Re: sendero 300winmag
Most factory LR rifles cost from $500 to $1000 or so, Savage vs Weatherby might be the range. Unfortunately we cannot depend on the factories putting out the absolute best designed rifles with the best available materials - dollars dictates design, manufacturing techniques, assembly and materials used. Buying a factory firearm is a crapshoot, you might get lucky and get a tackdriver but there are odds against it. If you are handy you can usually improve the base accuracy significantly with time and patience.
Sako and Tikka rifles are frequently superb out of the box performers, I would give them some consideration as well as a Winchester Stealth or Coyote in .308 Win. For even less money you could consider a Savage tactical or LR model, but you would probably require an aftermarket trigger and stock to make it perform to its potential.
Buying a good LR factory rifle, like a Sendero, Stealth or others can be a wise move if you are just getting into LR, your personal ability might need a lot of honing and a factory LR can be a good learning platform. Many shooters on this forum suggest getting into this game with a good .308 Win., learn the basics and then go from there into the big boomers. Many shooters have also learned the hard way that you should spend about an equal amount on optics as you did for the rifle (I learned that up from this forum).
Custom built rifles should be significantly better in many ways - not just accuracy. The magic that is performed by a good gunsmith is not something picked up from a book or just because you know how to run a lathe and mill.
Custom built LR rifles are better, and they should be because you are looking at from $1500 to the sky's the limit. Get someone like George Gardner to build a LR rifle and you are looking at $2500-$3000 and the rifle is worth every penny. He will accurize old Betsy for much less but we are talking custom here. These guys on this forum can recommend some excellent smiths, depending on what you want, where you live and how much money you can spend.