Don’t know how universally my comments here apply but can offer some experience on two rifles in these chamberings.
Montana 1999 in 6.5x55 – the 1999 has a 3.1” magazine but with the very long throat they give it that’s still too short to seat the longer bullets out to kiss lands and still fit in the magazine. Shorter bullets, specifically a Hornady 100 gr. A-Max, don’t even have their bases inside the case mouth if seated to an OAL which would touch lands. To its credit the rifle is still accurate with loads fitting the magazine but I have a nagging suspicion it can't meet its full potential hampered like that.
Tikka T3x .260 – This one is the classic .260 conundrum since it comes with a 2.8” magazine, you can load to 2.805 or 2.81” max. The relatively short Hornady 100 gr. A-Max is only .01 off the lands at a 2.8” OAL, even the Hornady 129 Spire Point is only .015” off the lands when seated to touch. But the 123 gr. A-Max is a full .090” off the lands at 2.8”. In mine the 100 and 129 grainers shoot just fine, half inch or better for 4 shots in fact (it is a Tikka, after all), but that 123 grain bullet needs another .08” room, it will shoot great but only if you load it too long to fit in the mag. I tried a long magazine to see if that would work – even in their "short action" i.e. short bolt stop the bolt comes back far enough to allow a 3.1” OAL and still pick up the rounds - but feeding becomes problematic with bullets tending to nose dive and shift around in the magazine too much.
As can be inferred here I've only worked with the three Hornady bullets so far, 100 A-Max, 129 SP and 123 A-Max. I would imagine heavier streamlined bullets and especially the really long VLD types would be even more affected.
I understand that the Tikka 6.5x55 uses their full long bolt throw and 3.4” (or 3.34?) magazine, so you should be able to seat the longer bullets out to hit the lands in that. In fact, knowing what I know now, the MRC 1999 would be the better match for the .260 and the full length Tikka a better match for the 6.5 Swede.
Sure do wish Ruger would see fit to chamber the 6.5 Creedmoor in their standard SA American, to include left handers. Or Tikka would chamber the Creedmoor in their short mag T3, that would be one super accurate and well matched combo.
