There are very few benches that don't move. I have video off a concrete bench with channel iron legs poored into concrete that shows the bench move durring recoil. Most light weight protable benches are weak and wobbly. Now I am not saying the ground in the best for all instances, but I will say it is better than any portable bench as far as I am concerned.
I do all my load work, sight in and practice prone of a bipod and a good rear bag. Two days ago I put 5 into a .446" group at 200. I advise any hunter to learn to shoot prone off a bipod and rear bag and practice it.
You may find this interesting.
The importance of being Solid
PS: I strongly recommend you keep all mats out from under the bipod and the rear bag. I choose to just get dirty.