Jamming vs Touching

[jefats]

Been seeing a lot lately on jamming bullets into the lands various amounts. This was always a "NO NO" in the old days when you were taught to seat the bullet out as close to the lands as possible without the ogive touching them. So, can someone explain this jamming concept? What makes it more confusing is the various depths of jamming i'm reading. What do you get out of jamming that you can't get from seating so the ogive is just "touching" or kissing as we used to say?

Ready to be enlightened.



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[BoydAllen]

When you load to touch, or longer, which would be some distance into the rifling, with a given amount of powder, the pressure is higher, by 5-6,000 psi, than if you are loading shorter than touching, but if you reduce the powder charge, and work up your load with the longer seating depth, then there should be no problem, as long as you increase your load in small steps, and pay close attention to whether you are seeing and or feeling signs of excessive pressure. The reason that shooters do this is accuracy. I have found that once a reasonable powder charge has been arrived at, that there is a lot more to be gained by small changes in seating depth, than by fiddling around with small changes in powder charge weight. For instance, my loads for my varmint rifles work best when bullets are seated somewhere in the range of .006 to .010 longer than touching, as measured with the Sinclair tool that uses a rod, some stop collars and and action adapter, along with a bullet and a fired case. Do not simply change your seating depth without reworking the load. If you are already at maximum pressure with a seating depth that does not have the bullet touching, and you seat to touch or longer, you pressures may exceed the safe maximum. In order to work accurately with small differences in seating depth, you may need to buy some measuring tools. They are not particularly expensive, so be sure that you have the right tools for the job before you start.

 

[lpreddick]

i'm just now trying jamming after years of being in the "jump bullets to lands" school. i have usually seated bullets with .001-.002 thous neck tension. if i seat these bullets long to jam into the lands, i suspect they simply continue to be seated by the contact with the lands. how much neck tension is needed such that the bullet does not move further into the neck? is this latter situation what most benchrest shooters strive for or is there always some bullet movement? i know trial and error is possible but i have heard of the stuck bullet when extracting a jammed round. also, is slight runout corrected when the bullet is into the lands? if there is differing neck tension among several rounds, will accuracy suffer? thanks.

 

[BoydAllen]

The amount of difference in loaded and unloaded neck diameters is not the only thing that affects the amount of force that it takes to move a bullet in the case neck. The thickness, and hardness of the necks, and length of engagement in the neck also figure in.

First of all, to accurately work with seating depths, you need the tools necessary to measure the length of loaded rounds off of their ogives, with calipers that measure to .001. It also helps to have a way to easily find the length at which bullets touch the rifling, if that is the reference that you are going to use. Do you have these tools?

As far as pushing the bullet back, benchrest shooters use jam as a specific length that is where a bullet is pushed back to if loaded a little long. This length is specific to a particular barrel, and amount of neck tension. If you want to find out how much difference there is between touching and this length, first measure length (off of the ogive) that has the bullet touching the rifling, then load a bullet so that it is perhaps .020 longer, measure the rounds OAL (again, off of the ogive), chamber and unchamber the round, and then remeasure it. If the length is shorter after chambering you have the jam lenght for that bullet, barrel, and neck tension, and the difference between that length, and the one measured with the bullet just touching the rifling, is the range within you can vary seating depth seating into the rifling. Some of the longer bullets may have a tendency to stick when they are pushed back into the case, while determining what jam is. A light film of bolt grease on the bullet, in the area where the rifling will mark it, may help prevent this. If .020 longer than touching does not cause the bullet to be pushed back when the round is chambered, either resize that case and try loading a little longer, or use another sized case for a second try.

 

[Killshot]

I've understood that a jam will properly center the bullet in the bore, so that it's rotation is around the center axis of the bullet, leading to stable, accurate flight.

The amount of jam will affect the amount of pressure and velocity imparted on the bullet as Boyd stated. I believe this allows additional control (beyond changing the powder charge level) in finding the barrel's node or Optimal Barrel Time*

* more here: http://www.the-long-family.com/OBT_paper.htm