Quote from BR Central:
"The first, and simplest, method for forming 6BR Improved brass is to seat a bullet long into the lands, with strong neck tension, and then fire the case with a near full-power load. Make sure the bullet seats firmly in the lands so the case can't move with the firing pin strike." To each his own.
There are lot of old wives tales out there. Here is the point:
Premise: Unless there is no head clearance (space between the boltface and cartridge head, sometimes called "headspace"), you will not do the best job of fireforming.
Test: If you think you have sufficient neck tension to achieve this, seat a bullet a bit long and close the bolt. If you can close the bolt, then there wasn't enough tension.
No one I know has ever been unable to close the bolt.
But consider a false shoulder: If you get it too "long" you cannot close the bolt. You have to push it back a bit more until the bolt will close.
Now if anyone doesn't agree, they are essentially claiming the premise is wrong. That's really what most arguments are about, one person makes an assumption, often unstated, that the other disagrees with.
If you disagree, I consider the following:
We all know that the energy in firing a primer will move the case forward in the chamber -- that is, if it can move. Take a case with a bit of excessive head clearance. Fire it. The primer looks flat and a bit uneven.
See if seating a bullet long takes the splattered look of the primer away. Now try it with a false shoulder, tight enough it takes some effort to close the bolt.
What has happened is the primer has remained against the boltface, whereas the case has moved forward a bit Then when full pressure was reached, the case moved back, fully reseating the primer. But in the intervening milliseconds, pressure has expanded the primer a bit, so the "reseated" primer is enlarged, flattened, distorted, whatever you want to call it.
What those of us who say seating the bullet hard is insufficient are claiming is that fireforming is a dynamic process. How the case reacts to pressure over the time it takes to fully build, can be variable, and the final results show that variability. If you want to take forward movement of the case out of the equation, you can't get there by just jamming the bullet. The case shoulder itself must be used.
Proof comes from firing a set of cases with excessive head clearance (before fireforming) and measuring the clearance after. If one set has less variance after, then that was the better technique.
Benchrest shooters (who rarely shoot at the ten ring) think even, minimal head clearance matters. Whether or not it does is a whole different argument, including getting into pretty much untestable waters, the effect of varying bolt thrust on accuracy.