Razmuz,
Just a few questions if you'll bear with me.
What exactly is "won't shoot"? 2", 4", 6" and at what distance?
Are you using factory ammo or handloads?
What weight bullets? How many different brands?
Have you checked the action screws for tightness?
Have you checked the barrel channel for pressure points?
How bad is your trigger pull? I know the M77 MK II's are pretty awful in this regard, and it can make a difference.
What bench technique are you using, if any? I find best results with skinny barreled rifles if my rest is placed under the magazine, thus relieving any pressure on the barrel.
I have quite a few Ruger rifles, and most are adequate in the accuracy department, giving an average of 1.5-2" at 100 yds with my handloads. I have a Compact in .260 that I'm working with now, and it is very finicky on powder/bullet combos. I have a friend that has a rifle like yours in .257 Rob'ts that he loves, because it will shoot factory ammo into 1.5" at 100 yds, which is above average for modern production rifles. You are correct that these rifles do not come with factory iron sights, and if this barrel shoots, I think I would keep it, just for the oddity.
If you handload, you may want to try seating the bullets out to within .010 of the lands, as this sometimes helps improve accuracy.
Anyway just a few thoughts from someone who has been frustrated in the past by factory rifles.