30WCF:
The main concern with building a double this way is weight. A double in .30-30 would be a delight at 6 to 7 pounds or so, but by inserting the rifle barrels inside a shotgun you are already starting with the full weight of the shotgun and adding the weight of 2 additional rifle barrels. Even though you turn them down pretty thin, its all weight added forward of the action.

Now that you've successfully done a couple this way, why not try a proper sleeving job for your next project. I assume you are aware of Ellis Brown's book on the subject, seems like you will have already done 3/4 of the work anyway, and missed out on the fun of rib-laying! You should still turn the barrels down quite thin especially towards the muzzles, something most gunsmiths are quite reluctant to do judging by the e-mails I receive. They will not 'whip' or 'flip' (?) because they are stiffened by the ribs and second barrel.

Regular posters will be bored with this reference by now, but I tossed up a story about a .577 built this way:
http://www.hotkey.net.au/~orrs/GreenerStory.htm

BTW, welcome to the forum, mate! I can already tell you will like it here.