If a bullet is seated .010" longer than a length that places the bullet at the rifling how deep will the imprint on the jacket be? and .020" longer? How much longer would a bullet have to be seated to leave a .010" imprint in the jacket if it were then un-chambered assuming the imprint isnt 1:1?
"Jamming" bullet into the lands?
- Thread starter TnTom
- Start date
BoydAllen
Gold $$ Contributor
The length of the marks will depend upon the neck tension, the shape of the ogive (ogive number in the area where the contact takes place), and the throat angle. I am always curious about questions that have answers that are so easily determined by a little simple experimenting, using materials that are common, and readily available. The short answer to the question is "it depends".
Three questions by TnTom on imprint depth:
#1. If a bullet is seated .010" longer than a length that places the bullet at the rifling, how deep will the imprint on the jacket be?
#2 If a bullet is seated.020" longer, than a length that places the bullet at the rifling, how deep will the imprint on the jacket be?
#3 How much longer would a bullet have to be seated to leave a .010" imprint in the jacket if it were then un-chambered assuming the imprint isn't 1:1.
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They're all dealing with DEPTH of an imprint.
The softness of the jacket material, the force being used to create the distortion, and the protrusion height of the rifling's lands, are all going to make a difference.
It's been my experience that what I can see is a slight surface rubbing or blemish visible only with the aide of a 10X jeweler's magnifier.
I'm not sure how you would measure it or what type tool would be used given that the surface is somewhat rounded. Why are you asking questions about the measurement of an imprint's depth? What are you looking to do or establish with this measurement?
#1. If a bullet is seated .010" longer than a length that places the bullet at the rifling, how deep will the imprint on the jacket be?
#2 If a bullet is seated.020" longer, than a length that places the bullet at the rifling, how deep will the imprint on the jacket be?
#3 How much longer would a bullet have to be seated to leave a .010" imprint in the jacket if it were then un-chambered assuming the imprint isn't 1:1.
------------------------------------------------------------------------
They're all dealing with DEPTH of an imprint.
The softness of the jacket material, the force being used to create the distortion, and the protrusion height of the rifling's lands, are all going to make a difference.
It's been my experience that what I can see is a slight surface rubbing or blemish visible only with the aide of a 10X jeweler's magnifier.
I'm not sure how you would measure it or what type tool would be used given that the surface is somewhat rounded. Why are you asking questions about the measurement of an imprint's depth? What are you looking to do or establish with this measurement?
Let me explain where I am trying to go with this and why I am asking the question. I have made the setup, taken the measurements and have my results and I understand why. I have no interest in trying to prove anyone wrong.
I have read numerous threads about "jamming the bullet" varying amounts into the rifling for improved accuracy (by varying amounts). I am not addressing the credibility of the practice. I am trying to determine what is physically happening when the bolt is closed.
I use .0025"-.003" interference between my bullet o.d and case i.d. Setting the bullet out .010" beyond the distance to the lands I interpret that as having prepared the round to be "JAMMED" by .010. When the bolt is closed on the round and extracted and re-measured it has been set back into the case by .010" . Nothing has been jammed into the lands rather the bullet has been set back that distance into the case. Some slight insignificant burnish marks. I understand if there were some indentation is probably wont be 1:1
Increasing the interference between the bullet and neck to a point that prevents the bullet being set back into the case causing embedding of the bullet it into the rifling is what I am trying to understand. Is this a common practice? Is this what is referred to as "jamming" a round into the lands?
I spend quite a few hours in my shop utilizing my fabricating resources and have invested significantly in measuring devices and probably have accumulated about 40 years in both machining and metrology. I build most of my jigs and fixtures and all of my seating dies and
I usually shoot with some jump and occasional load to the lands but really don't understand this "Jamming" thing.
Possibly someone can shed some light on what or how they are doing it and what their end results are before they fire the load.
Thanks
Tom
I have read numerous threads about "jamming the bullet" varying amounts into the rifling for improved accuracy (by varying amounts). I am not addressing the credibility of the practice. I am trying to determine what is physically happening when the bolt is closed.
