

Thank y'all for the response, I certainly but a decent squeeze on the tool and feel the stop when it bottoms out. Making sure it was fully seated was taught to me early on but was wondering if there was more to it than that.

 

[Mikecr]

There is more to it than leaving short, or bottoming in contrast. There is more to it than anybody can 'feel' (even with the $$$ tools).
Primer manufacturers recommend 2thou crush. That is touch bottoming + 0.002" further. I have found over the years that Feds/Wins/Rems like 2thou, but CCIs like 5thou crush. CCIs do not provide the same grouping results for me at 2thou crush.
There is another issue in this, in that primers back out over time. Before I go hunting with ammo made 6mos earlier, I re-seat them 1thou, and so far this has worked well.

You can't feel this kind of precision, and anyone who measures it learns this much for sure. But seating to bottom by feel does at least provide reliable function, and I'm pretty sure most people assume that primers fire the same if they fire at all..
That is not true.

You may have noticed while reviewing attributes of primers, that it's common to wholesale swap them, searching for best results. Also, that there are sometimes lot-to-lot differences causing setback in tune.
This is not one primer actually being better than another due to case capacity or powder. The test for that fails while we end up using different primers for the same cartridges and powders.
What passes tests here is different (possibly unique) striking from each shooting system. It's my contention, because I've seen it, that you can pick any primer and tune the striking for optimum results -from that primer.

Primer striking is difficult science. I'm aware of very little information out there about it, and zero standards. But for sure, primer seating affects the outcome, regardless of your striking.
I have a Taurus pistol in 38spec that happens to be very very accurate. Unfortunately it is just as unreliable as accurate. It has a headspace issue causing ~99% of ammo available to misfire. This, even with full indentation of misfire primers.
The difference in measure between reliable firing and failed is 3thou rim thickness!
And get this: a 2thou thicker rim(1thou under confirmed reliable) will fire nearly every one, but these shots are thrown from grouping..

Headspace and striking are important, and if you care to measure seating to crush there is only one tool to do so: the Indicated K&M seater.
It's also a pretty good seater, measurement aside.
I also uniform primer pocket depths to help with consistent striking. Sinclair offers a really good tool for that.

 

[cape cove]

Could you eloborate on primers backing out in time, reasons etc. Tks

 

[Mikecr]

When you size metal, you're adding dynamic energy to it.
It's no longer at lowest energy balance, as everything in the cosmos seeks.
Our brass alloys spring back from sizing, relaxing most of this right off the bat, but not all of it. There is a bit more energy that releases over time, as seen by either continued free dimension changes, or tension force changes.

Over time, necks change dimension or tension, case shoulders loose bump, sized case bodies change, and so do primer pockets & cups. All of this opposite of last action (energy applied). Primers are put into an angled tension with pockets, and creep outward.

 

[birdiemc]

When you size metal, you're adding dynamic energy to it.
It's no longer at lowest energy balance, as everything in the cosmos seeks.
Our brass alloys spring back from sizing, relaxing most of this right off the bat, but not all of it. There is a bit more energy that releases over time, as seen by either continued free dimension changes, or tension force changes.

Over time, necks change dimension or tension, case shoulders loose bump, sized case bodies change, and so do primer pockets & cups. All of this opposite of last action (energy applied). Primers are put into an angled tension with pockets, and creep outward.

Taking things to a whole new level...now I gotta consider the kinetic energy the metal in an assembled round...what you say does make sense though on micro level.

 

[Mikecr]

Well, if you consider neck tension, you're dealing with this. Same with shoulder bumping.