6BR 105 Seating depth

[Obtunded]

Just a straw poll, I am currently trying to load develop another 6BR with a 1:8 6 groove. I am using Lapua 105's and I'm not seeing much consistency in groups between soft jam and 20 thou off the lands. Most shooters I have spoken with say their bullets like lots of jump, whereas my others have liked about 5 thou off.. What is your experience?

 

[Forum Boss]

Scenar 105s .010-.015" into the lands, That's measured from first jacket to rifling contact with a Stoney Pt.,now Hornady) OAL gauge. I've gone as far as .022 or so, but the ES/SD increased.

I've tried .015 off but the case pressured up more and the groups opened up a bit. This is with a PacNor 3-groove .236 bore.

Although every rifle is different I also think the realm of .005 off to .005 in,Assuming your zero point is correct) is the "no-mans land" where results can be erractic. This is because there are variances in bullet base to ogive measurements of up to .007" and even two or three thousandths with the best match bullets. Therefore if you're "just kissing" the lands, you may have some bullets that engage the rifling and some others that do not. That will give you pressure variables.

 

[Zale]

Editor, I got a dumb question. When talking about into the lands, does that mean seating them out however far past where they are just kissing? Does this mean they a "jammed" into the lands? If so, I would assume the bullets may move some from the original seating when the bolt is closed? Thanks for clarification... Z

 

[Forum Boss]

Editor, I got a dumb question. When talking about into the lands, does that mean seating them out however far past where they are just kissing? Does this mean they a "jammed" into the lands? If so, I would assume the bullets may move some from the original seating when the bolt is closed? Thanks for clarification... Z

There is some confusion regarding what is meant by "jam". That is why I defined the "zero" point to which I referred--namely the point where, using a Stoney point gauge, the bullet first touches the rifling.

Other people use the term "jam" to mean the point where you can't shove the bullet any farther into the rifling and it starts to push the bullet back into the case. Some folks call that "hard jam".

I don't like to use the latter term because it is NOT anywhere near constant from gun to gun, as it is very dependent on: 1) neck tension; 2) jacket hardness; 3) bullet profile; 4) land configuration.

Some people don't like the Stoney Point gauge and find that it is not particularly repeatable. I agree it takes a developed touch. However, I can take any of my rifles, and using the same bullet,saved from previous measurement), I can go in and get a "just touching" measurement that is repeatable to .002". And I have the entries in my log book going back to 2001 to prove it.

Normally, in getting the zero point, I will use the Stoney point with 3 or 4 bullets and measure each a couple times. You'll find one number predominate with maybe a .003" range. Pick the predominant measurement. That's your zero point. Then set your seating die for +.010, +.015 etc. from there,measured from case base to ogive).

The problem with using the "hard jam" as a starting point is that this might be .020" from "first contact" in some rifles, .030" in others. Also, depending on neck tension, your bullet might start pushing back into the case anywhere from .015" past "first contact" to .030". In my barrel, with about .0015" neck tension, the bullet doesn't start to move until it's about .025" into the rifling past "first contact". With less neck tension, the bullet will start going back into the case earlier.

Sorry to be so wordy, but I want to be clear about definitions. When the editors, on this board, refer to .00X" into the lands, we are referring to positive OAL longer than the OAL where the bullet jacket first touches the lands. First jacket to lands contact is our zero point.

 

[Zale]

Thanks for taking the time to make it clear... It allows me to understand... I thought I knew, but now I do know. Thanks

 

[Obtunded]

I have always heard,and used) the term "soft jam" in reference to the point at which the bullet fist contacts the lands...

When seating that hard, what issues,if any) are you having with bolt closure and what neck tension are you using in order to avoid pulling a bullet out?

 

[Forum Boss]

I have always heard,and used) the term "soft jam" in reference to the point at which the bullet fist contacts the lands...

When seating that hard, what issues,if any) are you having with bolt closure and what neck tension are you using in order to avoid pulling a bullet out?

At .010"-.015" into the lands, there are zero issues with bolt closure. Likewise I can load and unload the round without moving the bullet. This is with a no-turn neck and about .0015" neck tension,after springback).

If I've been shooting hot loads, the bolt will be a little stiff to close on fired, unsized brass--but that is headspace tightness, not a function of bullet position. I FL size my brass each time and my die is set to "bump" the shoulder back about .0015". With sized brass and bullets seated .015" into lands, I can "feel" the contact but it requires barely little more effort than closing the bolt on factory-loaded 6BR ammo which is "jumping" about .030".

One thing I should add--seating into the lands is not some special magic. Some bullets and barrels seem to work best jumped .020" or more. But for 6BR shooters with match chambers, I advise them to start .010-.015" into the lands. Often that gives very good results right from the start and you can back out from there. Berger VLD bullets, in particular, seem to like being in the lands, .015" or more. But if your gun isn't grouping the way you wish, you might as well try jumping the load.