The long and short of it- truncated cones

[callgood]

On my first attempt to reload truncated cone bullets, I had a perplexing experience. I had finished reloading 100 Nosler 135gr JHPs on a Forster Co Ax with a Redding Competiton seater. No problems. I had it set for 1.25" OAL and they were right on.
I then switched to the 180gr Winchester truncated cones and on the first one I backed off on the micrometer, thinking I would set the first one a little long and adjust down, in case it seated the two bullets differently. I adjusted until the first one was 1.25 and then set the second one. It was too short, about 1.248. This kept up. I would back off and set one too long, adjust it down, and the next one would be short.
After I loaded the 50 I had planned, I thought about it and decided the thing to do might be to seat them all long, then go back and adjust down and run them all thru again. This would be a PITA, but at this point I can't think of anything else. I am going to contact Redding and see if they have an explanation/cure.
Has anyone had this experience?

 

[Ol` Joe]

What seating stem do you have? My dies (Lyman, but RCBS I know sells seater plugs for different nose shapes) have two seaters, one for round nose and the other for semi wad cutter -truncated- bullets. I`d bet you`ve the wrong one in your die for the bullet.

 

[Mal H]

Your two stage approach might be deforming the tip or portion of the ogive where the stem is pushing of the adjustment bullet (about 2 thou). Then when you follow it with a single stage to get to 1.250, the seater now pushes down the same distance, but it is now .002 too low. I've seen that happen when I reload soft bullets.

Is the TC plated?

This is in the "well that's obvious category" - try making your first adjustment a little too high, (.002 obviously), and finish off the batch. Then go back and seat that one down a little if you feel the need.

BTW, If I get only .002 variance in a batch, I feel good about the session especially with lead bullets. The variations in the tips can be more than that. It's the exact amount of the bullet that is seated inside the case that counts more (even so, .002 isn't that much of a variation for handgun bullets).

 

[callgood]

The seating die is a Redding. The stem is domed, looks like it would be better for the Nosler JHPs I initially reloaded with "better" results. I'll see if Redding makes a flat stem for truncated cones, wad cutters, etc. The Winchesters are not plated, but jacketed. Next time I load, I'm going to run them through twice and see if that cures my "problem"- (they should all be so serious!). By then, I will have the results from the 5 loadings from this session, and will see if any one groups best. Thanks for the input, I think I may have learned something.