You said that you were using " light magnum" cartridges. These shells will create more recoil. Use standard 150 grain 30-06, you should feel a difference.
Also, you mentioned shooting off of a concrete block. Your rifle will " bounce" to some effect from any hard surface. The sleeping bag isn' t the best either, a little too soft in my opinion.
Considering what you are using for rests, I' m impressed with your groups. Fill several old socks with dry sand and tie the ends shut. These work great. The sand is hard enough to get a solid rest from, but will GIVE when the gun fires and not cause the rifle to bounce.
Good luck, let us know how things work out for you.