are these rounds used for cowboy action?

I'm not much of a CAS shooter but this is my understanding.

Rifles are limited to designs of 1899 and earlier which eliminates the 348 as a factory option.

There isn't a 358 Win mag at this time- if you mean 358 Win or 358 Norma mag, see the above.

I also believe the rules were changed many years ago limiting rifles to pistol cartridges. Someone better versed should be along soon to give a better post.

If that was the case people wouldn't be allowed to use Henry Big Boys, a brand new 21st century design. Unfortunately, Henry somehow got an exception for the ugly things.

But you can't use a .348. Or a .358 Win Mag.

Maybe I'm late to the party, but the answer is still no.

See sassnet.com for a downloadable copy of the rule book.