The current 20g slugs have more energy than the 45-70 and 30-30. You think either of those are too light for deer?
Only the very low pressure 45-70 loads. The most powerful 20 gauge slugs max out at almost 2400 ft/lbs. A 45-70 (Ruger #1 or similar) maxes out at almost 4000 ft/lbs. A Marlin can handle the 300 Grain Buffalo Bore loads reaching 2350 fps. A 20 gauge can only come within a couple hundred fps of that. I'm not knocking the 20 gauge, I might use mine out of a treestand this year, but a 45-70 can certainly have more energy than a 20 gauge slug.