Salopian,s assessment ot the gun trades out-sourcing practice is excellent. Even the great Joe Manton out-sourced the Forging of breech blocks for his stanchion guns to Fuller. All of which and much more on the trade between Birmingham & London is detailed in the Diaries of Colonel Peter Hawker.
Other factors that forced increased out-sourcing on many makers were;
1/The introduction of;steel to replace damascus as a barrel material.
2/The success of various patent actions.
It is obvious that production of steel barrel blanks was out of the question for individual gun makers. At the same time if a customer demanded a specific action for a new gun[ie; Anson and Deeley] many gun makers would purchase actions, "In the white", for finishing in their own works or alternatively a complete gun bearing their name; rather than risk loss of business.

Roy Hebbes