Thanks a lot Folks!

I makes much more sense.

What is also important is the ratio between case volume and caliber. A big case with a wide neck will require a faster powder, compared to the same case necked down. eg S335 is great in a 308 win, but S365 works better in .243 win.

You will be surprised at how much difference small changes in a case dimensions and capacity can make
Why some powders do better in certain cartridge cases
QuickLOAD will indicate this when doing simulations

Have a look at what the powder column in early 458 Winchester rounds looked like

What is also important is the ratio between case volume and caliber. A big case with a wide neck will require a faster powder, compared to the same case necked down. eg S335 is great in a 308 win, but S365 works better in .243 win.

You will be surprised at how much difference small changes in a case dimensions and capacity can make
Why some powders do better in certain cartridge cases
QuickLOAD will indicate this when doing simulations

Have a look at what the powder column in early 458 Winchester rounds looked like

I lapped the bolt face of my 204Ruger. I must have taken off less than 0.1mm, and this was enough of an increase in head space (case capacity) to completely throw off my load. I had to add another 0.2gr to get the load back on track.

What is also important is the ratio between case volume and caliber. A big case with a wide neck will require a faster powder, compared to the same case necked down. eg S335 is great in a 308 win, but S365 works better in .243 win.

You will be surprised at how much difference small changes in a case dimensions and capacity can make
Why some powders do better in certain cartridge cases
QuickLOAD will indicate this when doing simulations
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I think you removed more than you think, or there was some other factor involved.

Have a look at what the powder column in early 458 Winchester rounds looked like

I lapped the bolt face of my 204Ruger. I must have taken off less than 0.1mm, and this was enough of an increase in head space (case capacity) to completely throw off my load. I had to add another 0.2gr to get the load back on track.