Primer depth for accuracy

[Ron Blain]

I went through this about 25 years ago and I just seat till the primer bottoms out and I hand prime and can tell by feel when a primer is bottomed out in the primer pocket. Below is an image of some CCI 200 large rifle primers. Looking close we can see the anvil sits just above the primer cup, all primers I have looked at up close and personal under magnification look this way.

Seating till I feel the primer bottom out normally results in about a 0.003" gap between primer and case head with the primer below case head. Figure the primer pocket has a depth and the primer cup has a height and both dimensions have a tolerance. I never noticed any difference in my group sizes dependent on primer seating depth.

However, it never hurts for anyone to see how it plays out for themselves. I also never saw a difference over the chronograph to speak of.

Just My Take
Ron

 

[gunsandgunsmithing]

At this point I don't really know what depth is totally bottomed out. I am assuming that since it was at the extreme end of my seating depths - it was close. Flush was obviously not fully bottomed out although there were no failures to fire. Clearly more to investigate but I felt that by initially looking at the extreme ends I could get an idea if there was something worth looking into in more detail.

FWIW and to put that in perspective...a human hair is about .004"

 

[mikecr]

So you size the brass more than is needed then expand back up. Isn't this contrary to your goal. The neck wants to go back to it's fired dimension after sizing. Over sizing and then expanding doesn't change that. Does it establish initial neck tension? Yes It doesn't change the way the brass wants to travel which is back to its fired state. As always YMMV

No, brass counters LAST sizing action.
Spring back from inward sizing is outward, spring back from outward sizing is inward.
Firing is just upsizing. It's the first sizing in a reloading cycle with your best die (your chamber).

 

[NorCalMikie]

I went through this about 25 years ago and I just seat till the primer bottoms out and I hand prime and can tell by feel when a primer is bottomed out in the primer pocket

Can't get much easier than that. Primers seated NOT all the way to the bottom of the pocket will cause you nothing but problems.
First FTF cause the primer got pushed to the bottom of the pocket well lead you off in another direction. Firing pin bad, bolt head needs to bushed, brass all junk, etc.
Primer seated all the way to the bottom of the pocket will bypass all those problems.
KEEP IT SIMPLE and don't make more work for yourself.

 

[Webster]

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.

Just checked Tony Boyers book. I cannot even find a mention of primers. He must have messed up. I'm sure he has a few opinions on primers.

 

[Dave M.]

Just a little reality check. A friend owns several world records in short range group. He never brushes the insides of his case necks, and only cleans primer pockets when he happens to think about it. He seats primers by feel with a hand tool. There is no primer depth consistency trophy. We tend to obsess over things that we can measure, whether they matter or not. How many of you own and regularly use wind flags?

I never clean the inside of my necks, but annealing after each firing burns some of it off. I uniform primer pockets on all new brass and then never clean them again. I do make sure the hole is free and clear of all obstructions. I consider myself to be a pretty good shooter and I have NEVER measured a primer pocket depth or primer in my entire life, and I never plan to. For F class open shooting I feel this is a complete waste of time. I’ve shot many clean relays, and literally dozens of 197+ relays and my missed points were on me, not primers or primer pocket depth.
Dave

 

[Dave M.]

Just checked Tony Boyers book. I cannot even find a mention of primers. He must have messed up. I'm sure he has a few opinions on primers.

Exactly….and we know he couldn’t shoot for shit right? Lol. One of the best shooters ever and not a mention of primer pocket depth in his book

 

[JimSC]

I am far from a expert but here is my take on this. I measure the thickness of 5 primers, generally I find they are close, mostly around plus or minus .001. Lets say they are LRP' measuring .130 in thickness. I measure the depth on the primer cups and for the sake of this discussion they are .135 in depth. That tells me to touch the bottom of the cup seating depth should be .005 below flush. I use a Holland perfect primer seater so I set the seating depth to .006 below flush. That is about as simple as it gets. Whether it makes a difference, darned if I know. But it only takes a minute or two so why not

 

[mikecr]

Simple is not what this thread is about or seeking. There is no rocket science conquered in getting primers to fire. The question is about primer preload to results.

