When you find a nice strong shotgun your 30-30 can be built on a shotgun as small as a 28 ga, but a 20 ga is far more common to find in good shape, and in strong actions. I built two 30-30 doubles on shotgun actions, one on a SKB over under 20 ga, and the other on a Lurona s/s 20 ga. Both turned out fine. The SKB was easier because it was a Mono-block tto start with so simply reaming out the old shotgun barrels was about all that was needed to get it going, The Lurona however was a little more dificult, because I had to cut the barrels off 3 1/4" in front of the breech, and ream, and square the front end to get the mono-block started. Full length liners is not the way to go, and especially in a "12 GA ACTION", that thing will wiegh a ton, and will be imposible to regulate. The bigest action I would use is a very trim 16 ga, and turn the sleeved in barrels down the real slinder profile, 24" barrels with a 1 in 10" twist. Regulate them to shoot side by side @ 100 yds with the right side of the left barrel group, and the left side of the right barrel group merging, with the aiming point exactly half way between the "CENTER" of each barrel's individual group. It is best to regulate the rifle with factory 150 gr factory ammo, then you can buy cheap ammo at any country store in the USA! Then work up a good handload that will shoot to the regulation, and you have a very versatile little deer, and black bear rifle for the lower 48!

I know how much fun building your own double rifle is, but for practicality, you would be money ahead, to simply buy a small caliber double rifle used. The home made one will cost you nearly as much, if not more, and the used factory double will be worth twice as much when it comes time to sell. I had a full commercial machine shop at my disposal, and mine still cost me a lot of money by the time they were finished!