Interesting idea. I am curious if anyone here has done it with the .45-70. I googled "45-70 sabot" and a few things came up that might be worth looking at, including a few recipes with .357 and .40 bullets.
On the .224 sabot in a larger case, your post made me think of the Remington "Accelerator" rounds. I thought they had stopped production, but Midway still lists them in 30-06 and 30-30:
30-06 (4080 fps claimed): http://www.midwayusa.com/viewproduct/?productnumber=220009
30-30: http://www.midwayusa.com/viewProduct/?productNumber=366971
Although they don't list .45-70 or .270, this place has components for .308 and others:
http://www.sabotreloadingpro.com/products
Sorry I don't have any experience with it, but I am curious to read the responses of others.
On the .224 sabot in a larger case, your post made me think of the Remington "Accelerator" rounds. I thought they had stopped production, but Midway still lists them in 30-06 and 30-30:
30-06 (4080 fps claimed): http://www.midwayusa.com/viewproduct/?productnumber=220009
30-30: http://www.midwayusa.com/viewProduct/?productNumber=366971
Although they don't list .45-70 or .270, this place has components for .308 and others:
http://www.sabotreloadingpro.com/products
Sorry I don't have any experience with it, but I am curious to read the responses of others.