Springs are not the same. The lower portion of the 13 round follower can be ground down on the outside so that it can nest inside the 13 round spring rather than the spring nesting inside the follower. The top eighth inch of the follower needs to remain unground in order to create a shelf on the follower for the spring to rest against. You will probably still have to cut one or two coils off the spring to give clearance for the fifteenth round. If the newly created follower shelf spring stop is placed too high, the follower will hang on the top front of the mag.