I personally feel that runout plays a much bigger role.
Big Grin,
.. I have a buddy that indexes his brass too..
But, he takes it a step further than that. He reloads at the bench with the same piece of Lapua brass over and over. He is retired and enjoys it....
There is no reason to stay at 2.800, you are not using the magazine and Remingtons usually have long "generous" throats, I bet you can tighten that group up a little. NOT taking anything away from your .5 group. Thats Great!
While you are not noticing it now at 200 yards, Heck, you are almost at 2500 fps, You should be able to find the next node up pretty easy moving at .3 grains (that’s not going to be Hot)
I can make 2600 out of my 20" FN. With no signs of pressure at all. I think you will find that faster relative burn rates work to your advantage with shorter barrels.
HTH