Is anyone else using the Lyman Mandrel die with 6.5 CM experiencing rather tight necks still?  With Hornady 143's set to AR10 mag length, I'm getting a lot of deformed bullets because my necks apparently are still too tight.  I even tested this with an empty casing, and it's still happening sometimes, so it's not powder compression.  I'm running Whidden dies, without running the neck expander.

Since I wet lube when I initially resize with the 0.288 bushing, I wet tumble them to get all the lube off.

After drying, I run a Lyman .260 Rem Mandrel (which should work for 6.5 CM).  But when I seat, it's pretty darned tight and often the seating die will deform the bullet at the end.   I don't understand, as this process works excellent in .223 reloading, but not so good here.

So I checked the mandrel vs the ball.  The expander ball (that I'm not using), measures 0.264" diameter.  Yet the Lyman Mandrel shaft only measures 0.260" diameter.  Is that normal?  I'm beginning to wonder if the mandrel that Lyman uses for this caliber round is too small.

Does anyone else have thoughts on this?  What are other people doing?

Ogive

Yes

Already done - not it

uh... No

First off what is you neck wall thickness?  It varies brand to brand!

I would get the right measurements first.

So Neck wall thickness X 2 plus bullet diameter.  
For an autoloader you want 0.003" neck tension..... subtract that from above figure
That is your bushing size

I would do this
Universal Decap
Size using the Sinclair Expander mandrel die to uniform the case neck
FL Size with the bushing die to bump the shoulder and set the neck tension

If you are using ss pins and such...you are creating even more resistance when seating the bullets because the case neck is TOO CLEAN!

Just to follow-up, I think it was a combination of wet tumbling, and the Lyman M-die mandrels are a bit small, making for a very tight neck with 0.003"+ of swelling once seated.  So I ordered a Sinclair Mandrel die and expander, and those open up the neck an extra 0.002" (ish).  For the sizing, that I do before the M-Die, I also ordered a bigger ring for my neck sizing portion (0.290", vs the standard 0.288" I've been running); but for now am going to stick with the 0.288", and see what happens.  I also ordered some of the neck powders, as an option to use while seating, though will sit on that at first.

I have a lot of already processed brass, so for the next few hundred rounds, will just try it with the bigger Sinclair mandrel.  When I get to seating some more bullets, I'll post on if that fixed it.

i use the sinclair mandrel and it's not been a problem