I have converted a Feinwerkbau P70 from 7.5Joule 10 meter to 16 Joule FT version. See my website: https://airgunaccuracy.wordpress.com...-field-target/
Once you have ordered a 20 Joule FWB reg spacer spacer and absorber cover plate it is very straightforward. Allthough a reg tester might come in handy. But read the website and you know anything you need. The most difficult part is the scope mount. But I'm sure Jon Harris will help you out with that.