Primer pocket ideal seating depth?

[MTGunner]

I am sure this has been discussed so many times that it forces some to roll their eyes back in their heads. But, I have to ask about two particular cartridges. I am loading Hornady brass for my 7MM Remington Magnum. Some of the pockets don’t seat the WLRM primers below the case rim. It becomes difficult to measure OAL. My groups with 140 BT range from 1.00-.750 inch. Not bad but would like more consistency.
Second cartridge is my 260AI using Lapua brass and WLR primers. Some seat nicely, others not so.
BTW, I use an RCBS primer pocket cutter. I clean the pockets after each firing.
Therefore, I am asking this knowledgeable forum their thoughts and opinions. Thanks in advance for any and all input. MTG

 

[straightshooter1]

I am sure this has been discussed so many times that it forces some to roll their eyes back in their heads. But, I have to ask about two particular cartridges. I am loading Hornady brass for my 7MM Remington Magnum. Some of the pockets don’t seat the WLRM primers below the case rim. It becomes difficult to measure OAL. My groups with 140 BT range from 1.00-.750 inch. Not bad but would like more consistency.
Second cartridge is my 260AI using Lapua brass and WLR primers. Some seat nicely, others not so.
BTW, I use an RCBS primer pocket cutter. I clean the pockets after each firing.
Therefore, I am asking this knowledgeable forum their thoughts and opinions. Thanks in advance for any and all input. MTG

I also use an RCBS primer pocket cutter to uniform and clean my primer pockets. Typically, I'll have .003 of recess from the base of the case. To get that recess my primer pockets are uniformed to .124" deep. And I use a 20th Century hand primer seater.

What tool are you using to seat your primers?

 

[RegionRat]

In addition to what @straightshooter1 is asking...
What was your initial primer pocket depth stat?
What was your after-cutting primer pocket depth stat?

Here is what they should be...

 

[straightshooter1]

In addition to RegionRat's chart, be aware of different primer dimensions between different brands . . . like these:

 

[BJames]

Jesus, you guys are over thinking it. Do your primers protrude above the case rim? Yes, uniform pockets - or seat deeper, my personal fav. No, who gives a shit.

 

[memilanuk]

@straightshooter1 do you know what the (data) source of that chart is? I'm curious because my BR4s are way taller than that

 

[straightshooter1]

@straightshooter1 do you know what the (data) source of that chart is? I'm curious because my BR4s are way taller than that

It came from Speer/CCI Technical Service. I copied the pic from the Accurate Shooters forum.

Is your measurement including the protrusion of the anvil?

 

[memilanuk]

Yes it is. I wondered if that was the pertinent detail there: cup height vs overall primer height. Still, it's unfortunate they didn't include an SD for those values.

A recent example of what I'm looking at: Lapua 223 Match brass. Pockets uniformed with a fixed carbide tool (K&M, I think). Pocket depth is consistent @ 0.1215". A random sample of BR4 primers (ie the first three I pulled from the box) measure 0.122" for all of them. Using a CPS to seat primers to - 0.002 below flush, or ~0.002" crush (as I understand the concept).

I'm getting more variation in primer seating depth (using Accuracy 1st tool) than the numbers above would indicate. Either there's more variation in the primer heights than my ad-hoc sample showed, or the rim thickness is throwing a wrench in the works.

 

[RegionRat]

While the pocket dimensions have a reasonable tolerance range, the specs tend to add to the confusion when the range on the primer height shows values that can exceed the pocket depth.

What the SAAMI spec is missing, is an explanation that the overall height of different primers vary due to the design of the cups and anvils. Primers are designed with differences that all assume some amount of compression in the height upon installation.

Some models are designed such that the initial anvil height sits high in the cup, and others can start partially telescoped into the cup. What isn't always visible, is how the cup and anvil will sit in relation to each other when the primer is installed such that the cup is at least 0.003" below flush in the pocket.

While the SAAMI note mentions a minimum installation of "flush" to 0.008" below, I would always recommend a minimum of 0.003" below flush as a nominal goal to have the allowance for variations that might mean a sample may result in less due to dispersion from friction, defects, or dimensions.

I'm getting more variation in primer seating depth (using Accuracy 1st tool) than the numbers above would indicate. Either there's more variation in the primer heights than my ad-hoc sample showed, or the rim thickness is throwing a wrench in the works.

I'm not sure how much variation you are finding in the assembled dimensions, but there are always some small variations in the rim thickness. However, the rim thickness variations should be less than 0.002" and are often way less than that, which should prevent most "fixed" primer seat tooling from creating a variation of more than 0.001".

Sometimes, folks seat by feel and sometimes their tools are set to a "fixed" dimension. If you are finding large variations in the seating depth with a fixed tool, check the rims for damage. Often a semi-auto can be hard on brass to the point of bending rims. If you are seating by feel and seeing large variations, best to inspect with a tool like the Accuracy and determine if your "feel" is "calibrated" and that the primer pocket depths are uniform.