I am going to try and cover a lot regarding the magazines here. I am sorry about the novel, but hopefully, it helps clarify some stuff. The majority of your aftermarket chassis and stocks will use AICS pattern magazines. Could look at this as a industry standard.
Now for Tikka's for many years you would NOT find aftermarket stock or chassis for the long action options. Tikka action's are the same action no matter if the rifle is chambered in a SA or LA cartridge, and it is more of a "medium" length action. About 10 years ago, we looked at seeing if we could offer a LA option, which is the same inlet as the SA since it has the same action, but you could not use the shortest AICS magazine length, which is 3.715" due to the magazine being longer than the load port on the bottom of the action. If anyone were to run too long of COAL, quite a bit of the bullet would be hung up under the action. The next thing is due to so much of the magazine being under the receiver, it caused feeding issues even if the COAL was short enough. So we took a tikka action and milled out the bottom to see if we could then make it work and considered offering that as a free service for anyone wanting a chassis for them. That also led to complications, so it was a NO for any tikka-long actions.
Then, eventually, someone made an AICS-type magazine for long-action Tikka rifles, and that was MDT with the 3.56" magazine. So companies could start offering that option. Unfortunately, over the last few years, no other companies have made any magazines for them. So, the MDT 3.56 magazines are the ONLY magazines available for the aftermarket crowd. We have asked quite a few other magazine manufactures if they would and no luck so far. We and others have also considered using factory magazines, but that is an entirely different conversation, and there are always trade-offs.
XLR DOES NOT MANUFACTURE MAGAZINES AND, AT THIS TIME, HAS NO INTEREST IN GETTING INTO THAT MARKET.