I saw testing that used up to a 90 grain bullet in a 22/22 mag style case. They just shortened the case to keep the overall length the same. These were subsonic rounds used in a suppressed firearms that required a special rate of twist barrel to fly without key holing.
The heaviest bullet in a rim fire that I know of that is commercially available is the Aquilla 60 grain in a 22 short case, made for subsonic velocities in a suppressed firearm. The bullets are almost as long as the case!
View attachment 451037