I found this post because I too am working with a light Remington Mountain Rifle(new SS in .280), and have found that it seems very finicky to form and wanted to see some insight to shooting this type of rifle.
I noticed this rifle has a very whippy front barrel at the shot. Laying prone, or on the bench, the rifle recoils up and not back. Maybe if I was shooting a bigger magnum it would push harder. Flinch is not my problem, I shoot 7mm's and even regularly my friends 300 Weatherby pushing 180's 3250. Not saying it can't happen.
I found a load capable of multiple 1/2 moa 200 yd groups using 168bergers around 2700fps. My groups generally stay within 1.25 moa with the occasional flyer, so I'm not complaining.
My response here is to acknowledge that I think form can have a significant impact on such a whippy gun, in my experience. A good trigger helps. That said, I don't think form along takes you from sub-moa to 3moa. Neither does slight changes to load development. Complete switch in bullets/powder, maybe. I have found that crushing over stretched cases can as well(particular to my belted 7mm). Most of the time I find this kind of problem is either 1) Equip: Loose actions, bedding, etc. or 2) Major flinch.
If you have time to have a buddy who can shoot, try it, that may tel you how much you contribute.
If you have access to a good gunsmith, I'd have them bed it and go through it if it has not been already.
Lasty, yes it could be the scope, but throwing another one on can also quickly eliminate that.
I've had this happen at least 3-4x and usually it's the gun or the brass getting too long without me noticing. I have a very reliable A-bolt that shoots the same bullets very well longer distances(.9moa to 100yds, but .6moa to 700yds), but it has a boss and if it comes loose at all, the groups at least double, maybe triple.
I'm going to post a question about form on a whippy gun seperately now so as not to hijack this post.
Brian