I recommend the Tatsu .22/.25 1/2x20 #S31. This will suppress the Katran Long .22 while being short on the long barrel. This is coming directly from Utah Airguns recommendation, As i have not had one to test.
If length is not an issue the Sumo .22/.25 1/2x20 #S61 will also be a great suppressor on the Katran Long.