I use .0025"-.003" interference between my bullet o.d and case i.d. Setting the bullet out .010" beyond the distance to the lands I interpret that as having prepared the round to be "JAMMED" by .010. When the bolt is closed on the round and extracted and re-measured it has been set back into the case by .010" . Nothing has been jammed into the lands rather the bullet has been set back that distance into the case. Some slight insignificant burnish marks. I understand if there were some indentation is probably wont be 1:1
Increasing the interference between the bullet and neck to a point that prevents the bullet being set back into the case causing embedding of the bullet it into the rifling is what I am trying to understand. Is this a common practice? Is this what is referred to as "jamming" a round into the lands?
I spend quite a few hours in my shop utilizing my fabricating resources and have invested significantly in measuring devices and probably have accumulated about 40 years in both machining and metrology. I build most of my jigs and fixtures and all of my seating dies and
I usually shoot with some jump and occasional load to the lands but really don't understand this "Jamming" thing.
Possibly someone can shed some light on what or how they are doing it and what their end results are before they fire the load.
Thanks
Tom
Nikon 3D does it in a moment.BoydAllen said:
The depth varies as the imprint moves along the profile.Yes I meant deep but its somewhat irrelevant sorry i asked. I am primarily interested in the practice and is the practice meant to embed the bullet into the lands by a certain amount. When "jamming a load by .010 or .020 is the intent to engrave the bullet by that amount or that amount beyond the initial contact with the rifling or is this simply a method to assure the bullet seated TOO the lands not INTO the lands seating the bullet deeper into the case by what ever amount?
Thanks for the feedback
Tom, Here is some info on JAM that I've gather over time and placed in my Benchrest Folder. It might be helpful here. If you're pushing bullets back into the case you had best obtain a bushing die so that you can increase the neck tension and prevent it. Personally I seat bullets to three lengths. One where the bullet just touches the lands, where the bullet is engraved with a "square mark", and where the mark is "twice as long as it is wide" per jackie's suggestion below.
Here's the info I gathered:
Call me "old school" but to me any time you are are using the term, "Jam" means bolt closure is seating the bullet by .XXX".
For seating of say 0.020" into the lands, it is just that: seating "into the lands" (ITL) and is not the jam seating.
So if you tell me you are seating your bullets at ITL=.015" I understand you to mean they are engraving the lands by .015".
If you say your at Jam=.015" I understand you to mean the “bolt-closer” is jam-seating the bullet the last .015".
If you say your OTL=.015" I understand you to mean your "off the lands" by .015" with no engraving. ... Donovan Moran
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I like what Donavan said. "Jam" is a relative term. Saying ".010 into the lands" seems to be a more accurate description as to what is happening.
In 100-200 yard Benchrest, many of us have found that moving a bullet into the lands, at what first glance would seem like a ridiculous amount, is where many bullets, especially the "double radius" variety, like to shoot.
For instance, I tell shooters that, with the combination I shoot, I "make the marks appear twice as long as they are wide". I really do not know how far into the lands I actually move the bullet to achieve this ... I never measured it. I can feel the bullet lightly "crunch" as the bolt closes.
It is one of the strange quirks that appear when using 133 with this particular style of bullet. ... jackie schmidt
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Before one can tune a rifle, there must be a starting point. Many use the "JAM" length as a reference to start loading development.
By "JAM" I mean the maximum length that can be obtained ... without the lands pushing the bullet deeper into the case.
Once this is determined, one can tune by moving the bullet only one direction ... deeper into the case. I, like many, have found that many bullets shoot best when seated .008" or .010" off the jam.
After determining a starting point, let the conditions and the results on the targets dictate what tuning is needed.
Joe Krupa, Precision Shooting, August 2008, page 51, under: Where to Start.
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One of the things many of us have discovered with the current crop of double radius bullets, when using 133, is that they like to be jammed to the point that you can actually feel the bullet crunch into the lands when you close the bolt.
However, shooters who use these bullets, especially the Bruno boattail, with 8208, find that just off the lands to just touching works best.
With a standard .237 4-groove Krieger and 133, I jam the bullets to where the [marks appear to be about twice as long as they are wide.]
These seem tight, but remember, the portion of the bullet that contacts the lands is just about the same shape as the 1.5 degree lead. It really isn't 'tight' in the sense that the bullet is getting compressed more, the marks just sort of skim along the ogive.
Other shooters use different methods with these bullets and 133, but this is where the best agging capability seems to be. ... jackie. Hope this helps.
