A good grouping means that, after shooting, the arrows are positioned close together and often touching on the target.
This can damage the nocks.
Learn how to replace the nocks on your DISCOVERY 300 arrows in this video.
Check that your arrow is in good condition when making repairs (and before every shot).
Never shoot with a damaged arrow, inspect the shaft and nock.
Upon the moment of release, a damaged arrow can break and cause injury.