I too have this same struggle. I'm trying to launch my Berger 185 VLD's just off the lands and can't with the magazine restrictions.
I'm also looking to fit in a Manners T4 stock for the Tikka, and lastly, I'm looking to use the CDI bottom metal with AI mags for the 3.65 COAL like you are.
Since there are 2 action lengths restricted purely by the bolt release (everything is actually on a long action) my thought process is this.
If I sell my 300 Win Mag and buy a 300 Win Short Mag...I'll keep the same ballistics which is fine. But in theory, I will achieve what I'm looking for. The ability to seat my bullets to the lands.
If I use a 300 WSM
, (medium action) I can file down the bolt release (or you can buy the full length for $65) so I have the full length of the long action at my disposal but with a much shorter brass length (300 WSM vs the long 300 WM). I'll have the full length of the long action CDI bottom metal and mags to seat out my bullets to whatever length I want.
Do a web search for "Tikka T3 Bolt Stop Modification" and click on the first YT link and watch it.
Am I missing anything? It's quite possible I am, but as long as I purchased a long action mag for my medium length caliber (now working in a long action due to the bolt stop fix) it will give us our desired overall length.
Please let me know if this makes sense...it makes sense in my head but that doesn't mean anything!