If you had access to die or ammo sales, i would think that the 7mm Rem Mag or 300wm would be the highest of any common magnum. I would not want to know what ATRS has invested in bringing our the MH or MV, or what they would see it costing to go ahead with the MM. I seem to remember that they just invested $250,000 in another CNC milling machine.
I certainly could not blame them for going with the most common magnum chamberings, with the highest ammo sales. They need to recover their investment, and certainly have my admiration for going forward with any of this in these uncertain political times.
My thought with going forward with the 2.5" family is that it is a family of cartridges. 264wm, 7 rm, 308 nm, 338 wm, 358 nm, 416 taylor, and 458 wm. Much like the MH incorporated all the 308 win family.
I think that if one used some google fu, you would find that there are some semi auto firearms already out there using 338 Lapua mag, as well as the Chey tac calibers. Not sure if they are available in Canada, but they overcame some challenging engineering issues. Non of these are what most people would consider portable. Each probably require a bank loan for most people.