I have a question, it might be a stupid one though so bear with me. Could I buy just an action of a rifle, say a remington 700, if i wanted to build my own rifle. And would an action fit a barrel of the same caliber but different brand?
As to the first question yes you can buy an action without a rifle. You'll pay through the nose for it (for example Brownells - charges $400 for a new Remington Action - you must ship to an FFL), and you can get a new Reminton Rifle with the action for $400.00.
As to the second quesiton, I'm not positive, but think you should be able to. Individual company's actions are different, and their stocks are must be different to accept their actions, but I recall that they all have standard sized and spaced threads to facilitate replacement. If I'm wrong we'll both know shortly.
If you are talking about mating, say, a Winchester barrel with a Remington action, that basic idea anyway, I'd say no.
Each manufacturer has different length threaded sections for their actions, and the barrels chamber area is machined for their receivers, and bolt faces(different extractor cuts, shapes etc.).
So finding one from another manufacturer that would just mate up to a Remington receiver is doubtful.
Even were this to happen, you still have to get the chamber face machined to match the Remington bolt face, which would then require a finish chambering reamer to be used to complete the job of headspacing.