Most double action revolvers are specifically designed to have slight rotational play in the cylinder.
This is to allow the bullet passing from the chamber to the bore to force the chamber into alignment with the bore.
This includes the S&W, Ruger, Dan Wesson, Taurus, and the newer Colt's like the Mark III through Anaconda guns.
The only DA revolvers that lock tightly are the old Colt actions like the Detective Special, Python, etc.
Pulling the trigger and checking for rotational play on a S&W, Ruger, etc is NOT a valid test for anything. That's only valid for the old Colt's.
While a revolver may seem to be tightly locked with the trigger pulled, there's enough backlash built into the design to allow the cylinder to move enough to align with the barrel.
Bottom line, Your Ruger is designed to be slightly loose.