I have an ammo puzzler with a Model 27 (no dash) that I recently got. I have the gun sighted for .38 Special 154gr FMJ (Magtech and S&B, so far) to give me an impact point about 1 1/2 inches above point of aim at 15 yards. When I shoot .357 Magnum through the gun (Geco 158gr FMJ, so far), I get a point of impact at 15 yards about 5 inches above point of aim and about 4 inches left. The rounds are grouping, just at a very different impact point. I’ve shot .357 Mag through my Model 19, Model 66, Model 28, and Model 686 with about the results you would expect: a slightly lower mean point of impact without any drift left or right, so I'm puzzled by the extreme difference I'm seeing with the Model 27. The Model 27 looks to be in excellent shape, with good lock-up and a shiny bore. I had a gunsmith check the timing, and he said it was fine. I have not had the cylinder alignment checked, but I’m not seeing signs of a major misalignment issue. I did have to crank the rear sight left to get the .38 Special rounds centered on the target, so I suspect that centering the rear sight would center groups with .357 mag. Any thoughts as to what could be causing such a large and consistent shifting of the impact point between .38 Special and .357 Magnum? Have others seen the same thing?