I have a longer end cap for my shadow IV that is actually a whistle. It looks exactly like the CRK one, only its longer. I believe it was made by TAD gear. The extra ength is not really needed on the shadow IV, but the whistle option is cool, especially as an extra survival item. They may have made them for the smaller CRK OPR, but I am not sure. A good machinest could make one for sure.