You will need this adapter for the Taipan veteran.
https://donnyfl.com/products/taipan-veteran-shroud-adapter-with-1-2-x-20-unf-threads?_pos=3&_sid=bb5217e81&_ss=r

No plenum

The Sumo .25/.30 1/2x20 #S9 will be the perfect match for this airgun with no plenum.
https://donnyfl.com/collections/ldc/products/1-5-x-6-5-inch-sumo?variant=33070855487531

With plenum
With the Plenum I recommend the KOI .25/.30 1/2x20 #S38 with a plenum
https://donnyfl.com/collections/ldc/products/1-22-x-7-inches-koi?variant=6604174393383