Measuring Primer Seating Depth

EricBu

EricBu

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Apr 2, 2021
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A customer brought me a partial box of 45 Colt high performance defensive boutique ammo and a Taurus revolver a few weeks ago. His issue was light striking primers, and he was concerned it was the gun, but wanted to have it checked before he bought more of this particular ammo, or sent his revolver back to Taurus. So I broke down a few rounds to see what they were about. It was apparent that the company was using Ginex primers after I decapped a couple of pieces I'd broken down. So I immediately busted out my calipers and checked the seating depth...because metric primers are often not seated deep enough. I thought, you know....this would be a good time to take some pictures in case there are folks out there who don't know how to measure primer depth. Slipped my mind until this morning, so here it is, better late than never. Of course, this is just one method, there's lots of other ways, some much more accurate, some less so. But almost all reloaders and handloaders have a dial caliper, and almost all dial calipers have a depth gauge...so this is likely a method that virtually everybody can use right away without additional expense.

Now, we know there are dozens of folks out there groaning...."you don't measure, you seat until they bottom out"....well, sadly those folks are wrong. Why? Because a metric primer "feels" bottomed out, when it isn't. "well, it just needs to be flush"....no, sorry, again....wrong. Depending on which reloading manual you own, the specification for primer depth is from .003 to .008. Myself, after 35 years of doing this, have determined that .003 is the absolute minimum depth to function reliably 100% of the time in all properly functioning firearms, but that's MY OPINION....most books seem to lean towards .004 to .005. Edited to add, the other reason to make a habit of QC'ng your ammo and checking primer depth...is when you get a piece of brass that's getting long in the tooth....you'll catch it here as well...you may see your primer seated .010 or even deeper...because the pocket has streatched or malformed. This is just as bad as not seating deep enough.

So back to the story...

This particular boutique ammo used Starline brass. New Starline brass can have relatively tight pockets, leading me even more to the conclusion the problem was seating depth. So I took a couple of pieces of brass I'd broken down and checked them out, then ultimately measured all of the rounds in the partial box. What I found:

Out of 10 rounds, 2 were .001 proud (above flush), 6 were .000, and 2 were .001 deep. None were within the specification above. How did we measure this?

1. The butt of your dial caliber has what's called a depth gauge. This is a slender metal rod that extends out the base of your caliper at the same measurement as the gap in your calipers. By butting it up against a primed piece of brass, you can extend that depth gauge, and get a reading how deep it is below the base of the caliper.

depth-gauge.jpg


2. I like to push the mouth of the piece of brass up against a flat surface, I loosen the friction lock on the caliper, extend the depth gauge, line it up with the center of the primer, and push the caliper forward and take the reading.


close-up.jpg

3. I usually rotate the brass and take the measurement several times. The overall all measuring process looks like the last pic below. You'll note that the measurement is .001, not deep enough. You want a reading of .003 minimum, deeper even depending on your gun. Only once you've measured primers and verified the depth, can you start looking at other things...a primer with too hard of a cup for your application, a gun with a problem, or a bad batch of primers. Relying on feel....then everything else is just a guess, unsubstantiated claim, or excuse.

measuring.jpg

Hopefully this helps somebody out. I did this because I was working on the related task above and it was timely, and I don't know that I've ever seen anybody put pictures up on how to use the depth gauge on THR before.

So what happened with my customer? I called the ammo company, talked to one of their folks. They'd actually had another report from the same lot, and knew of the issue. I gave them my customers address and they shipped him two new boxes of ammo, and they functioned perfectly in his gun, and he was super happy, so I folded up my cape, put it away until next time...and had a celebratory ice cold adult beverage.
 
EricBu said:
A customer brought me a partial box of 45 Colt high performance defensive boutique ammo and a Taurus revolver a few weeks ago. His issue was light striking primers, and he was concerned it was the gun, but wanted to have it checked before he bought more of this particular ammo, or sent his revolver back to Taurus. So I broke down a few rounds to see what they were about. It was apparent that the company was using Ginex primers after I decapped a couple of pieces I'd broken down. So I immediately busted out my calipers and checked the seating depth...because metric primers are often not seated deep enough. I thought, you know....this would be a good time to take some pictures in case there are folks out there who don't know how to measure primer depth. Slipped my mind until this morning, so here it is, better late than never. Of course, this is just one method, there's lots of other ways, some much more accurate, some less so. But almost all reloaders and handloaders have a dial caliper, and almost all dial calipers have a depth gauge...so this is likely a method that virtually everybody can use right away without additional expense.

