Tungsten comes in 6" and 7" and WC has made back caps for both, not sure about CK, but they probably do, too. Then again if you have a 17/18 torch, you can get stubby parts for the front that might make 7" not fit. Have a look at the parts book for your torch and see what is available.
WC has had at least 5 different length caps for the WP 20 that I know of.