Try spraying some 'Easy Off' oven cleaner first. Spray on cold muffler and let it sit a few hours, and then wash it off.

This is a good trick to remove hardened rubber/etc. from stainless steel motorcycle exhaust mufflers. It will probably work well on your muffler tips as well.

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Currently driving: 2011 E70 x35i , 2011 E90 335d
Previous BMWs: 2007 E90 328i , 2005 E53 3.0i, 2011 F10 535i (6spd manual)