Now, we know there are dozens of folks out there groaning...."you don't measure, you seat until they bottom out"....well, sadly those folks are wrong. Why? Because a metric primer "feels" bottomed out, when it isn't. "well, it just needs to be flush"....no, sorry, again....wrong. Depending on which reloading manual you own, the specification for primer depth is from .003 to .008. Myself, after 35 years of doing this, have determined that .003 is the absolute minimum depth to function reliably 100% of the time in all properly functioning firearms, but that's MY OPINION....most books seem to lean towards .004 to .005. Edited to add, the other reason to make a habit of QC'ng your ammo and checking primer depth...is when you get a piece of brass that's getting long in the tooth....you'll catch it here as well...you may see your primer seated .010 or even deeper...because the pocket has streatched or malformed. This is just as bad as not seating deep enough.

So back to the story...

This particular boutique ammo used Starline brass. New Starline brass can have relatively tight pockets, leading me even more to the conclusion the problem was seating depth. So I took a couple of pieces of brass I'd broken down and checked them out, then ultimately measured all of the rounds in the partial box. What I found:

Out of 10 rounds, 2 were .001 proud (above flush), 6 were .000, and 2 were .001 deep. None were within the specification above. How did we measure this?

1. The butt of your dial caliber has what's called a depth gauge. This is a slender metal rod that extends out the base of your caliper at the same measurement as the gap in your calipers. By butting it up against a primed piece of brass, you can extend that depth gauge, and get a reading how deep it is below the base of the caliper.

View attachment 1198898


2. I like to push the mouth of the piece of brass up against a flat surface, I loosen the friction lock on the caliper, extend the depth gauge, line it up with the center of the primer, and push the caliper forward and take the reading.


View attachment 1198899

3. I usually rotate the brass and take the measurement several times. The overall all measuring process looks like the last pic below. You'll note that the measurement is .001, not deep enough. You want a reading of .003 minimum, deeper even depending on your gun. Only once you've measured primers and verified the depth, can you start looking at other things...a primer with too hard of a cup for your application, a gun with a problem, or a bad batch of primers. Relying on feel....then everything else is just a guess, unsubstantiated claim, or excuse.

View attachment 1198900

Hopefully this helps somebody out. I did this because I was working on the related task above and it was timely, and I don't know that I've ever seen anybody put pictures up on how to use the depth gauge on THR before.

So what happened with my customer? I called the ammo company, talked to one of their folks. They'd actually had another report from the same lot, and knew of the issue. I gave them my customers address and they shipped him two new boxes of ammo, and they functioned perfectly in his gun, and he was super happy, so I folded up my cape, put it away until next time...and had a celebratory ice cold adult beverage.
Click to expand...
Excellent! Thank you for the informative post.

I have all manner of measuring devices - 0-1” and -0.005 - +0.005” dial indicators, anvil micrometers, depth micrometers, a few sets of calipers - and what you showed is just plain good enough. More “precision” is not needed.

I can’t vouch for the boutique ammo maker but for me, until I see a little perfectly centered crush ring on the primer’s face, I don’t consider it properly seated.
 
Fingers can feel surface differences as small as 13nm.
I have feen happy just ensuring that primers are seated below flush just by wiping my thumb across the base.
It doesn’t take long to develop a feel for what is enough. Proud primers stand out immediately.
 
Atavar said:
Fingers can feel surface differences as small as 13nm.
I have feen happy just ensuring that primers are seated below flush just by wiping my thumb across the base.
It doesn’t take long to develop a feel for what is enough. Proud primers stand out immediately.
Click to expand...
13 billionths of a meter? Did autocorrect bite you?
 
CQB45ACP said:
13 billionths of a meter? Did autocorrect bite you?
Click to expand...
He’s correct but the key word there is “can”. 13nm is the minimum found in some medical research for hypersensitivity. It’s not a typical level of sensitivity but it’s not abnormal.

I can login to JAMA and find an article for you if you want.
 
GeoDudeFlorida said:
He’s correct but the key word there is “can”. 13nm is the minimum found in some medical research for hypersensitivity. It’s not a typical level of sensitivity but it’s not abnormal.

I can login to JAMA and find an article for you if you want.
Click to expand...
No that’s okay. I had no idea. Dated a girl once who was that sensitive. Relationship didn’t last long at all.

