I had a buddy who rebarreled his sons .243 Win and bought the Core Lokt bullets to reload to break in the barrel. Had the same problem, nothing would group better than an 1" on a blue printed action with custom barrel. I told him to weight his bullets and he found a pretty wide variation in weights as well. It wasn't as large as mine, but when he sorted by weight the grouped okay as well, nothing spectacular but well enough he wasn't thinking of taking the rifle back to the smith.
You can't go comparing accuracy of reloads to factory ammo, especially in a new rifle. If you want to go down that path, I have an experience 100% opposite yours -- where my squared-and-trued .243 was printing groups smaller than 0.5" at 100 yards, within 15 rounds of the first shot down that barrel. (excluding the single
test load fired by the gunsmith.) Untested reloads, factory ammo... everything. ...including factory loaded 100 gr Core-Lokts.
Every barrel reacts differently.
Every reload is different.
Reloads are not factory ammo.
As far as weight deviation goes....
Right around 3% is the industry standard for anything but premium bullets. With a 100 gr Core-Lokt, that means you'll see about 98.5 gr to 101.5 gr.
With a 150 gr Core-Lokt, that means you'll see about 147.7 gr to 152.3 gr.
But, even some premium bullets will still see deviation around 3%. Right now, I'm shooting an average group of 0.490" at 100 yards with 140 gr Nosler Partitions in .270 Winchester. They weigh 137.5 to 143 gr. (Nearly 4% deviation.)
I don't know who makes the Core Lokt bullet for Remington but they don't have the same standard of QC other bullet makers have.
Remington makes them for Remington.
Some rifles just don't like certain loads.
In this case, I still think it's a stabilization issue, though.....