Severql things can be going on, and all of them at once too, so as you make changes, do one change and then shoot the gun enough to see how that actually helped, or made no difference. I have four of these and have been through a good bit getting them to shoot.
Canting....... this is where we hold the gun at different angles at each shot fired. You must be certian that each thime the trigger releases, the sights are not only on target, but the crosshairs are vertical/horizontal each time. Having high blood pressure, I watch the scope move around every beat of my heart. It is interesting to feel the trigger release, and know that shot is off
Gun rest....again, be sure that the gun is rested in the same position on your bags each shot. I shoot using two folded towels over a fire brick for a forearm rest. With eash session, the towels are somewhat fluffy, and as I begin to shoot the stray shots will begin to fall away. The more I shoot, the more the gun compacts itself into a groove in the towel and the better the gun shoots. I took long ago to striking the towels with the edge of my hand to get a compacted groove started in them
Trigger quality.....Factory NEF triggers have no quality, unless it is the quality of awful. Get a trigger job done or do it yourself. (
www.perkloafm.com) for do it yourself instructions. Mine brea at about 3 pounds (maybe a teensy bit lighter) and most days I wish they were lighter than that. For most purposes though, 3 pounds is as light as needed, 4 pounds might be better. But it must be crisp and clean with no creep, very attainable with the NEF lockwork.
It sounds as though you've addressed the scope mounting.
Some suggest an o ring in the forearm bolt hole between the forearm and barrel to create a free floating barrel. I didn't care for the free floating forearm I got using that solution. Perhaps a better answer would be to cut out a rubber washer from a bicycle inner tube that fits over the forearm screw lug, but lies against the lug shoulder to creat a bit of free space. My personal solutio has been to take a rasp to the hole in the forearm and open it a little, then tighten the forearm screw ONLY snug against the forearm. My forearms will actually move a teensy bit. Not rattling loose, just not binding anywhere.
Many have complained of getting poor groups with the 243 and 90-100 grain bullets. Start with some lighteer bullets and see what you get. GO SLOW. Take another rifle with you and shoot a round form your NEF and then a round or two from your other. givethe barrel time to cool between shots. The heavy barrel takes a bit to heat up, easy to fire 3 or 4 off before you notice heat build up, if then. It will take as long or longer to cool the barrel back down.
Now obviously if you are out prarie dog shootin' the action can be fast and furious, but to get to an accurate load, may take a bit slower approach.
Finally. a friend of mine has the synthetic stock and standard weight barrel version in 243. Haven't shot the gun at over 75 yards yet, but it did show great promise. His trigger needs work though before much else can be decided. Ihoe this will help to give you a direction in your quest for accuracy.
I believe it was Townsend Whelen that said "only accurate rifles are interesting" If that is true, then why do I spend all my time shooting my inaccurate rifles?