Edit: sounded creepy rereading...what I meant was emotionally sensitive AND it was a joke.
 
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I once had a .25-06 and the brass I got for it -RP- had excessively deep primer pockets. A bottomed LR primer was out of reach of the firing pin. Seated flush, obviously floating in the pockets, they would all go off. I wonder if that is why I never got it down to MOA. I didn't really need the rifle, so I just traded it off rather than chase cases.
 
Jim Watson said:
I once had a .25-06 and the brass I got for it -RP- had excessively deep primer pockets. A bottomed LR primer was out of reach of the firing pin. Seated flush, obviously floating in the pockets, they would all go off. I wonder if that is why I never got it down to MOA. I didn't really need the rifle, so I just traded it off rather than chase cases.
Click to expand...
The Lee-Enfield has a field service replaceable firing pin. It also happens to be easily adjustable as a consequence. I have replaced many and at times had to adjust them to prevent primer piercing. I never ran into a rifle that wouldn’t reach deep seated primers. That would encourage me to make a trade I think.
 
Blue68f100 said:
To me seating depth is not a good reference unless all of your pockets are the same depth, and you know what it is. You MUST preload the anvil for a primer to go off. If you have deep pockets (0.008") and seat the primer 0.003" but you still have a gap your going to have a problem.
Click to expand...
Yep, primer seating depth is determined by primer pocket depth.
 
Blue68f100 said:
To me seating depth is not a good reference unless all of your pockets are the same depth, and you know what it is. You MUST preload the anvil for a primer to go off. If you have deep pockets (0.008") and seat the primer 0.003" but you still have a gap your going to have a problem.
Click to expand...
Well, yes....but if you have deep pockets you're out of spec anyway and as I in fact mentioned, if you measure and see that it's seated too deep, you've now got a definitive measurement that will lead you to a resolution of your actual issue. Instead of like the majority of the "I can feel when it's seated enough" guys.....who discover things like a deep pocket or otherwise incorrectly seated primers at the range when their boomstick no go boom. There's a reason there is a specification, to high or too deep both equal a potential to failure, and in both cases a simple measurement saves some pain at the range. This is especially true for new loaders who likely haven't developed a refined sense of feel for when a primer is in fact seated deep enough.

I mean, I love all you folks who push back against using the tool you've got in your drawer...because "something else might be wrong" or "you can feel when it's bottomed out", or whatever ....that's cool, you do you. I've heard folks tell me they don't need feeler gauges to adjust valves, or a torque wrench to put a head back on, or a crush gauge when putting a crank together......and I've pulled a lot of those guys out of the desert on a strap behind my bike......and believe me.....they don't want a repeat of that, and they don't make the same mistake again.

To be clear...I learned without tools...because I had too. A decent set of calipers when I started reloading was half a months pay for an E-1 in the Army. I didn't have a scale, used only scoops. No case gauges, no real measurement tools at all. It's different today. And new reloaders coming into the hobby are different today. It's something we, as the "old men" in the room have to understand. Telling a 25 year old that he needs to "feel how the primer seats" is all fine....but a youtube video can't show "feel", and trying to tell them just to keep trying until they get it right is a bit of a turn off to a lot of kids these days...being raised on complex problem based videogames that require a lot of logical problem solving skills doesn't leave them much patience for something based on a feeling...but show them how to use a measuring instrument, and give them a set of standards, and they are amazingly fast learners. I can teach a definable process to the young folks I take under my wings to bring into the fold. I can have them seat a primer...then I can take that piece of brass and SHOW them how the primer isn't seated. Then we can do it a few more times.....and then they can actually see in a measurable manner what it felt like to seat the primer correctly. This is the path forward for our sport.
 
Last edited:
Walkalong said:
Yep, primer seating depth is determined by primer pocket depth.
Click to expand...
I like to square them and deburr the flash hole. Truth be told, after years I quit doing both - felt it was not the most important step in loading.
Have gone back to doing it. Old habits die hard.
Chasing accuracy and technique is like arguing how many angles can you fit on the head of a pin…..everyone is right and no one is wrong.
 
