The newer T3 models' receivers are long action, but are still chambered in short action cartridges. For example, you will see a T3 with its long action, and assume that it is chambered in .25-06, .270 Winchester, .30-06, 7mm Rem Mag, .300 Win Mag, etc. However, that gun, although having a long action receiver, may be chambered in .223, 7mm-08, or .308. In such a case, the bullet is shorter than the long action magazine, right? Yes. So Tikka puts a "stop" in the magazine so it is rigged for short action cartridges. I am not a fan of a "one size fits all" action, but Tikkas are still very well made guns and are good accurate shooters.
In the older Whitetail Hunter series, the Tikkas had both long action and short action receivers. I have one in 7mm-08 and love it. It appears that Tikka went with the one size fits all action in order to same money on production costs.
Zachary