So you must be talking about pre-calibrated turrets like the Leupold CDS, Kenton, or Huskemaw
, which are marked in yards instead of an angular unit like MOA or mil.
In that case, you're right. Each turret is only good for one bullet at one speed at one altitude at one temperature at one angle in one rifle. Your turret marked for a 300WSM will be wildly inaccurate on a 7mmMag, or anything else for that matter. You have to give the turret maker (Leupold, Kenton, Huskemaw, whoever) information before they make your turret. That information might be just load data or it might be actual tested dope. Of course if you give them tested dope, it'll be a more accurate turret than just giving them muzzle velocity and ballistic coefficient.
That's why I don't own any of the pre-calibrated turrets. I understand their value, but their limitations turn me off to them. I'll stick with a mil or MOA marked turret and verified ballistic tables.