Tikka now makes an 'M+' magazine that they are shipping with T3x rifles chambered in 6.5 Creedmoor. They are the same as the 'M' size magazine except they have a shorter spacer in the back and longer follower to suit. It should be possible to buy these and swap out or modify the bolt stop to suit as discussed in hughston's post above.
I have been looking into this topic. The internet consensus on T3/T3x magazine internal lengths seems to be roughly as follows:
M (308) 2.83in (71.8mm)
M+ (6.5 Creedmoor) 2.98in (75.7mm)
LG (30-06) 3.37in (85.6mm)
CTR 2.92in (74.2mm)
There is also 'SM' for 223 and similar size cartridges.