OP, I'm sorry if this slightly derails your thread; not meant as a hijack but to shed some light on your idea.
Unless I'm horribly mistaken, all Tikka T3 rifles use one action size; the bolt stop is what dictates a Tikka to be long or short action (and appropriate bolt face diameter for the intended cartridge.) Magazines I believe share the same footprint but have internal differences (spacers for .223 for example.)
With Short action cartridges in the tikka t3, it's as simple as having the right bolt stop installed for the bolt throw length you need; this is dictated by Cartridge overall length, magazine style (tikka .223 mags for example hold rounds far forward whereas AICS mags hold rounds about 1/2" further back) or a combination of these scenarios.
Where things get a little tricky is when you try to use AICS long action mags in a Tikka T3 action; I'm gonna use .300 win mag as an example because of the build I'm doing and some issues / fixes I found. With AICS long action mags, you have a CIP length Cartridge Overall Length of 3.7" ish to play with. With the tikka, you can only run a COL of 3.6" or less and need to modify the mags to make them work reliably. My solution of choice is to run an MDT chassis and use their proprietary tikka long cartridge mags (3.560" COL) and handload them at that mag length.
Just one more thought, I understand the desire to have a DBM system that uses good quality / steel magazines; does it have to be a CTR / Sako TRG based magazine? I only ask because AICS mags are easier to find / possibly less expensive and offer the same advantages.