Here's the info I gathered:
Call me "old school" but to me any time you are are using the term, "Jam" means bolt closure is seating the bullet by .XXX".
For seating of say 0.020" into the lands, it is just that: seating "into the lands" (ITL) and is not the jam seating.
So if you tell me you are seating your bullets at ITL=.015" I understand you to mean they are engraving the lands by .015".
If you say your at Jam=.015" I understand you to mean the “bolt-closer” is jam-seating the bullet the last .015".
If you say your OTL=.015" I understand you to mean your "off the lands" by .015" with no engraving. ... Donovan Moran
-----------------------------------------------------------
I like what Donavan said. "Jam" is a relative term. Saying ".010 into the lands" seems to be a more accurate description as to what is happening.
In 100-200 yard Benchrest, many of us have found that moving a bullet into the lands, at what first glance would seem like a ridiculous amount, is where many bullets, especially the "double radius" variety, like to shoot.
For instance, I tell shooters that, with the combination I shoot, I "make the marks appear twice as long as they are wide". I really do not know how far into the lands I actually move the bullet to achieve this ... I never measured it. I can feel the bullet lightly "crunch" as the bolt closes.
It is one of the strange quirks that appear when using 133 with this particular style of bullet. ... jackie schmidt
-------------------------------------------------------
Before one can tune a rifle, there must be a starting point. Many use the "JAM" length as a reference to start loading development.
By "JAM" I mean the maximum length that can be obtained ... without the lands pushing the bullet deeper into the case.
Once this is determined, one can tune by moving the bullet only one direction ... deeper into the case. I, like many, have found that many bullets shoot best when seated .008" or .010" off the jam.
After determining a starting point, let the conditions and the results on the targets dictate what tuning is needed.
Joe Krupa, Precision Shooting, August 2008, page 51, under: Where to Start.
------------------------------------------------------------
One of the things many of us have discovered with the current crop of double radius bullets, when using 133, is that they like to be jammed to the point that you can actually feel the bullet crunch into the lands when you close the bolt.
However, shooters who use these bullets, especially the Bruno boattail, with 8208, find that just off the lands to just touching works best.
With a standard .237 4-groove Krieger and 133, I jam the bullets to where the [marks appear to be about twice as long as they are wide.]
These seem tight, but remember, the portion of the bullet that contacts the lands is just about the same shape as the 1.5 degree lead. It really isn't 'tight' in the sense that the bullet is getting compressed more, the marks just sort of skim along the ogive.
Other shooters use different methods with these bullets and 133, but this is where the best agging capability seems to be. ... jackie. Hope this helps.
Outdoorsman said:
If I were to utilize this practice I would use the measurement to determine the ratio with a specific style of bullet in a specific bore and how much interference would be needed between the neck id and bullet od.
I would hope to discuss this with some one who does this "Jamming" practice in detail and how the result is monitored i.e. how is it determined that there has been a specific amount of interference created. Or are these procedures based around the old SWAG methods of reloading.
Outdoorsman
That is exactly what I was wanting to know. I really appreciate the excellent detail and the time you've taken to help me get my head around this. It isn't something I have any intentions of pursuing at this time in my shooting but it provides a good reference to base the practice on.
Measuring the indent can be accomplished but not easily without 3d capability. That you have established a standard or method to base your loads by is the bottom line. I really feel now that I understand. Thanks again.
That is exactly what I was wanting to know. I really appreciate the excellent detail and the time you've taken to help me get my head around this. It isn't something I have any intentions of pursuing at this time in my shooting but it provides a good reference to base the practice on.
Measuring the indent can be accomplished but not easily without 3d capability. That you have established a standard or method to base your loads by is the bottom line. I really feel now that I understand. Thanks again.
"JAM" is part of the tuning process normally referred to as Seating Depth. It's a measurement that can be seen on a micrometer when you measure overall cartridge length, as a micrometer seating depth reading on a Wilson bullet seater type die with micrometer top and is monitored on the target as a large group, a clover leaf group or a one hole group, or anything in between. All can be measured as a group size. It's definitely, not in the scientific wild ass guess category.
AlNyhus
Silver $$ Contributor
Hi Tom.
I think if we think of it as jam/seat, things become a bit clearer.
