I use a rear sand bag for the butt stock on the bench. You squeeze it to adjust the alignment. Actually it is filled with rice which is a lot lighter than sand. In the field, a bag of styrofoam beads or air soft pellets works. I will also use a styrofoam bead seat cushion for a rifle rest. They are great to sit on in cold weather and have a double use. I have a couple of them in camo that clip to your belt.
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"I would not be an old man if I had not been an armed young man." JTJ
I was taught to respect my elders but they are getting harder to find.