Never could get good grouping off sandbags; I found that I couldn't keep a uniform grip was the problem...and too, the group center was not the same as my two-handed, offhand field shooting positions. It varied from them as much as 2-3" at 25 yds.
I use, instead, Elmer Keith's old long range shooting position. In it, you sit on the ground or at most, a very low sitting beach chair, with back support. A tree or even fence post or rail will do as well. Knees are pulled up and the gun is extended between them, two-handed, finger on finger/thumb on thumb grip. Using it, I get better, tighter groups, and their point of impact coincides with my hunting, field positions.
I have used this technique since the mid-60's, and it's good enough to evaluate hand load accuracy. I'm 72 now and I can still get ~1" groups with open iron sights, from my guns when the loads right and the sun's behind me. Give it a try...it's handy in the field for deer or while squirrel hunting, and any old tree will support your back giving you the accuracy of a bench rest. Best regards, Rod