please define

[G3automotive]

Can some one please tell me in detail what jam means in regaurds to seating depth .... i am a newbe an i thought if your up on the lands there is no more room to lenthen the bullet ?? An woodnt making it longer just push the bullet deeper into the case when u closed the bolt . Making it back to the lands ? I get confussed when people say .015 jam into the lands .... how is that any diffrent then .015 deeper in the case seating depth an to the lands ???? Help

 

[Alex Wheeler]

True jam is the longest you can load a round before the rifling pushes the bullet back in the case. But most people call anything in the rifling "jam", so .010 jam would mean .010 longer than the touch point.

 

[ShootDots]

True jam is the longest you can load a round before the rifling pushes the bullet back in the case. But most people call anything in the rifling "jam", so .010 jam would mean .010 longer than the touch point.

Now the first part of what ZFast is saying is correct as far as most people are concerned. However, that is a nebulous concept. Every "jam" using that definition would be different based on 1.) neck tension; 2.) type of ogive (secant or tangent) and 3.) the "narrowness" if you will, of the bore diameter. On the other hand, the 2nd portion of his definition is what I believe to be correct. ANY portion of the ogive, past the point of "entrance of the rifling", would be a "jam". Then it is no longer a nebulous term. Whether the projectile has a secant ogive or tangent OR 1K neck tension or 4K, if the bullets' ogive passes INTO the lands, it is considered "jammed" at that point. So a VLD (secant ogive) that is set 12K INTO the lands would be "jammed 12K"... That is the easiest most consistent way to state "jam". At least this is my humble opinion. However, with a light enough neck tension, ALL bullets will "soft-seat" when pressed far enough into the rifling. However, even that would not negate the concept of "anything past the entrance" of the lands would still be a jam..

 

[cocopuff]

and here I thought a jam was with the ogive touching tight against the rifleing.where it can not move out any futher..But your saying if it is backed out .015,that it is still in the lands,but not touching.....I can see where people get corn fused with this...

 

[cocopuff]

True jam is the longest you can load a round before the rifling pushes the bullet back in the case. But most people call anything in the rifling "jam", so .010 jam would mean .010 longer than the touch point.

if the bullet is already jamed in,then how can it be .010 longer than the touch point? it's already touching..??

 

[25AI260]

cocopuff
"if you jam".. look at your bullet at the ogive with a magnify glass an you'll see the little scrape around the bullet from the rifling .. the further you can jam the longer the scrapes...
...
"more neck tension maybe needed"

 

[johara1]

I do it the old way …… i find zero or as Alex said the touch point. This is beneficial for checking throat advancement and as a start point if i move in to the lands or" jump "away from them. Jam is a term is variable it is totally worthless………… jim

 

[ebb]

What 25AI260 said, Ive got some loads that have rifling marks and 1/8 of an inch long on the bullet. Eventually the bullet is going all the way down the barrel, and if neck tension holds out you can jam hard and then some.

 

[BoydAllen]

Back in the day, the term was used, and I believe originated by benchrest competitors and later used and corrupted by those who did not understand the original meaning. For this reason it is best not to assume that a reader knows what you mean whey you use the word, and elaborate in some detail what is meant.

The old school, and to me correct usage is as a noun, not a verb. It is a specific loaded length, that with the neck tension that the load will be shot with, is the longest that a bullet may be seated in that particular barrel, without being pushed into the case as the round is chambered.

This dimension is determined by seating a bullet long enough so that it will be pushed back, chambering a dummy, or if at the range, at the firing line, loaded round. Usually I record the seater setting that I loaded the intentionally loaded the long round, measure the long round, chamber it, remeasure it, subtract that measurement from the original and adjust the seater accordingly. If I want to be say .006 shorter than jam, I add that amount to the setback from chambering the test round, adjust my seater to produce that length, seat the bullet of my test round, verify that is measures what I want it to, and then record the seater setting. With an arbor press seater, with a non-micrometer top, it is the combined length of the seater's stem and cap.

The tricky part of using jam as a starting place is when you want to seat your bullets shorter than it would take to touch the rifling. For this reason its usefulness is greatest when seating some distance into the lands.

The difference in loaded round lenght between just touching the rifling and jam, depends on the shape of the bullet's ogive, the leade angle, and wear. Because of this, it changes over time.

One should also be aware that recent use of the word, has changed it to a verb, as in I jammed my bullets. This use is very imprecise and usually tells the reader little that is useful other than that the bullet was seated to a length longer than it would take to just touch the rifling. Shooters who talk about being "jammed" some large amount may not have even checked to see if their bullets have been set back in their cases as the rounds were chambered. For this and a number of reasons, I stick to the older, and to my view more correct use and definition.

 

[sroemer]

The start of "The Jam" concept is where the ogive of a bullet touches the lands. Determining that is another story....

 

[BoydAllen]

Generally, in the traditional old school use of the word, one does not even refer to where the bullet is touching.There is not an amount of jam, there is how much shorter than jam one is seating, assuming that you are not at jam, which can have the disadvantage of causing bullets to end up stuck in the barrel if a live round is unloaded. Which points to the advisability of using a dummy round, to find jam. (Ask me how I know this.) If for instance, I am seating a bullet .006 shorter than jam, I would say that I am seating six thousandths off jam, or possibly shorter than jam. Jam is a specific measured seating depth for that barrel, bullet, and neck tension. Using the word that way precludes saying that a bullet is jammed by a certain amount. If one were applying that convention, one would either say that one was seating so many thousandths into the rifling, touching , or so many thousandths off the rifling. If you want to make it easier to see rifling marks, which is another way to look at seating depths that are long enough to mark the bullet, pinch a wad of 0000 steel wool around a seated bullet, and apply a good amount of hand pressure while you turn the round in the 0000 a couple of turns. The little marks around the bullet will make rifling contact much easier to see, and won't hurt a thing. Finally, given the way that the use of the word has been corrupted, it is best not to assume what someone means when they use it.