I had a bad experience a week ago with the top end of my Mamba breaking. A small piece in front of the mainspring housing broke off. The gun would no longer fire which was probably a good thing at that point and the top end would no longer close onto the receiver.
Volquartsen told me to send it back. This on Dec 23rd and I figured even with overnight there would be a problem with delivery. UPS surprised me and everything went smoothly.
Today, Jan 4 I received my gun back from Volquartsen with a new LLV upper and new compensator. Great service from Ryan and all the people at Volquartsen.