First, read the following thread:
http://www.longrangehunting.com/forums/f19/i-can-hardly-bring-myself-say-ruger-51205/
Many of the "issues" that Rugers have are caused by people not understanding that some of the unique features of the design require an approach different from the more conventional accuracy fixes used on other actions. Pay particular attention to what is said regarding action screw torque and sequence. I would be surprised if that does not yield some improvement.
If you have a wood or laminate stock and feel that it needs bedding work, Brownell's sells the pillars and the correct cutters to install them in your stock. I have done this myself, so if you have questions, I may be able to help.
Next, start with one of the accuracy loads posted here:
204 Ruger
It doesn't matter whether or not you are using Nosler bullets. Just match the bullet weight and powder. Use that load to do a seating depth test like this:
http://www.longrangehunting.com/for...accuracy-berger-vld-bullets-your-rifle-40204/
This seating depth test works with any bullet, Berger or not. Once you establish the preferred seating depth for a given bullet, it will remain the same regardless of powder or primer used.
With preferred seating depth established, do your powder charge workup with your chosen powder. I like to use a variation of the technique posted here:
OCW Overview - Dan Newberry's OCW Load Development System
It will take some time and elbow grease to work through these suggestions, but a minimum of expense. If you go through all of the above and your rifle still will not shoot, then you probably have issues that can only be corrected by a teardown and rebuild (or getting rid of the rifle).