I don't shoot offhand that much anymore (sticks have spoiled me) but when I shot my R9 .22 springer all the time I could get 2"-3" groups when I was in the zone. Since I went to mostly PCPs I don't shoot offhand much and my skills have faded...
I do most of my offhand shooting at 25-30 yards and a good 5 shot group for me is about 1 inch ctc with most of my groups being 2 inches +. I imagine at 50 yards a good group for me would be 2 to 3 inches ctc but most groups would be 5 - 6 inches. I may have to try it during my next shoot and find out for sure.
I don't know why or for what purpose you would want to shoot offhand at 50 yds however air rifle silhouette is shot all offhand and the farthest animal is the ram at 45 yds sooooo a really good offhand group would be one that hits a ram size target at 45 yards. Airgun silhouettes are one tenth the size of full size metallic silhouettes. Airgun ram silhouettes are of course shaped like a ram and they are about 3" wide from nose to rump and about 1 1/4" tall in the body with the head and horns being taller. Actual sizes and images can be found on the NRA website. Shooters routinely hit the rams and although it is difficult it is also very satisfying. It is a very good sport and I wish more shooters were interested. It is fun but not easy. More info about silhouette can be found on the NRA website or at steelchickens.com