What difference

[deadwooddick]

does it matter ?

Using an L.E. Wilson micro-top seater on an arbor press;
—123 Lapua Scenar
—NEW Lapua 6X47L brass
—neck expanded using (oops, not Redding it's Sinclair's Gen-II ) mandrel on lubricated case necks…

I'm getting .000 to -.002 variation in COL, measured w/ the Sinclair nut.

What am I doing "wrong" ?

 

[potatoe]

That's probably the difference in the bullets themselves, nothing wrong with your seater.

 

[Shynloco]

That's probably the difference in the bullets themselves, nothing wrong with your seater.

+1. So separate then into groups of measurement and shoot them in those groups.

Alex

 

[mikecr]

If loaded in the middle of your tested seating window, 2thou shouldn't matter.
As far as the cause, It usually takes a lot of variance in ogive radius to affect CBTO.
The same radius shift causing a seater plug to contact nearer bearing, would also cause your nut to contact nearer to bearing. This, countering the variance(in this example), because the bullet would not seat as deep(because the stem is enclosing more nose) -but- the nut would read very near the same CBTO because the bullet nose is also seating deeper into the nut. All that matters is CBTO from your nut matching tested best.
I wonder about the lube mentioned though. Changing seating force causes changing seater stem wedging, which is independent of ogive radius. This can cause what you describe.

 

[deadwooddick]

… If loaded in the middle of your tested seating window, 2thou shouldn't matter.

Whew, that's good to hear.

The same radius shift causing a seater plug to contact nearer bearing, would also cause your nut to contact nearer to bearing.

Then sorting bullets by bullet base to ogive (similar ogive radii) would provide a more consistent CTBO ?

I wonder about the lube mentioned though. Changing seating force causes changing seater stem wedging, which is independent of ogive radius. This can cause what you describe.

FWIW, Redding indicates that expanding mandrel use requires lube.

So… I "lightly" lubed the case necks the w/ RCBS stuff on a J&J cue tip , ran them into the Redding and then w/ a saturated cue tip, wiping the lube off the interior neck surface. I hate that process, but see no alternative, as unlubed, the mandrel picks up brass leaving the case neck w/ the appearance of being lightly scored.

 

[mikecr]

Then sorting bullets by bullet base to ogive (similar ogive radii) would provide a more consistent CTBO ?

No, base to ogive is meaningless to seating and CBTO. The only way to match CBTO is to seat and verify, with every round, that you have matched it. This, just like you're doing.
There are basic steps taken to reduce errors here: proper fitting seating stem, correct neck preps and sizing, controlling consistent seating friction, minding case fill of powder, occasional annealing if working brass a lot, use of a Sinclair expander mandrel(pre-seating).

FWIW, Redding indicates that expanding mandrel use requires lube.

I am not familiar with Redding's turning/expander/mandrel tool.
But it makes sense to lubricate an expander mandrel with new necks. After firing you'll have a nice carbon layer that provides consistent friction(no lube needed). Maybe your seating to exact CBTO will be easier with this.

On occasion I'll be off by a thou in CBTO. It's pretty rare & I don't need to readjust anything for that. I guess this is due to my efforts in matching seating force(as measure with a load cell on my mandrel), with tension adjustments, prior to each bullet seating. I don't ever mess with or do anything to change the carbon layer in my necks, and my sizing is truly minimal. So in my case, seating force does actually correlate with tension. With the same seating forces, my seater stem consistently wedges to the same datum.
These are the kind of efforts needed for nats azz precision in seating.

You're already using the right seater die setup. Jim Hardy just mentioned mouth chamfering in another recent thread. This too can influence seating force as well as runout. And you're close enough in CBTO,, good start.

 

[deadwooddick]

No, base to ogive is meaningless to seating and CBTO. The only way to match CBTO is to seat and verify, with every round, that you have matched it. This, just like you're doing.
There are basic steps taken to reduce errors here: proper fitting seating stem, correct neck preps and sizing, controlling consistent seating friction, minding case fill of powder, occasional annealing if working brass a lot, use of a Sinclair expander mandrel(pre-seating).
I am not familiar with Redding's turning/expander/mandrel tool.
But it makes sense to lubricate an expander mandrel with new necks. After firing you'll have a nice carbon layer that provides consistent friction(no lube needed). Maybe your seating to exact CBTO will be easier with this.

On occasion I'll be off by a thou in CBTO. It's pretty rare & I don't need to readjust anything for that. I guess this is due to my efforts in matching seating force(as measure with a load cell on my mandrel), with tension adjustments, prior to each bullet seating. I don't ever mess with or do anything to change the carbon layer in my necks, and my sizing is truly minimal. So in my case, seating force does actually correlate with tension. With the same seating forces, my seater stem consistently wedges to the same datum.
These are the kind of efforts needed for nats azz precision in seating.

You're already using the right seater die setup. Jim Hardy just mentioned mouth chamfering in another recent thread. This too can influence seating force as well as runout. And you're close enough in CBTO,, good start.

Mike and others,

Many Thanks.