There would only be few of us interested in a non-threaded, sporter weight barreled 300 RUM. Even fewer people who didn’t want a Wyatt’s mag box.
Personally, if I were going with a gunsmith build, I would want to pick out all of the pieces myself. However,
@p_ham appears to be wanting to build a spec gun that would appeal to many. Throw together a 6mm Creedmoor, a 6.5 Creedmoor, a 6.5mm PRC, or a 7mm PRC in a rifle described by
@Wind Gypsy, with a couple of stock choices, shoot a 1/2” to 5/8” group with Hornady Match or Precision Hunter ammo, post a photo of the target with the rifle, and, assuming a decent price point, it would not last long. I would stick with the OEM Tikka sporter profile out to the point the muzzle flares so it would be easier for people to switch stocks in the future. Having the option to order a second barrel at the same time would be a plus.