This is somewhat similar to a project I hope to start (complete?) this summer .......
I have a .308 Handi-Survivor, in which I'm going to transplant the store-stock and store-arm to a 12 gauge Topper or Pardner.
Plans are to cut the 12 Ga bbl to 18 1/4 inches and equip it with a .45/70 insert-adapter from McAce Sports out of Anchorage, AK. I'm also considering of getting a gunsmith (a VERY compitent one of course!) to solder a set of "express sights" on that barrel top, too.
In the Store-arm I can store at least four two-inch 12 Ga aerial marine or boating-type flares (red ones for signaling 'distress'). In the Store-stock I can store at least a dozen two-inch length 12 Ga hunting shells (#6's or 7 1/2's) not unlike the mini-shells available from Aguila, but I plan to reload my own with data that's published and easily available. I also plan to carry four to six 12 gauge slugs as well.
Some commercially available 12 gauge slugs generate 3000 foot-pounds of muzzle energy! Plenty to kill a bear?
Also at least five to eight .45/70 cartridges will be carried too, but since I haven't worked out all the details in how to carry ammo ....... well! .......
But the express sights are an effort to better aim the .45/70 rounds.
Also for carrying ammo is the sling that loops can be sewn onto, but I also a have a 5-round piece of plastic from a Tac-Star Side Saddle accessory I'll adapt to affixing to the left outside of the Store-stock.
The whole idea with my project camp/truck/survival gun is having a weapon than can kill a grizzly (biggest threat I preceive) at somewhat close-range, or some self-defense, or hunting for camp/survival meat including big game, not just grouse or birds.
I will of course be carrying a .357Mag revolver when carrying this gun, so having a first [.45/70] shot to deal with a bear isn't totally unreasonable.
The mini-12 gauge shells should have the range of a 28 gauge or slightly more.
The McAce .45/70 insert/adapter is fully rifled and 18-inches long! Should give more performance than a T/C Contender with a 14 or 16 inch bbl!