Boyer shot 6PPC with competitive loads. You should understand that this is a tiny cartridge with competitive load pressures of 75Kpsi+. The function of that is not for higher velocities, but to be deep in diminished pressure returns. Diminished returns equals diminished variance of returns.
So in that case normal variances in powder weight, or neck tension, or primer firing, or case capacities, etc, are well overridden by at least 10kPsi of extra pressure peak.
That's also limited to point blank ranges, and with accepted case life of 3-4 shots(with loose pockets after the first).

For LRH, we use far larger hunting capacity cartridges. No 6PPC potentials are viable for us.
So all the little variances actually matter here, especially at long range.
I suggest that primer preload and striking are part of our variances.
I've proved it to myself, and you could do so as well.

 

[garandman]

Based on this, I can be a world record holder too.

 

[BoydAllen]

This evening while I was in my reloading room working on seating depth settings for my 6BRA, I surveyed the primer depth from a set of 40 of those cases that are ready for powder and bullets. I pulled and measured a half dozen from all over the set, and they all measured from .0085 to .009, this with Federal 205s in unmolested Lapua 6BR primer pockets, seated with my usual force, by feel, using a top quality hand priming tool set so the handle does not touch the tool body.

 

[Skeptic123]

This evening while I was in my reloading room working on seating depth settings for my 6BRA, I surveyed the primer depth from a set of 40 of those cases that are ready for powder and bullets. I pulled and measured a half dozen from all over the set, and they all measured from .0085 to .009, this with Federal 205s in unmolested Lapua 6BR primer pockets, seated with my usual force, by feel, using a top quality hand priming tool set so the handle does not touch the tool body.

That's an awesome level of precision. How did you measure the seating depth?

 

[JimSC]

I have no idea on how Boyd does his measurements but I use a Starrett 25-141 dial micrometer with the contact point removed. The spindle fits perfectly into a small primer pocket and the stem is flat on the bottom. It is incremented in .001 increments but if the pointer is between marks you can assume that is .0005. They can be found on EBay for less than $50 used. The one I am using is easily 40 years old and functions flawlessly

The Holland Perfect Primer Seater is reasonably priced also, and is easily adjusted. A complete unit is $280, if you already own a RCBS bench unit $150

Here is a article on the US Armys Marksmanship Unit opinion on primer depth

Primer Seating Depth Uniformity and Accuracy « Daily Bulletin

USAMU shows how to measure primer seating depth for precision reloading. High primers can cause feeding problems or dangerous slam fires.

bulletin.accurateshooter.com

 

[Webster]

Exactly….and we know he couldn’t shoot for shit right? Lol. One of the best shooters ever and not a mention of primer pocket depth in his book

I talked to Tony Boyer about 5 years ago. He used the term internet shooter.

 

[Ccrider]

I talked to Tony Boyer about 5 years ago. He used the term internet shooter.

Easy now. You gone hurt my feelins.

 

[mikecr]

I pulled and measured a half dozen from all over the set, and they all measured from .0085 to .009, this with Federal 205s in unmolested Lapua 6BR primer pockets,

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

 

[RegionRat]

 

[BoydAllen]

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

Depth of seated primer below case head. Sorry for not being more clear.

 

[RegionRat]

Ever notice that the drawings for many famous cartridge designs include some funny dimensions between the cartridge and the chamber? Well, primers are another place where the committee doesn't explain themselves very well either....

Small Primer dimensions run beyond what would work if you take a close look at certain combinations.
On the diameter, the MMC (Maximum Material Condition) fits would be an interference of 0.0035" but at the (LMC) other extreme we are looking at line-to-line.

A look at small primer depths shows at MMC we have 0.117" - 0.126" = -0.009" and at LMC we have
0.123" - 0.115" = 0.008"

They didn't stop to explain the primer cup height dimensions at all within that drawing space, nor did they explain why the diameters can go line-to-line. There really should be some words with that specification that sheds light into why the primer could theoretically stick up by 9 mils.

This is what happens when committees are dominated by producers rather than consumers.

 

[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.