RE: "AT MY WITS END"
Sad to hear it's still not working.
Maybe the rifle just doesn't like all three brands of ammo you're trying, as all of us know, guns are very picky sometimes, but I will admit I've never heard of one THAT picky, which just goes to show my lack of experiance and wisdom in these matters.
The only thing I can think of is the ammo, in my rifle, Federal Powershoks shoot better than anything out of my Remington .308, and they shoot pretty well out of my dad's Browning A-Bolt.
I'd just try it.
I think the reason your friend suggested the muzzlebreak, is because Browning makes a special muzzlebreak that comes with some of their rifles, including A-Bolts, and I'm sure you've heard of it, the BOSS system.
That's what my dad has on his Browning A-Bolt, and it works good, and it only worked good when he tightened the action screws and made sure everything was in check on the rifle, but once he got that finished and used lock-tight on the screws, he was able to adjust the BOSS system muzzlebreak, andit made his groups smaller, like a cloverleaf.
It works, but I've heard alot of complaints concerning the noise the muzzlebreak BOSS does, so Browning has the BOSS CR (Conventional Recoil), which doesn't serve as a muzzlebreak, but does give you the benefits of greater accuracy.
Perhaps the BOSS system will help you, a BOSS should cost under $100 on eBay, and you'll have to get a gunsmith to install it for you.
That, if anything would, would help your accuracy, I'm sure, but it's all up to you on what you decide.