100% agree to have enough force on the reel to snap 15lb line. The mag has to be touching.Yes, i was just trying to give you understanding how to set one up. Snapping 15lb line sounds like the mag is touching the spool?? How long does the reel spin for. Could try it that way. spin it, find on the mag where it spins for say 6 seconds, mark it with a pen. Should be almost impossible to blow up.
Fitted a few mag kits over the years and it's tricky to get it right.