For example, when I set up seating depth for .025 jam/seat, the bullet certainly doesn't move 'into' the rifling .025. The actual number (.025 in this case) is simply a reference number and is really only accurate as it relates to that particular combination of bullet profile/land width/throat angle.
People get lost in this after observing that the rifling marks get shorter in length as the bullet is seated further back into the case neck...so they assume this means the bullet has actually entered the rifling by whatever amount the jam/seat is.
Hopefully I haven't misinterpreted your question or pointed out something you already know.
Seating depth is a subject where semantics can get in the way. -Al
I think if we think of it as jam/seat, things become a bit clearer.
For example, when I set up seating depth for .025 jam/seat, the bullet certainly doesn't move 'into' the rifling .025. The actual number (.025 in this case) is simply a reference number and is really only accurate as it relates to that particular combination of bullet profile/land width/throat angle.
People get lost in this after observing that the rifling marks get shorter in length as the bullet is seated further back into the case neck...so they assume this means the bullet has actually entered the rifling by whatever amount the jam/seat is.
Hopefully I haven't misinterpreted your question or pointed out something you already know.
Seating depth is a subject where semantics can get in the way. -Al
BoydAllen
Gold $$ Contributor
Fellas,
For a particular bullet, neck thickness, neck tension, throat configuration, and barrel, jam is the longest length that you can seat a bullet to without its being seated farther into the case. Generally, it is determined by seating to a length that is judged likely to cause the bullet to be pushed farther into the case as the bolt is closed, measuring, chambering, and then remeasuring, to both verify that the bullet was seated deeper, and and what jam is for that particular combination.
If a bullet is seated shorter than jam, its seating depth is described as so much off jam, meaning that the seating die is set to produce an OAL that is the specified figure shorter than jam. All of the jammed .015 stuff is IMO incorrect use of the term. Such a dimension would be more properly be described as .015 longer, beyond, or over touch, or so much off jam. Even in the discussion of the term, after a correct definition has been given, the same poster has inadvertently misused the term. This sort of misuse has become common and leads to confusion. I am not trying to tell anyone what reference measurement he should measure his seating depth on. You can use touch or jam. They both work. I am just trying to say that it is better to have one definition for a word, and stick to it, rather than making it up as we go along. Lest anyone think that I am holding myself up as the final arbitrator, I am not. My understanding of the proper and classic use of this term comes from a great deal of reading of Precision Shooting, going well back into the days when the editor was part time, the subject material much more narrow, and covers black and white.
As to the observations of the thread starter, my results are different. Depending on how large the ogive number is in the area on the bullet that will be marked, I can seat longer than the length where contact is made, and have the bullet show varying lengths of marks, depending on how much longer than touch they are seated, without being pushed back into the case, right up to jam.
A good followup question might be to ask posters to tell us if they have a record of the distance between touch and jam, and the particulars of the setup, bullet, neck tension, etc.
For a particular bullet, neck thickness, neck tension, throat configuration, and barrel, jam is the longest length that you can seat a bullet to without its being seated farther into the case. Generally, it is determined by seating to a length that is judged likely to cause the bullet to be pushed farther into the case as the bolt is closed, measuring, chambering, and then remeasuring, to both verify that the bullet was seated deeper, and and what jam is for that particular combination.
If a bullet is seated shorter than jam, its seating depth is described as so much off jam, meaning that the seating die is set to produce an OAL that is the specified figure shorter than jam. All of the jammed .015 stuff is IMO incorrect use of the term. Such a dimension would be more properly be described as .015 longer, beyond, or over touch, or so much off jam. Even in the discussion of the term, after a correct definition has been given, the same poster has inadvertently misused the term. This sort of misuse has become common and leads to confusion. I am not trying to tell anyone what reference measurement he should measure his seating depth on. You can use touch or jam. They both work. I am just trying to say that it is better to have one definition for a word, and stick to it, rather than making it up as we go along. Lest anyone think that I am holding myself up as the final arbitrator, I am not. My understanding of the proper and classic use of this term comes from a great deal of reading of Precision Shooting, going well back into the days when the editor was part time, the subject material much more narrow, and covers black and white.
As to the observations of the thread starter, my results are different. Depending on how large the ogive number is in the area on the bullet that will be marked, I can seat longer than the length where contact is made, and have the bullet show varying lengths of marks, depending on how much longer than touch they are seated, without being pushed back into the case, right up to jam.
