Paul,
On this exact mission a few years ago, I purchased some wadcutter ammo once fired brass, two
different brands. I carefully measured how far down inside the parallel sides went, using pin gauges and it
was clear that this brass was a bit different than much of the common .38 Spl brass out there, much of which
started tapering internally fairly high up.
I grabbed some of my new .38 Spl brass by Starline......AHHH! Their brass is just like the WC brass,
plenty of depth of the parallel sides to take a normal full WC seated flush without sizing down the
base of the bullet. So, unless they have changed things recently, Starline is good.---as usual, IME.
Hope this is the info you needed.
I have some Starline .357 brass, too, but have never checked it for internal taper. Perhaps I should.
Bill