Primer depth for accuracy

[mikecr]

Ok a primer height of .115 seated .008 below case head into a pocket depth of .123 has 0 preload(crush).
A primer height of .126(still in spec) seated the same .008 below case head into a pocket depth of .117(still in spec) would be 17thou of preload.
So 8thou below case head by itself could mean anything from 0 to 17thou of preload!

IMO, high consistency to cause such a huge variance in preload is nothing to moonwalk about.
Anything bottomed will fire, but it could be questioned whether results are better with set depth below case head, or set primer sensitizing.

I know preloading is important, as I've tested and witnessed different preloading affect results.
I've also tested pin travel, and tuned it with a set preload, to see that affect results.
To do that I had set same crush(prior tested best for primer) with same primer heights/same pocket depths/same seating below case head. The testing would not have worked with any preload spread(much less 17thou potential).

For the striking testing I adjusted released pin protrusion from boltface.
My pin had slipped in it's cocking piece, with no markings, and so I had no idea where the correct setting should be. That's what led me to this.
I made 20thou jumps at the range with 3sht grps each, and then locked into center of a window with 5thou adjustments. All primers fired but grouping went open-close-open, and like bullet seating testing, it was huge to results.
Ended up this tragic situation took me to results better than I ever could have reached with load development alone.

With this I say, if not already, there is a lot more to primer seating and striking than merely bottoming and reliable firing. I also suspect that there are scientific minds on this board who could define attributes of primer striking, and possibly develop standard testing of it (separate or summation).
And maybe it could lead us to another level.

 

[Skeptic123]

Ok a primer height of .115 seated .008 below case head into a pocket depth of .123 has 0 preload(crush).
A primer height of .126(still in spec) seated the same .008 below case head into a pocket depth of .117(still in spec) would be 17thou of preload.
So 8thou below case head by itself could mean anything from 0 to 17thou of preload!

IMO, high consistency to cause such a huge variance in preload is nothing to moonwalk about.
Anything bottomed will fire, but it could be questioned whether results are better with set depth below case head, or set primer sensitizing.

I know preloading is important, as I've tested and witnessed different preloading affect results.
I've also tested pin travel, and tuned it with a set preload, to see that affect results.
To do that I had set same crush(prior tested best for primer) with same primer heights/same pocket depths/same seating below case head. The testing would not have worked with any preload spread(much less 17thou potential).

For the striking testing I adjusted released pin protrusion from boltface.
My pin had slipped in it's cocking piece, with no markings, and so I had no idea where the correct setting should be. That's what led me to this.
I made 20thou jumps at the range with 3sht grps each, and then locked into center of a window with 5thou adjustments. All primers fired but grouping went open-close-open, and like bullet seating testing, it was huge to results.
Ended up this tragic situation took me to results better than I ever could have reached with load development alone.

With this I say, if not already, there is a lot more to primer seating and striking than merely bottoming and reliable firing. I also suspect that there are scientific minds on this board who could define attributes of primer striking, and possibly develop standard testing of it (separate or summation).
And maybe it could lead us to another level.

Good info. How much change in preload was required to show a measurable difference in group size? Also here are some actual numbers to go with your thinking. Latest batch of Lapua Blue Box 6br brass has consistent .121 primer pocket depth. My last batch of CCI 450 primers coincidentally are .121 tall. When I seat them with .005 preload or crush I get my best group results. I don't know if this is firing pin strike related or an ignition factor. Also I find that no matter how hard I try to feel what seating depth I'm creating, the actual measured results are quite varied. I'll go back after measuring and reseat the high ones and that gets them all (with minor exceptions) to roughly the same crush. Perhaps I just need a better seater.

 

[INTJ]

If you want learn about this (see it for yourself), get an indicated K&M and test crush for one primer -vs- another. It's the only way you'll come to know the following:
1. Every primer going bang is just the beginning
2. You cannot consistently seat primers by feel
3. You cannot set primer crush without measuring primer crush, which is accounting for primer height to pocket depth -at the same time.
4. Different primers actually have different optimum preloads, with much depending on your striking sys.

This ^^^^^

I used to seat primers then sort them using a primer depth gauge. I would then sort them
into the various seating depths and try to correct the shallower ones.

Then I stated using the K&M primer gauge. This tool measures each pocket depth and each primer height, and then has a gauge the shows you the exact amount of crush.

Using this method reduced the ES in my best 6 BRA LR BR loads from low teens to single digits.