A good followup question might be to ask posters to tell us if they have a record of the distance between touch and jam, and the particulars of the setup, bullet, neck tension, etc.
AlNyhus
Silver $$ Contributor
All good points, Boyd.
My 'jam/seat' number is based off the point where the rifling marks just disappear from the bullet...that's the 'zero' or 'just touch' point. Reference to the 'jam/seat' as .025, just means the seating stem is .025 shorter than the 'zero'.
Others may establish their 'zero' in other ways, like at the point at which the bullet won't move any further back into the case neck. This 'zero' may well be different than another persons method or understanding of 'zero'.
Other than the correct way to measure group size, the 'correct' way to measure seating depth (jam/seat, if you will) probably causes more confusion than any other topic.
Good shootin'. -Al
My 'jam/seat' number is based off the point where the rifling marks just disappear from the bullet...that's the 'zero' or 'just touch' point. Reference to the 'jam/seat' as .025, just means the seating stem is .025 shorter than the 'zero'.
Others may establish their 'zero' in other ways, like at the point at which the bullet won't move any further back into the case neck. This 'zero' may well be different than another persons method or understanding of 'zero'.
Other than the correct way to measure group size, the 'correct' way to measure seating depth (jam/seat, if you will) probably causes more confusion than any other topic.
Good shootin'. -Al
A lot of semantics going on. Jam becomes clear to me in reference to Outdoormans explanation.
I had never considered the term jam or practice which seems to be more specific or applicable to BR shooters though I sure not limited to. They seem to be well versed in the term and practice. My experience is limited to either loading to the rifling or back from the rifling a certain amount, or jump, a term / practice I am familiar with.
After a zero is determined, in my reloading practice, zero being the distance from the BF to bullet touching the rifling +/- nothing, anything greater than that specific length would be some degree of interference and anything less would be jump. Joe Krupa Precision Shooting as referenced by Outdoorsman keeps it quite simple and applicable to what I would consider typical.
"twice as long as wide" is a measurable and adds validity "ITL". As Donovan defines it jam finishes setting the bullet into the case. Distinctly two different meanings and helps reduce semantics.
Great feedback
I had never considered the term jam or practice which seems to be more specific or applicable to BR shooters though I sure not limited to. They seem to be well versed in the term and practice. My experience is limited to either loading to the rifling or back from the rifling a certain amount, or jump, a term / practice I am familiar with.
After a zero is determined, in my reloading practice, zero being the distance from the BF to bullet touching the rifling +/- nothing, anything greater than that specific length would be some degree of interference and anything less would be jump. Joe Krupa Precision Shooting as referenced by Outdoorsman keeps it quite simple and applicable to what I would consider typical.
"twice as long as wide" is a measurable and adds validity "ITL". As Donovan defines it jam finishes setting the bullet into the case. Distinctly two different meanings and helps reduce semantics.
Great feedback
AlNyhus
Silver $$ Contributor
TnTom said:
Well...yes and no, Tom.
The width of the lands and rifling profile becomes an issue in that ratio.
I'll give you an example: Using the same .025 jam/seat dimension as I explained earlier...and with identical throat angles... the marks I get on my Lilja 3 groove .30's are quite a bit different than what I see on my Rock .30's. And both of those are different than my Kostyshyn .30's.
Apples to apples is always the best comparison.
Hope this helps. -Al
Understood. the ratio was just an example Outdoorsman mentioned as a result of the load he had prepared. The ratios would be different depending on any number of things.AlNyhus said:TnTom said:
Well...yes and no, Tom.
The width of the lands and rifling profile becomes an issue in that ratio.
I'll give you an example: Using the same .025 jam/seat dimension as I explained earlier...and with identical throat angles... the marks I get on my Lilja 3 groove .30's are quite a bit different than what I see on my Rock .30's. And both of those are different than my Kostyshyn .30's.
Apples to apples is always the best comparison.
Hope this helps. -Al
The jump or jam to me is where you seat to fine tune your workup. In my world I measure the bullet used with the Stony Point setup, the reading I get is the "touch". I take that measurement and try different seating depths, if I seat it .010 longer I call that 10 thousands jam. If I seat it .010 short, I call it a 10 thousandths jump.
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