Sighted in my 16" Rossi 92 with factory iron sights. At 25 yds the 158gr 38 FMJ's hit dead center about 1.5" high, all holes touching. Switched to 158 gr 357 mag FMJ's and the group 1/2" below center as expected but was about 1.5" to the right, again all holes touching. Drifted rear sight and got group centered, switched back to 38's and now they were 1.5" to the left. I repeated this several times with the same results. What would cause this ? Is this normal ? I'm at a lose. I ending up splitting the difference and now 38 is 3/4" left of center and 357 is 3/4" right of center. Very confused. Any thoughts ? -Thanks

Slower velocity bullets drift more.  Perhaps there was a crosswind.  Otherwise, I have one more idea.

Years ago I was shooting a friends SMLE.  I kept hitting consistently to the left.  I'm a southpaw he's right handed.  It seemed that as the rifle recoiled it pulled off to the opposite side when the upper body shifted (we were shooting standing) and thus had a different point of impact.

So, could the difference in to recoil account for a different shifting of the rifle on firing, thus throwing off the POI?

You right handed?  The heavier recoil of the .357 mag is pivoting the rifle to the right before the bullet leaves the barrel.  If not, I'm at a loss.