Internal dimensions of .260 Lapua case vs. .260 Remington

[RedRyder]

I have new Lapua brass that I'm going to use to try to improve my groups. Although, they're pretty good to begin with. Having said that, I've always used Remington cases. My typical load for the Rem cases was 42.0g I4350. H4350 shoots just as well. But as I was going to start weighing out charges, it occurred to me that the internal dimensions of the Lapua cases might be different, i.e., smaller, which would increase pressure exceeding what I'm generating with the Rem cases.

1) Am I overthinking this?

2) If I'm not overthinking this, how should I proceed? 10% less powder seems like quite a bit

Thanks for your considered responses.

 

[ShtrRdy]

From my experience I ended up very close to the same powder charge. I would load a few rounds at 0.8 gr less than you were using and a few at 0.4 gr less. You could load a few at the same charge weight just to see.