Lionmarc makes a barrel for the Challenger 2, as fas as I know, and Accurate Armour made a resin one for the Chieftain and Challenger 2.
How different are the barrels of the Chieftain, the Chally 1 and the Chally 2?
EDIT!!!
I did my homework, the Chally 1 and the Chieftain have the same gun, Royal Ordnance L11A5 while the Challenger 2 has the newer L30.
So a Chieftain barrel suits the Challenger 1.