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Well, after jimm asked me to check this thread out I will add this....
1. Belted ammo varies widely in measurement from the case head to the belt (which is where SAAMI measures headspace)from manufacturer to manufacturer. Win & Lapua are typically smaller and Federal is thicker. This is why most smiths say to neck size and headspace off the shoulder for more consistent accuracy.
2. Since the 300 Win Mag headspaces off the belt SAAMI doesn't have a set dimension for the shoulder and reamers can vary a bit in this area, as can brass....I have seen Lapua brass be as much as 20 thou short.
3. Since there is no "datum" line set for the 300 shoulder any measurent from a Stoney Point tool or a Redding indicator does not tell you how short or long your fired brass is.
4. The brass will be different, especially when it was used in a ladder test with different powder charges, resulting in different pressures. Plus as others mentioned...different case capacity, wall thickness, grain structure, etc. All of these will determine how much or little the brass flows durng the shot and how much it springs back.
Now with this said, the Redding or the Stoney Point tools will tell you how consistent your brass is....and that is the name of the game....not that it is 1 thou or 10 thou back.....once you find something that works you want to duplicate it with every shot and those tools let you duplicate the headspace of every piece of brass to insure the needed consistency.
And just to stir the pot some more....the bump and chamber method is very inaccurate due to the plunger type ejector of a Remmy pushing on the case and the camming power of a Remmy bolt. It requires a "feel" and can be used to check shoulder bump but is not accurate for how much you are bumping it....
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Just to add to Chris' last comment...the shoulder may also not be what you feel binding...so you may have already pushed the sholder back a bunch, but the case may still chamber hard due to binding somewhere else...
JB