I have a Ruger Single Six with the interchangeable cylinders and RARELY use the .22 Mag.
It is my understanding that the .22 Mag was designed for rifles and is not much better than a .22 LR in pistol length barrels.
Someone correct me if I am wrong.
Will do.
.22 WMR (Winchester Magnum Rifle) from the PMR-30's 4.3" barrel matches equivalent grain .22 LR energy from a 16-20" barrel. So basically you get .22 LR RIFLE power from a pistol. If you switch to the 5" barrel available for the PMR it consistently exceeds the maximum energy of .22 LR. And of course when comparing pistol length barrel to pistol length barrel it isn't even close to a contest.
As for the Ruger Single Six, the longer the barrel the more the WMR would overshadow the LR. If you have the 6.5" barrel version then you should easily be able to get 30 grain WMR consistently over 1600fps. Check out some gel test comparisons on youtube too. They show the difference a lot more effectively.
As to the OP, if you want a Semi-Auto then your choices are very limited. I believe only 3 have ever been made, the PMR-30, the Grendel, and the AMT. Neither Grendel nor AMT is are in business anymore, so you are out of luck there. However, I High Standard acquired the rights to AMT's guns and I think they may still be making the AMT .22 magnum semi-auto. Google all that to find out more about all these firearms.
Personally though, I would go with a revolver. There are some great ones out there in .22wmr, most convertible to .22lr for cheap range-fun.