Re: Does bullet jump vary from bullet to bullet?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: hue001t</div><div class="ubbcode-body">i wasent going to ask but can someone break down bullet jump 4 me a lil?</div></div>
Jump is the distance between the point on the bullet that will contact the lands and the lands themselves when an unfired cartridge is loaded into the chamber. If there is any jump the bullet is not touching the barrel at all until when loaded and the bolt is closed. Thus when fired the bullet must "jump" forward over this distance without the barrel providing guidance/stability before it will contact the lands and begin traveling down the barrel in contact with the grooves. With zero jump (the bullet either lightly touching or jammed into the lands) the bullet is held in position by the barrel before it is ever fired. This space in the chamber between the end of the case neck and the beginning of the barrel lands is referred to as 'throat' or 'freebore.'
Issues with this first, obviously, are pressure. A bullet that is pressed against the lands and must deform and fill the grooves before it can move at all will obviously result in a higher peak pressure in the cartridge than a bullet that can allow a little of that pressure to subside as it is propelled into the grooves at some speed with very little initial resistance. Also there comes the issue of concentric circles. Ideally you want the bullet to enter the lands perfectly centered in the barrel. Whether this is best achieved by having the bullet stabilized by the lands of the barrel before it is ever fired (jammed into the lands) or if it is best achieved by allowing the bullet to "funnel" into the lands (relatively) free of the position of the cartridge is a question beyond my experience.
However different bullet designs will respond different to different jump distances because of the shape and profile of their ogieve (the point at which the diameter of the bullet matches the diameter of the lands and so will first contact the barrel) and the shape and angle of the lands.
Another issue to consider is that if you choose no or very minimal jump then you have left very little tolerance for error in your seating. Seat a bullet a little long and it may be wedged quite tightly into the lands when you close the bolt (possibly becoming stuck there should you choose to unload without firing) or load it a little short and you may have a load with markedly different pressure profile because it is not touching the barrel like your other shots.
Some benchrest shooters choose to leave all bullets seated long and then "soft seat" them by using the lands to push the bullet back to a consistent depth every time when the bolt is closed. Issues with this are that you must first have relatively light neck tension (so that the bullet can be pushed in relatively easily) and it must be very consistent (variances in neck tension, and thus resistance to moving, will cause the bullet to change how much it moves into the case vs how much it deforms and presses into the lands of the barrel.) Also when seated this way you pretty much MUST fire any cartridge you load because unloading will pull leave the bullet stuck in the lands as the case is extracted, spilling gunpowder into your action and requiring something to push the bullet out of the barrel before another cartridge can be loaded. That fact, and the relatively low neck tensions that make the ammo somewhat fragile for field use, are why most practical shooters don't soft seat.
