Can the Hodgdon load data be that conservative on their loads? They list 41.1 as max for the 140's.
I'm maxed out at 2.833 OAL with the normal tikka mag
I see that, but they also list up to 43ish grains with 150/153gr bullets too
I recommend the M+ mag for the 260, you can really increase your loaded cartridge length which helps get more powder in the case, and shortens the long jump to lands which robs some pressure / velocity
https://nechakooutdoors.ca/tikka-t3x-magazine/