Quilly, the main difference in Mauser type actions is if it is a large ring or a small ring Mauser action. There are a few variations of Muasers that have a slightly different action length, but this will not apply to you FN.

What you are looking for is stock your your FN, which is a large ring. The "ring" of the Mauser action is the forward part the receiver where the bolt lugs lock into and the barrel is threaded into the receiver. If I am not mistaken, the large ring has a diameter of approximately 1.350" and a small ring approximately 1.150". I am shooting off the top of my head so these figures may not be exact, but close.