I have extensions on a contender and an encore, no problems with either.....However...I sent the contender frame into T/C for work on the "rimfire" pin. I had removed the hammer extension before sending it back. They did fix it, and under warranty , but sent a note back with the frame. Can't quote it, but it basically said : We see evidence of a hammer extension has been in use on this frame, T/C does not recommend using, may void the warranty. Take it for what its worth
Now for $75 , you can send your Encore frame to them and they will install a " swing hammer ", guess these are on the newer Encore frames. May be food for thought ! Good shooting