Case Stretch

[beltfed]

We presume you have necked down 45-70 Starline brass???
Indeed, you may have thick neck walls.

I would always OUTSIDE neck turn. It will result in more consistant circumferential
thickness uniformity since you are turning against the pilot.
Inside neck reaming the neck is unsupported unless it is done by one of the old
systems where you ream thru an aligned hole inside a support die.

Also, FWIW, I have stuck with sizing down R-P or WW 45-70 brass for
my 40-65 that has fairly snug chamber neck. No Problem with stretching
and no need to trim them repeatedly.
PS, The bullets you sent fit my case necks well with the paper you patched them with. I hope to try them this weekend -tomorrow looks good for weather. Hope I can slip away for a while.
What paper did you use?
Arnie