Using the K&M tool shows you that there is a lot of variance in primer cup height and maybe our uniformed primer pockets aren't as uniform as we think. Also, "feel" when seating primers isn't 100% reliable either.

Now does a 5 FPS improvement in ES translate to smaller groups at 1000 yds? Maybe. I certainly hope that my tuning node is much wider than 5 FPS. That said, if I have a 20 fps node, a 5 FPS reduction in ES gives more room for those other things that cause velocity to vary.

Now since using the K&M gauge I have shot smaller groups. But that isn't the only ingredient in the cake........

Regardless if all that, I now think that using a primer depth gauge is a pointless exercise. For most shooting disciplines, as long as the primer is seated with some crush and is below the base of the case, everything is good. For LR BR and ELR, seating to an exact crush may help with smaller capacity cases.

 

[BoydAllen]

Ok a primer height of .115 seated .008 below case head into a pocket depth of .123 has 0 preload(crush).
A primer height of .126(still in spec) seated the same .008 below case head into a pocket depth of .117(still in spec) would be 17thou of preload.
So 8thou below case head by itself could mean anything from 0 to 17thou of preload!

IMO, high consistency to cause such a huge variance in preload is nothing to moonwalk about.
Anything bottomed will fire, but it could be questioned whether results are better with set depth below case head, or set primer sensitizing.

I know preloading is important, as I've tested and witnessed different preloading affect results.
I've also tested pin travel, and tuned it with a set preload, to see that affect results.
To do that I had set same crush(prior tested best for primer) with same primer heights/same pocket depths/same seating below case head. The testing would not have worked with any preload spread(much less 17thou potential).

For the striking testing I adjusted released pin protrusion from boltface.
My pin had slipped in it's cocking piece, with no markings, and so I had no idea where the correct setting should be. That's what led me to this.
I made 20thou jumps at the range with 3sht grps each, and then locked into center of a window with 5thou adjustments. All primers fired but grouping went open-close-open, and like bullet seating testing, it was huge to results.
Ended up this tragic situation took me to results better than I ever could have reached with load development alone.

With this I say, if not already, there is a lot more to primer seating and striking than merely bottoming and reliable firing. I also suspect that there are scientific minds on this board who could define attributes of primer striking, and possibly develop standard testing of it (separate or summation).
And maybe it could lead us to another level.

I think you have to qualify who you pay attention to. The guys that I pay attention to (short range group) hold records, and are members of the hall of fame, or consistently do well in matches. As far as I am aware none of them seat by depth. They seat by feel with hand tools. The numbers that I put up were seating by feel. All of the cases in the set were from the same lot, and all of the primers from the same box. There is some preload on them, medium not heavy. Personally, I am satisfied with a total variance of half a thousandth. IMO some really enjoy making simple things more complicated. For me the complicated things are staying in tune through ambient changes, and reading wind flags when there is no dominant pattern, they point in different directions, and switch often.

 

[cwebster]

I'm trying to find anyone's actual testing experience regarding primer seating depth. With the Accuracy One primer seating gauge and the Primal rights seating tool, among others, it is possible to mostly control or at least ascertain what each rounds seating depth is. But.....does it really make any difference in accuracy. Does .001 more or less crush translate to more accurate loads? Does it affect ES or SD? Is it quantifiable? Thx.

Seat till it bottoms out. Worry about something more important......

 

[Dave M.]

What did your numbers represent?
Small primer heights= .115-.125
Small pocket depths= .117-.123

That would be the depth of the pocket itself without the primer in it.

 

[X-Caveman]

Are there any primer seating or measuring tools that use the functional datums - the case shoulder & the bottom of the primer pocket? The seating tools I know of use the front face of the rim in the extractor groove as the datum (meaningless in practice). Likewise, you can measure from case head to cup of a seated primer, but that isn't what's important when the firing pin falls (unless everything else is PERFECTY consistent and the chamber remains immaculately clean). It seems there's a mismatch between how we seat them, what we can measure, and what really matters during ignition.

Feeling a little cam over / crush with my hand tool seems pretty reasonable given all of the uncontrolled & immeasurable variables.

Of course this is only my opinion, and I'm a nobody.

 

[Mulligan]

Are there any primer seating or measuring tools that use the functional datums - the case shoulder & the bottom of the primer pocket? The seating tools I know of use the front face of the rim in the extractor groove as the datum (meaningless in practice). Likewise, you can measure from case head to cup of a seated primer, but that isn't what's important when the firing pin falls (unless everything else is PERFECTY consistent and the chamber remains immaculately clean). It seems there's a mismatch between how we seat them, what we can measure, and what really matters during ignition.

