Here is the adapter needed for the REX rifle .45/.50
https://donnyfl.com/products/gamo-tc45-m18x1-adapter-a59?_pos=2&_sid=cd57d7604&_ss=r
I recommend the Emperor .45/.50 M18x1 #S5B.
https://donnyfl.com/collections/ldc/products/2-x-10-5-inches-emperor-v3?variant=14050945105963