Marks on bullet vs jam ?

[Link]

I measured the jam marks on the Berger 115 gr 30 cal bullet and got .050 +-. Then I backed off .020 on my micrometer top die and the marks were gone. I would call this .020 jam ? Does this compute in your book ?

thanks in advance. Link

;D

 

[CatShooter]

The lengths of the marks do not indicate the amount of jam.

If the angle of the leade is the same as the angle at the contact point of the ogive, then you can go from no marks to long marks in just 2 or 3 thou.

Jam is measured from the first contact length, on out.

 

[Tim Singleton]

Much discussion comes up with this topic. Certain terms like Jam mean different things to some people.

The method I was taught is this. Using a fired case in your chamber resize set the bullet long with your seater. Chamber the round the rifling will push the bullet back in the case. Measure base to ogive repeat a couple times to confirm.

This is the jam length for that bullet and neck tension. You then shorten the OAL come off the Jam or work into the Jam seating the bullet longer

Just how I was taught

 

[bsekf]

I agree with the kitty shooter but will add that the definition of "JAM" is whatever you want it to be and is nothing but a starting point when you are playing with bullet seating. My "JAM" is when the bullet first touches and there usually is no mark, + 0.01 will put a mark on it, + 0.10 will mark it and push it into the case.

Now, if you really want to get anal, when you start jumping bullets you are lengthening the barrel and the harmonic's change, so you may change your accuracy nodes.

Bill

 

[Tim Singleton]

I agree with the kitty shooter but will add that the definition of "JAM" is whatever you want it to be and is nothing but a starting point when you are playing with bullet seating. My "JAM" is when the bullet first touches and there usually is no mark, + 0.01 will put a mark on it, + 0.10 will mark it and push it into the case.

Now, if you really want to get anal, when you start jumping bullets you are lengthening the barrel and the harmonic's change, so you may change your accuracy nodes.

Bill

You are correct that the term means whatever it does to each person a starting point. What jam means to you. Is what I refer to as touch

 

[geraldgee]

For me, I find more consistency using the Sinclair Seating Depth tool then any of the other method Ive tried, including the one suggested above and the hornady tool with a modified case (least accurate for me).

I take 4-5 different measurements using different cases and bullets and average the measurements. I am careful not to pressure the bullet into the lands. I am only looking to find the lands. Once I determine where they are, I then determine what direction to take with respect to seating depth.

When I come to my average COAL using the Sinclair tool, I'll make a dummy
round to determine my CBTO. This has worked very well for me.

Good luck - Gerald

 

[BoydAllen]

The use of the word jam to describe bullet seating depth originated many years ago in short range benchrest, and Tim's definition is how it was, and still is used in that sport.

Unfortunately since then, largely because of the internet, the definition has been corrupted to mean so many things that one should really explain which definition is being used ever time a post is written.

There are advantages to sticking to standard definitions. It promotes better understanding, without requiring long explanations.

Personally I use jam to mean exactly what Tim described.

If I seat a bullet some distance into the lands, but not at jam, I can either describe it as so many thousandths longer than touching, or some number of thousandths shorter than jam, or off jam.

The other thing that I have seen on the internet is fellows saying that they are seating their bullets jammed, without specifying just exactly how their seating depth relates to touching or classically defined jam. It almost seems to me that some do not understand that very small differences in seating depth can have have large effects on tune, and accuracy, and that they lump all seating depths that mark the bullet together.

 

[Link]

Yup I'm one of those guys that misuses the word jam. I have read the definition before and you guys are right. When I use jam it means anything deeper that touching. When I say jam .010 or jam .020 into the lands it seems easier for me to describe the fact. [ however wrong ] . We also know neck tension can play a large factor in true jam. I am using a .004 neck tension and might be able to push the bullet out the firey end.

I did find the fact that .020 of jam equaled out to .050 or so of land mark on the ojive [sp] amusing. I had never really played with that before.

thanks Link

 

[Tim Singleton]

At first it amazed me how much a small amount of seating depth changes the group size. This is typically how I start. I will go back to the .005" off Jam and go in and out .003" to fine tune. I test at 150 it shows the best depth better

You wouldn't think these groups even came out of the same barrel.