Feeling a little cam over / crush with my hand tool seems pretty reasonable given all of the uncontrolled & immeasurable variables.

Of course this is only my opinion, and I'm a nobody.

The K&M priming tool index’s off the bottom of the primer pocket
CW

 

[Straightshooter1]

Are there any primer seating or measuring tools that use the functional datums - the case shoulder & the bottom of the primer pocket? The seating tools I know of use the front face of the rim in the extractor groove as the datum (meaningless in practice). Likewise, you can measure from case head to cup of a seated primer, but that isn't what's important when the firing pin falls (unless everything else is PERFECTY consistent and the chamber remains immaculately clean). It seems there's a mismatch between how we seat them, what we can measure, and what really matters during ignition.

Feeling a little cam over / crush with my hand tool seems pretty reasonable given all of the uncontrolled & immeasurable variables.

Of course this is only my opinion, and I'm a nobody.

The Accuracy One tool works great for me; fast and accurate measuring, not only for the primer pocket depths but also for how far below the face of the case head the primer has been seated.



PS: Those two pieces you see in the middle are used to zero the indicator for various calibers and the one below used for the SRP's

 

[mikecr]

Yeah, don't zero on machined pieces.
Instead, zero on actual pocket depth and actual primer height for the pocket and primer you're seating.

 

[INTJ]

I don't know why I keep bringing this tool up. If I would keep quiet about it, it would be to my advantage. This is THE tool that measures pocket depth and cup height of each primer and case you are seating. Then it has a gauge to show you the exact level of crush. No other method is as precise for establishing a consistent primer crush.

 

[BoydAllen]

Rereading this thread this evening, I have to laugh. I tell you that a bunch of guys who probably shoot better than anyone on this thread, seat primers with hand tools by feel, and immediately get asked about numbers. When I seat a primer I do not approach it with fear and trepidation. I squeeze the handle with moderate firmness and after I feel the primer bottom, put a little more pressure on it. That's it. You fellows need to find something else to obsess about. Learn how to tune on the fly at a match and read wind flags, those are the skills that win matches. If you do any shooting at 200 and under and don't own a set of flags how would you know what is causing your groups to be lager or smaller?

 

[jimmymac]

Hey @BoydAllen

Move along pal. You are needed over on aisle 3 in the weighing to one granule of powder makes a difference thread.

 

[Webster]

Hey @BoydAllen

Move along pal. You are needed over on aisle 3 in the weighing to one granule of powder makes a difference thread.

Need a clean up on aisle 3.

 

[RDavies]

Rereading this thread this evening, I have to laugh. I tell you that a bunch of guys who probably shoot better than anyone on this thread, seat primers with hand tools by feel, and immediately get asked about numbers. When I seat a primer I do not approach it with fear and trepidation. I squeeze the handle with moderate firmness and after I feel the primer bottom, put a little more pressure on it. That's it. You fellows need to find something else to obsess about. Learn how to tune on the fly at a match and read wind flags, those are the skills that win matches. If you do any shooting at 200 and under and don't own a set of flags how would you know what is causing your groups to be lager or smaller?

What distance do you shoot at? Ive never shot at 200.

 

[BoydAllen]

What distance do you shoot at? Ive never shot at 200.

Typically at 100, sometimes at 200. When I shot competition it was at those distances for group. If you go back in this thread you will see where it measured some random primer seating depths in a set of primed brass and came up with a total variance of .0005. The primers were seated by feel.

 

[gunsandgunsmithing]

@gunsandgunsmithing
Mike the free bore discussion is on a different thread. Smiles

Oops! Not sure how that even happened.

 

[Mulligan]

logical question, the only thing to add would be this if you ponder tuning or rifle performance improvements based off a "personal thought" just take the time to test it regardless of what others have found often what gets me further is not at all what others are doing. This question of primer depth, compression, or pre loading anvil to a equal position or deeper crush depth is know different see it through let your rifle and you decide not your shooting community.. In closing I suggest tinkering with this idea at no closer than 400 yards further would be even better happy tuning. your looking for clean grouping with lack of spitters 100% of shots not 85% of your group, not sub small grouping but clean grouping. good luck

Shawn Williams

A lot of wisdom in this post.
Thanks Shawn.
CW