The M77 MKII and the RAR and the Ruger Precision all use the same tenon and threads. I have no experince with the tang saftey M77 in terms of rebarreling one. Keep in mind that Ruger does not use a washer type recoil lug and has flat bottoms on most of their actions prior to the RAR/Precision.
You can add a barrel nut to any action if that is how you want to roll!
The RAR is machined from bar stock not investment cast so that plus the stock use of a barrel nut makes it more user friendly to pre-fit and more likely to be more precise.
Since all surfaces on the RAR are machined into the receivers round bar stock it is faster and cheaper to make and you are more likely to get a product that is less in need of major re-machining. The fat bolt makes for less surface interactions and less total machine time and simpler machine operations. The fewer times you need to switch machines or re-position from one fixture or jig to another the less tolerance stacking occurs. So the fewer operation you need to do and the fewer specialized machines being used the more likely you are to get CNC as an option. It is a classic example of Remington 700 and the saw cut receiver face instead of being precision machined.
To hit price point you have to be mindful of machine operations in total. Man hours. Machine hours. Profit margin. Price point you want to hit. If the design is a very smart design you can build it cheaply and have hi levels of precision. You have to really put a lot of thought though into what is a must have and what is not a must have. The simpler the design the less machine time you need and the more precision you can design in because you can now afford to use more very precise and consistent machinery.Intricate designs with lots of complications have to go. Often a part that would be made from a forging with lots of machine operations like a bolt body with intergral lugs and handle will be broken up into a lot of tiny parts made from castings, billets, round bar stock, MIM etc and built up into a finished bolt from many times the number of parts which greatly reduces cost!
Keep in mind the Remington 700 and the Savage 110 were the Ruger American, Savage Axis of their day. They were both designed to be much cheaper to make than the Mauser, Springfield 1903 and Model 70. Sadly today they are clearly not cheap enough and do not have the profit margin that the OEM's think they need in spite of costing 3X as much as they did 20 years ago with terrible surface finish, cheap stocks and lower precision and quality control compared to 20 years ago. It has always amazed me how the companies in Japan that make rifles manage to make them with high end materials and superior precision and quality control compared to the USA and can sell them at such great price points to others that important them under different brands and sell them at a profit! Japan has no natural resources and has to important every raw material. They have high rates of pay, high taxes, strict environmental laws, and cost of living is high as well. Japan is not China or Russia.
