What Dan said. I've found that the lighter the rifle, the more attention you have to pay to consistency in hold, cheek pressure, how hard you pull the rifle back into your shoulder, forearm hold, etc.
I'm not JB either but light rifles have given me fits over the years. Everything must be consistent. Things I find myself doing - more/less cheek pressure, more/less pistol grip pressure. I pay attention alot to these two things because nothing irritates me off more than have a good group going and then grip the rifle different and have a bullet 1-2" out. These things come to light when you shoot the exact same load over repeated bench sessions.
I get it that shooting off the bench is boring - but do you think you aren't doing the same mistakes shooting from field positions? I shoot from the bench at almost every session, at least a few shots, to ensure consistency in the items we are discussing. I normally shoot from the bench to get my mechanics down, then shoot from field positions.
You'll find some days sub MOA groups, some days 1.5-2" groups mostly 2-4 together and 1 shot 1-2" away opening up the group. At least thats what I've found. I've also noticed subtle shifts in elevation between bench sessions. If you shoot enough and long enough and pay attention, you'll notice these things and hopefully work on them. Of course I could simply be a crappy shot
