What's the difference between a Remington 700 short action stock and Remington 700 short mag stock ?

Barrel channel. One fits the sporter contour the other the magnum contour.

So basically a Lil help with a DreMel would fix the problem?

Yes, if the barrel channel is too small for the barrel. If the channel is too big for the barrel I fill in the gap with AcraGel then sand it for proper clearance.