They are not big enough.
An L frame S&W - the 696 - is barely big enough for .44 Special and not real heavy loads at that.
Dave Clement will convert a GP 100 to .44 Special and says it is ok for a 250 at 1200, Keith loads. $1250 on your gun.
There have been some L frame conversions to .41 Special (not magnum) that will do over 1000 fps with a 210 gr bullet.
So size is the limiting factor? Hmm...
Maybe one day Ruger will make a .44 Special Ruger GP100.