Soldering with irons works with any caliber, the only thing about doing smaller calibers is you will need more rods. The smaller diameter won't retain the heat as well or as long so you will have to swap them out more frequently...that means you have to have enough rods hot to let the first one reheat before you come full cycle and need it again. For 45 or 50 caliber rifles probably 8-10 rods minimum...the smaller you go the more rods you will need.

Colin