Fingers can feel surface differences as small as 13nm.
I have feen happy just ensuring that primers are seated below flush just by wiping my thumb across the base.
It doesn’t take long to develop a feel for what is enough. Proud primers stand out immediately
CQB45ACP said:
13 billionths of a meter? Did autocorrect bite you?
Click to expand...
Moreover, the amplitude of the minimum pattern distinguished was only 13 nm, showing that the human finger with its coarse fingerprint structure in the sub-millimetre range is capable of dynamically detecting surface structures many orders of magnitude smaller.
www.nature.com

Feeling Small: Exploring the Tactile Perception Limits - Scientific Reports

The human finger is exquisitely sensitive in perceiving different materials, but the question remains as to what length scales are capable of being distinguished in active touch. We combine material science with psychophysics to manufacture and haptically explore a series of topographically...
www.nature.com www.nature.com
 
Atavar said:
Fingers can feel surface differences as small as 13nm.
I have feen happy just ensuring that primers are seated below flush just by wiping my thumb across the base.
It doesn’t take long to develop a feel for what is enough. Proud primers stand out immediately

Moreover, the amplitude of the minimum pattern distinguished was only 13 nm, showing that the human finger with its coarse fingerprint structure in the sub-millimetre range is capable of dynamically detecting surface structures many orders of magnitude smaller.
www.nature.com

Feeling Small: Exploring the Tactile Perception Limits - Scientific Reports

The human finger is exquisitely sensitive in perceiving different materials, but the question remains as to what length scales are capable of being distinguished in active touch. We combine material science with psychophysics to manufacture and haptically explore a series of topographically...
www.nature.com www.nature.com
Click to expand...
When I did body work many years ago we felt the patch after sanding to see if it was right, we could feel what we couldn't see.
 
EricBu said:
Well, yes....but if you have deep pockets you're out of spec anyway and as I in fact mentioned, if you measure and see that it's seated too deep, you've now got a definitive measurement that will lead you to a resolution of your actual issue. Instead of like the majority of the "I can feel when it's seated enough" guys.....who discover things like a deep pocket or otherwise incorrectly seated primers at the range when their boomstick no go boom. There's a reason there is a specification, to high or too deep both equal a potential to failure, and in both cases a simple measurement saves some pain at the range. This is especially true for new loaders who likely haven't developed a refined sense of feel for when a primer is in fact seated deep enough.

I mean, I love all you folks who push back against using the tool you've got in your drawer...because "something else might be wrong" or "you can feel when it's bottomed out", or whatever ....that's cool, you do you. I've heard folks tell me they don't need feeler gauges to adjust valves, or a torque wrench to put a head back on, or a crush gauge when putting a crank together......and I've pulled a lot of those guys out of the desert on a strap behind my bike......and believe me.....they don't want a repeat of that, and they don't make the same mistake again.

To be clear...I learned without tools...because I had too. A decent set of calipers when I started reloading was half a months pay for an E-1 in the Army. I didn't have a scale, used only scoops. No case gauges, no real measurement tools at all. It's different today. And new reloaders coming into the hobby are different today. It's something we, as the "old men" in the room have to understand. Telling a 25 year old that he needs to "feel how the primer seats" is all fine....but a youtube video can't show "feel", and trying to tell them just to keep trying until they get it right is a bit of a turn off to a lot of kids these days...being raised on complex problem based videogames that require a lot of logical problem solving skills doesn't leave them much patience for something based on a feeling...but show them how to use a measuring instrument, and give them a set of standards, and they are amazingly fast learners. I can teach a definable process to the young folks I take under my wings to bring into the fold. I can have them seat a primer...then I can take that piece of brass and SHOW them how the primer isn't seated. Then we can do it a few more times.....and then they can actually see in a measurable manner what it felt like to seat the primer correctly. This is the path forward for our sport.
Click to expand...
I have more tools than the average person, been collecting them for 50+ yrs, and use them when called for.

What I was trying to get across is if you don't know what your working with on pocket depth the primer seating depth you get may not tell the whole story. I would bet you could measure a small sample of brass and find a range of 0.003"-0.004". So in order for a single depth measurement to work the pocket would all have to be the same depth. Not going to happen on range pickup brass, unless you take the time to uniform them. Now some do this on their match rifle brass, but not on handgun.

Once thing that has not been discussed is the shape of the pocket. Some pockets are rounded at the bottom where some are square. The rounded ones will cause the primer to stop earlier than a square bottom one. This may be where a lot of problems are so hit or miss.
 
One of the places I worked manufactured paper machines. The final thickness is determined by running what was called a calendar press (called calendar because it gives it the gloss finish of calendar paper). Paper was run through two chilled cast iron rollers at high pressure. We made these calendar presses for Bicycle Playing Card. Before sale the customer would inspect the machine. If the difference in thickness of the card sheets was any greater than .0005" the would reject the press. We were told any variation greater than .0005" would be considered marked cards because the dealers could feel the difference.
 
