Impact of seating depth on nodes

[TC]

Hi,

If I've conducted an exhaustive ladder test and then moved on to fine tune loads down to a final choice but I took no account of my headspace length, will my identified node likely be broadly the same if I subsequently decide to seat my bullets just off the lands or do I need to re-do the entire process ?

I'm just trying to understand how sensitive nodes are to seating depth and identify the correct sequence of steps I need to take in order to develop a load ?

Thx

 

[AP2020]

..seating depth will affect your nodes....remember nodes are your OBT,optimum barrel time.

 

[NorCalMikie]

Depends on what your rig likes. Same load, just change the seating depth and see what it likes.
It's all a guessing came anyway.
I've changed seating depth from .010 into the lands to .010 jump to the lands and saw no difference. Next time it might make a "big" difference. You won't know till you try. And it may depend on what day it is. ;D No hard and set rules. YMMV.

 

[TC]

Right, thanks. If it makes a difference to nodes and a final load selection then it would appear to make sense to determine optimal bullet seating depth for every change in powder gr in a ladder test ie

42.2gr fired with seating depth say -0.002, 0, +0.002 + -0.004, 0, +0.004 etc
42.4gr fired with seating depth say -0.002, 0, +0.002 + -0.004, 0, +0.004 etc

Would that be the optimal way to determine the correct load/seating depth combo ?

 

[RonAKA]

I went through a calculation on my 6BR with a 26" barrel, 3400 fps nominal, and 1 ms barrel time. I wanted to find out how much velocity increase would be required to keep barrel time constant if I increased the seating depth 0.036". It worked out to be 50 fps extra, or 3450 fps. So in approximate terms a 0.010" seating depth change could be nulled out with a 14 fps velocity change.

My conclusion is that within a reasonable range, it is a pretty small effect, and if you worried about it, could be compensated with a very slight change in powder weight. And, in fact it may be to some degree self compensating. Deeper depth means less case volume, more pressure, and potentially more velocity. In the end, I don't think my chronograph can measure close enough to make it a viable adjustment process. I change absolutely nothing and I can't seem to get the exact same velocity from one time at the range to another.

 

[spclark]

Yes, theoretically there's an "optimum" seating depth for each tested charge weight, but the latter likely has more of an effect on accuracy than does the former.

The idea behind ladder testing is to find velocity points with different loads where one or more close loads produce little or no change in point of impact. Once that's been determined, moving seating depth in or out will indicate whether further improvements in consistent accuracy are to be achieved.

Headspace - at least as far as bottle-necked rifle cartridges are concerned - shouldn't enter into the equation unless it's either too much (cases move forward upon impact of the FP which reduces primer ignition consistency) or too little (where rounds won't allow the bolt to close).

Bullets seated long enough to touch or "jam" into the lands will mask short headspace problems, particularly where neck tension is high.

I used to think seating depth should minimize jump where possible but this past season was pleasantly surprised to find the cartridges & bullets I use work best when seated anywhere from .020" - .060" off the lands. When I do load testing now I start with seating depth of .030" as a result & once I find a reasonable velocity that groups nicely I'll try +/- .005" & .010" variations for fine-tuning.

Range conditions - temperature, humidity, even lighting - can all impact chronometer readings on different days, even across changes from morning to afternoon on the same day. Instruments are merely a means by which we gauge some performance parameters, while results on target are what we're ultimately most concerned with.

 

[RonAKA]

I used to think seating depth should minimize jump where possible but this past season was pleasantly surprised to find the cartridges & bullets I use work best when seated anywhere from .020" - .060" off the lands. When I do load testing now I start with seating depth of .030" as a result & once I find a reasonable velocity that groups nicely I'll try +/- .005" & .010" variations for fine-tuning.

What cartridges and bullets work best this way? What neck clearance do you run, and do you size the whole neck or partial? Curious as I always keep coming back to a jam, but I have not done a huge amount of testing.

 

[spclark]

I think it's too general a consideration for me to answer that question, but for me it's true with everything I shoot: 6XC & 6HAGAR with the Berger 105VLD, Palma with the Sierra 2156 (new Palma) bullet. Neck tension's typically .0015" - .002", neck clearance is never less than .004", necks sized with bushing dies at least .1" but not typically more than .120" - .125".

I really, really HATE having to clean my action of powder, on the line, when I've gone with a jam & for some reason a cease fire gets called & I have a round in the chamber. Happens every time.

For me, whatever accuracy advantage there might be with that is overwhelmed by the wobble in my hold and the aggravation of having to get up out of position to take my rifle apart & get all the bits out of my lugs before I can chamber the next round.

 

[Steve Blair]

I really, really HATE having to clean my action of powder, on the line, when I've gone with a jam & for some reason a cease fire gets called & I have a round in the chamber.

Amen, brother.