I think because it only uses the shell holder to initially hold the shell. It then tightens against the case head and datums off of it.
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Effects of rim thickness dependence on primer seating depth
- Thread starter Sniper King 2020
- Start date
The Sinclair uses a shell holder too but has a mechanism built in to lock the brass in place which is why it uses a shell holder but is not rim thickness dependent. I would separate the two concepts (shell holder from rim thickness dependent) from your mind and focus only on rim thickness dependent. It would behoove companies that produce primer seaters to focus their design to not be rim thickness dependent and consider this effect when designing their tools.
I’d love to see a mechanical breakdown of how they infect off the head rather than the rim.
Me too. I thought about tearing apart the tools but wanted to respect their intellectual property. Regardless, the data shows what the data shows. I do know that Lee put a cutout of their basic design on their website so it’s easy to see how the Lee ACP holds down the brass while seating which eliminated rim thickness dependence.
Yeah, the Lee makes senses. I’m just having a hard time visualising how a standard shell holder isn’t rim thickness dependent. What’s pulling the case down to index on the head?
That may not be the only way to eliminate the dependency. Have you checked into the design of the tool? Maybe that would provide clarification?
Yes, but only under certain circumstances. For example, if I have lots of variability in the brass lot with rim thickness, I’ll use a rim thickness dependent tool only when fire forming brass and breaking in a barrel. For testing, I use the tool that gets the best most reliable results.
So, the K&M, to the best of my knowledge......
It has two stems that move up as you squeeze handle. One is a larger tube around the priming stem. The other is the priming stem.
The outer stem is taller than a primer when the primer is seated on the seating stem. This effectively replaces the bottom of the shell holder and it's a static height which basically becomes a datum. As you close the handle outside stem moves up and pinches the rim into the case holder. Once that pinches and stops, the priming stem continues moving up. Since the outer tube is now effectively the bottom of the shell holder, it works the same as the Sinclair.
It has two stems that move up as you squeeze handle. One is a larger tube around the priming stem. The other is the priming stem.
The outer stem is taller than a primer when the primer is seated on the seating stem. This effectively replaces the bottom of the shell holder and it's a static height which basically becomes a datum. As you close the handle outside stem moves up and pinches the rim into the case holder. Once that pinches and stops, the priming stem continues moving up. Since the outer tube is now effectively the bottom of the shell holder, it works the same as the Sinclair.
The key to rending the rim thickness null is actually having a platform above the primer that is a static height/measurement.
After that, it doesn't matter which method you use to force the bottom of the case into that platform.
After that, it doesn't matter which method you use to force the bottom of the case into that platform.
This is the stem of the K&M. Has a spring that allows the outside and inside stems to work independently once the outside stem comes to a stop.
I don't have a hard time understanding that it is depending on rim thickness.
The K&M? It's not dependent on rim thickness. At least until the primer bottoms out. After that, you basically can't control how much crush and such as there's no adjustments or hard stop.
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Guess I'm missing something, then.
If I'm understanding how these things work, the Lee uses a completely different method, with a backer rod inside the case. Let's set that aside for the moment.
Everything else uses a shell holder to contain/capture the case via the rim. The rim is what the priming system is pushing against, ultimately. The Sinclair and K&M have mechanisms to take up the slack between the shell holder lips and the case rim... but in the end, that lip/rim relationship is *still* the reference surface when you're pushing 'up' like that.
Assuming that is true, I don't entirely understand why one device using the exact same shell holder (for example, Sinclair) has so much 'better' variance than another (e.g. Primal Rights CPS). Or for that matter, why the Lee ACP is as consistent as it is - unless the dimension it references off of (case head thickness) is really *that* much more tightly controlled than the case rim thickness (in an admittedly outlier batch of brass).
If I'm understanding how these things work, the Lee uses a completely different method, with a backer rod inside the case. Let's set that aside for the moment.
Everything else uses a shell holder to contain/capture the case via the rim. The rim is what the priming system is pushing against, ultimately. The Sinclair and K&M have mechanisms to take up the slack between the shell holder lips and the case rim... but in the end, that lip/rim relationship is *still* the reference surface when you're pushing 'up' like that.
Assuming that is true, I don't entirely understand why one device using the exact same shell holder (for example, Sinclair) has so much 'better' variance than another (e.g. Primal Rights CPS). Or for that matter, why the Lee ACP is as consistent as it is - unless the dimension it references off of (case head thickness) is really *that* much more tightly controlled than the case rim thickness (in an admittedly outlier batch of brass).
Because they are using a surface independent of the primer to hold the bottom of the case up against the shell holder.
When you prim with the CPS, the primer itself is what pushes the brass up into the top of shell holder. That creates a situation entirely to the mercy of rim thickness.
The Sinclair and K&M have cylinders that pinch the brass up into the shell holder and then the primer moves into the pocket after.
The difference with the Sinclair and K&M though, the Sinclair plunger is completely independent of the outside cylinder. So you can adjust a hard stop wherever you want.
The KM has a spring and no hard stop. So you only have two choices....seat until you feel the bottom of the pocket (there's almost no variation in primer pocket depth from base of case in almost any brass), or always squeeze the handle until it stops and you get whatever crush the full travel allows. And the crush will be the same for every round......but you can't customize the crush, you get what you get.