 

[/VH]

I test at 150 it shows the best depth better

Tim,

I'm not catching what you mean by this.

 

[Tim Singleton]

I test at 150 it shows the best depth better

Tim,

I'm not catching what you mean by this.

It's easier for me to see what the group does at each depth at 150. It's clearer to me which depth the barrel likes than it is at one hundred

 

[1shot]

For what it's worth, I consider "jam" to be a square mark on the bullet. Lets assume the land is .052 across. When I have the bullet marked up with a sharpie for measurement purposes I start long and continue to shorten the seating depth until I measure .052x.052. I put that bullet in my comparator and measure base to the large side of the ogive. That is my "jam" or starting point. Down the road, it also tells me how much the throat has worn in "x" number of rounds. The last thing I want to do what some shooters call "just touching". I think we can all agree that touch can vary a lot, even between the same bullet of a different lot. I shoot predominately one bullet for each of my cartridges. If I pick up a different lot, I have something to reference between the lots. This has saved me a lot of time and components over the years and gets me to the center faster than always starting from ground zero.
i hope this helps,
Lloyd

 

[/VH]

I test at 150 it shows the best depth better

Tim,

I'm not catching what you mean by this.

It's easier for me to see what the group does at each depth at 150. It's clearer to me which depth the barrel likes than it is at one hundred

Oh, yards! I got it now.

 

[Dusty Stevens]

I, like other short distance shooters, refer to jam like tim and boyd. Its just like it says "jam" we back it off til the marks are exactly square and thats "touch". You can go from jam to touch to no marks in just a few thou according to reamer and bullet combo

 

['Freak]

Adding to the confusion; folk will say that they end up with .004” of neck ‘tension’ but they don’t say whether that means the bushing ID was .004” less than the neck OD with seated projectile, or no matter the size of the bushing, the resulting OD of the sized neck was then increased by the .004” from having seated a projectile. :-\

 

[tenring]

When I find the point where rifling marks from touching the lands show, I back off .005" then go forward .001" at a time. When I can see the beginning of touching with my 6x loupe, I mark it as kissing". From there its either jam or jump. Then I write it down.


Be advised that the marks may be very difficult to see on most factory barrels.

When you find the right neck tension with bushing dies, write down the bushing size. What it actually is, does not matter. You want to use or not use the same bushing next time.

 

[CatShooter]

Adding to the confusion; folk will say that they end up with .004” of neck ‘tension’ but they don’t say whether that means the bushing ID was .004” less than the neck OD with seated projectile, or no matter the size of the bushing, the resulting OD of the sized neck was then increased by the .004” from having seated a projectile. :-\

.... and the same with "2 thou of bump"

2 thousandths from where - is the case shoulder 4 thou off the chamber and you are going to add 2 more thou, or is the case a hard crush fit, and you are going to give it 2 thou less crush.

These "2 thou" of bump or tension are just numbers thrown around with no references... and mean nothing.

 

[BoydAllen]

When I refer to neck tension I have measured a sized neck, and the same neck over the largest diameter of a seated bullet. Also, I use a case that has reached the maximum that a particular chamber will allow as a reference for bumping my shoulders, so in my case, both numbers have meaning. The problems that you describe happen when shooters do not think through what they are trying to accomplish. I run into this a lot.

 

[Dusty Stevens]

And i think old competition shooters all use the same terms then when somebody hears that or reads it on the net they interpret it different. Its like that kids game where one kid tells something to another then they to another and when it gets down the line you realize theyre calling jam a touch and theyre guessing at sizing bushings without seating a bullet and measuring that. Its all interpretation of getting to a task

 

[C Valentine]

Whichever way you measure, the key is use the exact same method to establish your reference point every time. It may not be the same as someone else's, but it's the same for how you do it on your rifle with your method, your handling of the measurement tools, your lot of bullets and cases and your neck tension. If I have learned only one thing about this sport of precision shooting it is consistency, consistency, consistency! It is important in every aspect of reloading! My -.020 may be your -.018 or -.022.