Indeed, or it could be that it was never in true to start with. Many rifle barrels have bores that are out of true with the circumference of the barrel blank, so if the thread is cut true to the outside of the blank, then it is not true to the centre of the bore, and clipping can occur. Try a different mod, if it still does it then you either need to get the rifle replaced, or get a competent gunsmith to recut the thread.