I got to thinking about the different possible variables, there are some not given in the OP.
Aside from the temperature difference what are the other differences between this time, and the last time you shot that gun??
First question, is it the same ammo you used last time??
Does your Ruger have adjustable sights?? The Ruger Single Six does not have adjustable sights. The Ruger calls the gun with adjustable sights the Super Single Six. People frequently just say "Single Six" referring to either version, but when trying to (remotely) troubleshoot a problem, it makes a difference.
Along with cold ammo another likely factor is a cold shooter. The OP says the indoor range was less than 40F. Wearing gloves??? My fingers get cold and stiff easier today than they did back when I was a younger man, and cold stiff fingers simply don't have the same sensitive feel. It's not impossible the shooter was pulling his shots without realizing it.
Another question, indoor range...were you outside the last time you shot that gun? Lighting can make a difference.
But most 22 ammo has grease in the grooves of the bullet.
I think you are confusing .22 rimfire bullets with centerfire lead bullets. The .22 uses a heel type bullet, there are no grease grooves, and regular .22 ammo has used a wax coating, not grease, for about a century.
5" at 15 yds is extreme, more than usually seen, but since it happened, its clearly not impossible.
Cold gun, VERY cold ammo, cold range, cold hands, perhaps poor lighting, perhaps all those together were the cause.
How's the trigger pull on your Ruger?? Light triggers and cold numbed fingers (or wearing gloves) affecting your feel of the trigger often results in a change of impact/poor groups.
Repeat the test with everything at comfortable room temperature, changing nothing else if/when possible. If your results are different, then it was the cold. If not, then its something else. Sorry I can't be more helpful.