Sizing new brass?

[centerfire]

I think transferring any inconsistencies to the outside of the neck is a great idea. I would however caution against using these expander mandrels in a semi-auto.

I have a similar expander mandrel by KM and use it to neck-turn new brass for a 6.5*47 bolt gun. My concern is these mandrels are designed to mimic a bullet so after being forced into the neck to expand it you can then neck-turn your brass with the correct ID as if it held a bullet. If you are using these mandrels for loading however this means the expanded necks will not be holding the bullet with any more than about .001" tension. For a semi you want at least .003" to prevent the bullet flying forward under momentum when being forcefully loaded by either hand or gas cycling, or being pushed back into the case. After using the KM mandrel I can force bullets into the necks and pull them out gain by hand which means they are being held by only about .0005". This is fine idea if using a bolt gun but not for the semi auto action of an AR.

Reading the blurb on the Brownells website it says that the Sinclair neck turning pilot is .002" less than a bullet, and the expander mandrel is .001" bigger. So, .002 - .001 = .001 which is the tension the neck will hold any loaded bullet with if you then use them for loading. Very similar to mine.

What actual measured tension does your brass hold a bullet after it has been through the Sinclair mandrel? Be interesting to see if it matches the blurb.

Just an after-thought...if you wanted to use this idea in a semi you could spin a mandrel in a lathe and turn it down another .002"

There is spring back so you get about .002" of tension. If you want more tension just chuck the mandrel in a drill and polish off .0005 at a time with 1500 grit wet/dry paper. I have my 308 gas gun mandrel sized to .003" of tension. 6mm has .002" of tension.

There are multiple sizes available from Sinclair too. With that said, .002" has always been plenty of tension for me with AR15 gas guns. I don't see any set back or growth with .002".

ETA: To be clear, the "Expander" mandrel gives you .002" of tension and the "Neck Turning" mandrel gives you .003" of neck tension. I started with all Expander mandrels and reduced the size of the .30 to the same size as the Neck Turning mandrel.

[centerfire]

I found this an interesting video. If you're ironing out case mouths, this has relevance.