I probably wouldn't go for a .30 Herrett, personally. Not to say you shouldn't, but if I were looking to add a .30 caliber barrel to my Contender collection, I'd look at either the .30-30 AI or the .309 JDJ.
If you're not set on the .30 caliber, but want to go up from what you have, I'd take a serious look at the .375 JDJ or the .45-70. If you want to go down from what you have, a .222 Remington or a 6x45 would be a good place to look.