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Just gonna make up a number here. We'll use .005.
Let's say the KM tool. The top of the primer always sits .005 below the top of the outside cylinder. When you squeeze the handle, the cylinder and the primer stem (and by default the primer) move in tandem. They keep moving in tandem until the top of the cylinder pushes the brass rim into the top of the shell holder. It is now pinched. This whole time, the primer maintains its .005 below.
This is where the primer starts moving independently. And it *always* starts independently .005 from the top of the cylinder. Since the cylinder is pressed to a hards stop against the bottom of the case, by default, the primer *always* starts moving .005 from the bottom of the case.
So, its effetely turning the bottom of the case into the datum.
Let's say the KM tool. The top of the primer always sits .005 below the top of the outside cylinder. When you squeeze the handle, the cylinder and the primer stem (and by default the primer) move in tandem. They keep moving in tandem until the top of the cylinder pushes the brass rim into the top of the shell holder. It is now pinched. This whole time, the primer maintains its .005 below.
This is where the primer starts moving independently. And it *always* starts independently .005 from the top of the cylinder. Since the cylinder is pressed to a hards stop against the bottom of the case, by default, the primer *always* starts moving .005 from the bottom of the case.
So, its effetely turning the bottom of the case into the datum.
The CPS would work the same way if it had a cylinder around the primer that would move up in tandem with the primer until it pinched the rim into the top of the shell holder.
Now, there are obviously other issues that arise for consistency. When you have something like a cylinder moving in tandem, you need some sort of hard stop that has a datum off that cylinder.
If it's something like the Lee or the Sinclair, since you locked the case down before ever moving the primer and you did it via pushing the case down and not up......it's much easier to control adjusts with click systems and such.
IMO, the K&M is really only good for seating primers to the bottom of the pocket. The Sinclair or Lee you can do custom crushes and such.
Now, there are obviously other issues that arise for consistency. When you have something like a cylinder moving in tandem, you need some sort of hard stop that has a datum off that cylinder.
If it's something like the Lee or the Sinclair, since you locked the case down before ever moving the primer and you did it via pushing the case down and not up......it's much easier to control adjusts with click systems and such.
IMO, the K&M is really only good for seating primers to the bottom of the pocket. The Sinclair or Lee you can do custom crushes and such.
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We must be using the K&M very differently. I always adjusted the head so the primer would bottom out with a little room still between the lever and the body. Then squeezing it all the way home is where you get the 'crush'.
I haven't used on in a while and could be wrong on that.
But the way the KM and Sinclair pinch the rim does take it out of the equation. At least from a datum perspective.
Mmm... still not seeing it. They still pinch *up* against the rim, which is being held *down* by the shell holder. I can see how the way the priming ram is sleeved and the head is captured would eliminate some of the slop otherwise necessary to get the case in and out of the shell holder... but ultimately it looks (to me) like they still reference off the rim.
It seemed like the sleeve moved with the seating stem so sleeve and seating stem both move up together and after the sleeve engages the rim, the seating stem pushes up the primer. That’s what it seemed to be doing when I checked it out. Does that seem like the case?
That's exactly what's happening. And because the primer does not start moving on its own until the the rim is pinched and it starts at the same spot below the top of that sleeve every time, the rim is negated.
It's the same as taking the rod and holding the brass down like the Lee. As long as the bottom of the case is held down against a flat surface and the primer starts at the same spot below said surface, then it's not rim dependent.
The sleeve and the stem do move together, but the stem is not constrained in any way in the vertical plane by the sleeve. If it hard stopped against the sleeve, and you could adjust how proud it sat above the sleeve, then it would only ever seat to depth relative to the sleeve, but in its current configuration with just a spring, it’s absolutely rim thickness dependant.
To clarify, the KM tool is only designed to seat to the bottom of the primer pocket. Used in that way, it's not rim dependent. If you're trying to seat to a certain crush or below.....then the rim will come into play.
This is why KM offers a separate seater with an indicator.
So, the KM is kinda a hybrid. Not rim dependent for seating to the bottom of the pocket, but you won't be able to get anything consistent past that.
This is why KM offers a separate seater with an indicator.
So, the KM is kinda a hybrid. Not rim dependent for seating to the bottom of the pocket, but you won't be able to get anything consistent past that.
Granted, this is all moot if all you want to do is seat to the bottom of the pocket. As there's very little variance case to case in primer pocket depth.
To me, seating to the bottom of the pocket by feel is akin to bumping your shoulder by feel, without camming over to a stop, or hitting a hard stop on your press. I’m sure there are folks that can do it, but why would you?
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Thought I’d add to this for anyone looking for a precision primer seating tool that won’t break the bank and adjusts seating depth by 1/2 a tho.take a look at the derreco seating die,I’ve got one and it’s a great bit of kit.the issue I’m having is trying to buy The Accuracy One Precision Primer Gauge,as they will not ship internationally to the uk and no one else seems to sell them or anything similar,if anyone could help it would be appreciated
Not sure why
. The other issue is how to operationally define “feel”? I tried this “feel” method which BTW has more than one method (not surprising given how subjective the whole idea is anyway) and did a mini test with two of the feel methods I’ve heard of. I did it while evaluating a new rail gun design. Results for “feel” didn’t feel so good 
