I do have multibarrel barrel sets for my Tikka/Valley. As most regulars on the NE FORUMS know.

I really do like the side by side, double rifle, cape gun, side by side shotgun and paradox systems.

Tikka 512SD Under and Over, Valmet 412 system:
* 12-bore Under and Over, changeable screw in chokes
* 12-bore over .223 Remington, scoped. Fixed choke.
* 12-bore over 7x65R, to be scoped. Fixed choke.
* .30-06 Double Rifle U/O. To be scoped. I'd still possibly like to rechamber this to .30 H&H Super Flanged.
Just because I like the thought of it better. Also a rimmed case.
* 9.3x74R Double Rifle U/O, scoped.

No paradox barrels in this line up.

Also missing a decent big bore. The 9.3x74R is an excellent cartridge but at the lower limit for dangerous game.
I've shown a .450/400 custom barrel set that was for sale at one time.
I like the idea of a .500/416 NE or .416 Rigby No. 2.
However if the action can handle it, a .450 NE would be better, and a .500 NE more better still.
I think that would be overdoing it though.

I very much more would prefer a good side by side line up.

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Tikka 512SD:
* 12-bore Under and Over, changeable screw in chokes
Purpose: general shotgunning, waterfowl, it might be OK for steel shot. Clay targets, trap and skeet and simulated.
* 12-bore over .223 Remington, scoped. Fixed choke.
Purpose: small game, walked up. Rabbits, hares, foxes, ducks and birds. Roos.
* 12-bore over 7x65R, to be scoped. Fixed choke.
Purpose: Mixed hunting shot for small game, 7mm for medium game.
* .30-06 Double Rifle U/O. To be scoped.
Purpose: Medium game hunting.
* 9.3x74R Double Rifle U/O, scoped.
Purpose: Medium game, driven game, scrub and close bush hunting. Medium game but up to dangerous game,

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=341307&page=0&vc=1#Post341307
Tikka 512SD with 12-bore/.223 Remington barrels. Leupold 1-4x scope on top.