The last couple of times I shot the .308 from the bench, I forgot to bring the rabbit ear rear bag that I normally use. Instead I went with a couple of flat top bags that were available, with the support hand to the rear on the buttstock. I noticed at all ranges 100-300 yards that I had considerable horizontal stringing. I hadn't realized the rear bag would make that much difference, but will bring the rabbit ears next time.

I keep an extra one in my truck incase I forgot the one I normaly use. One thing that made my rear bag work so better was the use of heavy sand.

I agree. Rear bag setup and alignment of the front and rear rests is critical. At least that can make a relatively huge impact in rimfire benchrest shooting.

I did bring all my bags next time to include the rabbit ear, and the problem went away.