The 18 pdr and 25 pdr Mk1 are different Ordnance construction completely.
18 pdr was made up from Breech ring,Wire, jacket and A-tube.
25pdr Mk1 was made up from Breech Ring,Loose liner Securing screw,loose liner ,Jacket ,Muzzle Bush and Muzzle bush securing screw.
Definition of Loose liner is when the liner is completely protected by the Jacket,A Loose barrel projects (exposes) at the front through the Jacket.
In the case of the 25 pdr Mk1 it has a liner rather than a barrel/tube.
Rob..................rnixartillery.