Most other actions on the market offer two different sizes to accommodate the different lengths of ammunition. Tikka decided to make one medium action with a size in between what a short and long action would be.
To work with one size action, they shim the back side of their magazines to allow for shorter sized cartridges.
The only difference with the chassis is the size of the magazine well. For a .223, .308 or 6.5 Creedmoor you will need to select short caliber so that you can use .223 or .308 AICS style magazines.
For .300WM, 6.5x55 or 30.06 you would need to select long calibre. But because it’s not as long as a standard long action rifle, we had to produce a custom size magazine to reliably feed, they are listed on our website as the Long Action 3.560” length.