Samari46, I found that same stuff about a year and a bit back at a local industrial supply place. I've used it for a few reloading die projects now and it's been really handy. Next up is to use it with an O1 drill rod tool steel insert as a custom sizing push through die. Just got to get some real life house type things done first....
Overmax, I've found that sometimes folks that do things a bit differently reach good results because they follow a consistent approach to their "different way". I thank you for your detailed posts and I've saved them to my "paper patching notes" directory for careful consideration in how I go about all this paper patching in the end.
The rounds I'm shooting are 38-55. So a slightly tapered "straight wall" casing. I only partially size them at present to get the nose portion of the casing down to size so that when I push in the opening and flaring die I get a nice parallel neck ID for the length of the bullet. Or at least most of the length of the bullet. I do crimp but only enough to straighten out the very slight mouth flare, not to actually bite into the bullet. I found I needed a slight flare or sometimes the edge of the mouth would "plow" some lead off the riding bands. And in the case of PP'ed boolits it might tear the paper. I'll likely continue to do this flare thing but perhaps modify it to where the flare still fits within the chamber and does not require it to be crimped back in?
The custom made ID neck sizer and flare insert I made up is 0.3810. The cases spring back and come in at .3800 to .3803'ish I assume differences in this spring back are due to different hardness levels of the cases. I may try some lower heat level annealing such as done with shouldered rifle casings and see if that results in a more consistent size. But on the other hand that's pretty close already.
I can't find the slug I made for this barrel at the moment. So I'll likely have to make another. But I'm pretty sure the groove diameter is .381. And I seem to recall that the bore ID was .379? Or maybe .378? I'll have to slam another round ball into the muzzle and check...... The regular cast boolits I'm using at present bumped out to .382 are fitting into the chamber nicely. Neck tension on these (I know this isn't paper patching yet but still.... ) is firm but I can push them in with thumb pressure without leaving a long lasting divot in the end of my thumb. Is that about what you are looking for with your paper patched loads?
I'll likely still need to slightly size down then open up the necks. Otherwise the fire formed cases straight from the rifle are tapered and the next bullet in even if it does have a whiff of neck tension will only be touching right at the mouth. But like I say I aim for only enough of a sizing reduction that I can generate a short parallel portion for the bullet to rest in at the mouth. Checking a few cases ready for powder and bullet shows that I'm getting roughly a 1/4" long parallel portion behind the mouth of the casings. So a touch less length than the lower bore riding portion of the bullets I'm using.