Rimfirecentral dot com is a must-have resource for all things .22
In addition, familiarize yourself with:
John Stimson's High Standard pages
Manuals, and other info is there. He will be shutting down the forum soon [as in, days], so ask any questions there quickly, or forever hold your piece.
Now, that said- I've never heard of a .22mag cylinder/swap for the R108 Sentinel.
I have a R106 Sentinel myself, fwiw.
My understanding is that the cylinder frame of the Sentinel is an aluminum alloy, and is ok for high velocity .22lr, but Hyper-Velocity [over 1400fps] is to be avoided.
In that case, a .22mag is definitely to be avoided.
Also, most .22lr barrels are around .222 bore, while it seems .22mag barrels are around a .224 bore.
Again, I've never read of being able to put a .22mag cylinder into a Sentinel 108 and make it work.
That said, High Standard did offer the MKI and MK IV Sentinels with .22mag options, and they sold their western-style R106 with .22mag cylinders.
If I were you, I'd check around, but begin preparing to accept that the .22mag in a R108 Sentinel is not likely to happen.
If you REALLY want a .22mag/.22lr revolver, there are other options out there.
The High Standard western style R106 is one. So is the Ruger Single-Six Convertible [which I have also]. S&W made some DA/SA ones in the 60s or 70s, I believe, but I don't remember the number.
Good luck!
