The Model 12 is a poor choice for a take down-switch barrel, it always suffered from that feature. The barrel take down feature on the Model 12 is to take the barrel out for cleaning purposes.

The barrel is just threaded in and held in place with a tapered cross pin. Getting that cross pin to stay tight in a.22 has always been difficult. I can't see it staying tight in a .357.

The better action to use for a switch barrel would be the Westley Richards Sherwood small martini with the bayonet or twist lock barrel. Much stronger lock up. The down side is duplicating the barrel lock. You would need to find an orphan barrel for the second barrel.

I have a WR in 219 Donaldson Wasp. If I could find an orphan barrel I would build it into a 7-30 Waters.

The .22 Hornet and .357 Magnum are not cross compatible in the Martini. You would need two trigger groups separately adjusted for each size cartridge.

My switch barrel Model 12 is a 17 HM 2 and a .22 LR. I do not think the .22 Barrel has been on the gun since just after I built the gun.

Using the .22 Hornet series, go to 17 Hornady Hornet for 2nd switch barrel.

With .357 Mag go to .256 Winchester.

The other issue to address with a Martini switch barrel is sighting. The system works just fine with iron sights.

Scope mounting on a Martini switch barrel requires the scope mount to beva type that clears the action and fore stock. Even with that issue addressed the scope still has to be removed to take of the barrel unless the forestock is also removed.

Nothing insurmountable, just thing you need to know before you proceed.