Remington.
This is what I have found during research, so not 100% if it's true or not: Remington has always made their own barrels. Sometimes they seek experience, guidance, opinions, and designs from major barrel makers for specialty rifle models with a specific purpose in mind, that require specially designed aspects of the barrels. Like the M24 barrels and Remington 5R MilSpec barrels were designed in conjunction with Boots Obermeyer, who designed the Obermeyer 5R rifling and the 11.25" twist. Back in the 1960's I think they also did some collaboration with Mr. Hart of Hart barrels.
Not sure of any other instances. But this is what I've found during research...But I could be dead wrong. Wouldn't be the first time.