Seating Depth question (Jam vs Jump)

[rchouser]

Ok, I have moved into the 21st century. No more candles and jam with the bolt, then measure. I bought into the complete stoney point (now hornady) comparator and OAL guage tools.

I understand jump (from contact with the lands out). My question is on Jam (or jamming).

If I set the bullet length out further (increasing OAL) from where I already have bullet contact with the lands, won't I just push the bullet back into the case on bolt closure?

How consistant would this be if one jacket was more willing to be cut by the lands than another jacket. Your discussion, opinions, and experience would be appreciated. thanks rc

 

[clannagael]

I've often wondered about this myself.

I've heard guys at the range and at competitions saying "I'm seating .02 into the lands." But my experience with neck sizing to .002 and .004 tension leads me to believe that there's no way that there's enough neck tension to keep the bolt from seating the bullet further into the case. Even with Bergers that seem to have a thinner jacket. I tried an experiment once - I removed a .223 barrel from a Savage then chambered a couple of rounds by hand (obviously) with different bullets and all of them just seated further. The Bergers had definite land marks on them. The SMKs and Hornadys had marks too, but not as pronounced. Either way with light neck tension all of the bullets just seated further back.

While not exactly a controlled scientific experiment, the results were sufficient for me to believe that the optimum seating depth is just touching the lands.

Just my $.02 USD

 

[whiskey08]

definitely depends on neck tension, which ever is less the case or the barrel. At some point you might not be able to close the bolt. I barely size the end of the neck and jam a few bullets to find the lands. My bro brought over his stoney point guage, in every instance the stoney point was jamming the bullet farther into the lands then me using just a little neck tension then closing the bolt. He would never believe that my techique was accurate, it ended up being more accurate then his gauge.

 

[BoydAllen]

Use your Hornady tool, and a case with the drilled and tapped primer pocket to come up with an OAL, measured on the ogive, that is just touching the rifling. Load a dummy round that has the neck tension that you normally use, and has the bullet seated out say .030 longer (measured off off of the ogive). Now, making sure that you have applied a thin film of bolt grease to the back of you locking lugs, chamber and then remove the dummy. A slight film of grease on the bullet my help keep it from sticking. Remeasure the dummy round to verify that it is shorter than before you chambered it. Note the difference between its OAL and that of the one that you gauged to just touch. Let us know what the difference is. The length that chambering pushed the bullet back to is the jam length for that barrel, neck tension and bullet. I usually back off about .003 from that to do my pressure test series, to reduce the tendency of bullets sticking in the throat if a live round has to be unchambered.

The old school, and I think correct, use of jam is that longest that you can load a particular bullet, with a given neck tension that will not result in the bullet being pushed back in the case when the round is chambered. A seating depth that is somewhere between there and touch could either be described as so much longer than touch, or so much off jam. Different neck tensions, neck thicknesses, neck hardnesses, amount of shank in the neck, throat angles and bullet ogives result in different distances that a bullet can be seated beyond touching, without being moved when chambered. Use which ever system that gives you the most consistent results.

 

[daj]

I just drill out the primer pocket of a 2x fired case (ie one with correct headspace) and use the depth probe of a vernier calliper to push an inserted bullet forward until it kisses the lands. Add that vernier readout distance to the bullet length to get OAL to the lands or add it to a comparator bullet length to get the comparator OAL to the lands.

With Berger VLDs and Amaxs I add 20T to the above lengths for a 20T jam. Ejected unfired rounds still measure the same OAL - ie no push back, with a neck tension of 1.5T or more. Even Scenars with a more "abrupt" ogive do not push back.

YMMV with your barrel.