ORIGINAL: tykempster
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.
Sorry, I should have quoted the article I read better. From Outdoor Life, June/July 2006, p. 122
These days, shotguns are required for deer hunting in many locations, and hunters have learned that you don't always need a 10- or 12-gauge to harvest a deer. A 3/4-ounce 20-gauge Foster slug at 1,800 fps produces over 2,300 foot-pounds of muzzle energy, almost eaqualing a 300 grain .44-70 load and beating virtually all .30-30 loads.
The article goes on to talk about the new loads for this year including a 250g .45 caliber SST-MLloadfrom Hornady at 1,850 fps as well as a 3" 260g Partition from Winchester at 2,000 fps. Federal has a 5/8 oz Barnes Expander sabot slug at 1,600 fps. There is also a Managed Recoilload from Remington with a 7/8 oz (385g) slug that runs at 1,275 fps good to 100 yards for deer.
So, ya, to answer the question of this post, I think a 20 gauge shotgun is plenty to take a deer.