Blue68f100 said:
I have more tools than the average person, been collecting them for 50+ yrs, and use them when called for.

What I was trying to get across is if you don't know what your working with on pocket depth the primer seating depth you get may not tell the whole story. I would bet you could measure a small sample of brass and find a range of 0.003"-0.004". So in order for a single depth measurement to work the pocket would all have to be the same depth. Not going to happen on range pickup brass, unless you take the time to uniform them. Now some do this on their match rifle brass, but not on handgun.

Once thing that has not been discussed is the shape of the pocket. Some pockets are rounded at the bottom where some are square. The rounded ones will cause the primer to stop earlier than a square bottom one. This may be where a lot of problems are so hit or miss.
Click to expand...
GeoDudeFlorida said:
Yup. When I get donations of brass I run a pocket uniforming tool through each piece. It makes sorting by condition easier.
Using a pocket cleaner should work just fine.
Click to expand...
Some of us do exactly that. 😁
 
Blue68f100 said:
I have more tools than the average person, been collecting them for 50+ yrs, and use them when called for.

What I was trying to get across is if you don't know what your working with on pocket depth the primer seating depth you get may not tell the whole story. I would bet you could measure a small sample of brass and find a range of 0.003"-0.004". So in order for a single depth measurement to work the pocket would all have to be the same depth. Not going to happen on range pickup brass, unless you take the time to uniform them. Now some do this on their match rifle brass, but not on handgun.

Once thing that has not been discussed is the shape of the pocket. Some pockets are rounded at the bottom where some are square. The rounded ones will cause the primer to stop earlier than a square bottom one. This may be where a lot of problems are so hit or miss.
Click to expand...
Which is fine...but not relevant to using a tool to measure what you have. You're using exceptions to justify not measuring to validate a published specification. Like I said, you do you. Measure, don't measure...whatever works for you. What I'm trying to do is demonstrate to new users, or to people that are having challenges with certain types of primers...a method to check depth when they think it's seated deep enough based on feel, when in fact it is not. This was a post to demonstrate one method of measuring primer seating depth in a definable and useful way for folks who may not know how to, or that they in fact already have the tool to do it.

Again, to emphasize, regardless of all the noise here...if you cannot measure a minimum of a .003 primer depth, then your primers are not seated deep enough. Square pocket, round pocket, blue pocket, the number of molecules on a pin head...whatever argument you want to make...doesn't matter.

Btw, you hyper sensitive touch guys....You realize people aren't born reading brail right? They have to be taught the meaning of what they feel...which is why telling people how great your sensitivity is...ain't real helpful to the guy trying to figure out the problem with his reloads.
 
Last edited:
EricBu said:
Myself, after 35 years of doing this, have determined that .003 is the absolute minimum depth to function reliably 100% of the time in all properly functioning firearms, but that's MY OPINION..
Click to expand...
SAAMI set standards and is correct saying, flush to .008" below the case head. *When seated to bottom of the pocket.*

I have seen shallow 38 spec brass pockets, where the primer is above the case head. The defect was corrected by uniforming the depth of the pocket.

Primers can be mashed in the pocket and still fire, every time.
The RCBS Ram Prime unit gets the job done, if needed.

I have been loading for 59 years. Not that years makes anyone an expert.

From SAAMI. Screenshot_20240312-235753_Drive.jpg
 
Last edited:
Atavar said:
Fingers can feel surface differences as small as 13nm.
I have feen happy just ensuring that primers are seated below flush just by wiping my thumb across the base.
It doesn’t take long to develop a feel for what is enough. Proud primers stand out immediately

Moreover, the amplitude of the minimum pattern distinguished was only 13 nm, showing that the human finger with its coarse fingerprint structure in the sub-millimetre range is capable of dynamically detecting surface structures many orders of magnitude smaller.
www.nature.com

Feeling Small: Exploring the Tactile Perception Limits - Scientific Reports

The human finger is exquisitely sensitive in perceiving different materials, but the question remains as to what length scales are capable of being distinguished in active touch. We combine material science with psychophysics to manufacture and haptically explore a series of topographically...
www.nature.com www.nature.com
Click to expand...
Lets get small as Steve Martin used to say.

I wasn't doubting you...well, yes I was I suppose.

While it's true we can tell there is a difference, I'd be more impressed if someone could tell what the difference is.
 
Last edited:
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