Your bolded question above - a chamber that is cut tighter than normal (a neck that is smaller diameter than SAAMI calls for) may not allow the neck of your brass to relax enough to freely let go of your bullet. Or your chamber may not have the free bore that the SAAMI drawing is calling for. I have reloaded thousands of rounds - I do not check every one, but every fired round that I check, a bullet will drop cleanly through the neck into the case - not pushed through, it drops through. When your cartridge goes all the way up into your full length die, the neck is squished smaller than it needs to be. As you withdraw the case out of the die, it is pulled over the expander ball on the primer stem. That expander ball should be "opening up" your neck to about .002" or so smaller than the diameter of your bullet - in your case, your resized case should be about .282" inside diameter, all the way through, before you seat a bullet. When you seat your bullet, that will force the case neck open just a bit more - your bullets should be .284" diameter - that .002" or so tension is about the correct amount of "hold" that you want. So, if there were any abnormalities in your case neck thickness, they would show up on the outside diameter of the neck - right from the mouth of the case, all the way back to where the shoulder meets the neck. The SAAMI drawings for the 7mm STW gives that cartridge dimension as .3170" all the way from shoulder to mouth on the cartridge. If any part of your loaded cartridge's neck is larger than that, the neck needs to be turned, to be made thinner. At the same time, the SAAMI drawing calls for a diameter of .3200" at the junction of shoulder and neck and .3190" at mouth of the case for the rifle's chamber. If the chamber is cut to a smaller size, then the "standard" brass can not expand enough to allow a bullet to pass through after it is fired. That is why you will want to have a cast done of your chamber - you want your loaded brass necks about .002" smaller than whatever your chamber is, in the neck area.
As far as using information - they gave a recipe and tested it using their components in their rifle while measuring the pressure that they were getting. Especially that you now have reason to doubt that is a standard factory barrel and chamber (no marks on it), you will have to approach it as a "wildcatter" would - invent your own data, using information from other similar cartridges. But start by knowing how your chamber is different (or not) than a factory standard one.
Another thought occurred - SAAMI also specifies a maximum brass length - in this case 2.850" - .020" If your brass is longer than 2.850", the leading end of it might be squishing in on the bullet, making it harder to let go.
go here:
https://saami.org/wp-content/upload...99.4-CFR-Approved-2015-12-14-Posting-Copy.pdf Then go to Table of Contents and click on 7mm Shooting Times Westerner - top half of page is drawing for cartridge, with dimensions; bottom half of page is drawing for chamber, with dimensions.
