Good question, and let's look at this rationally.
The
Federal 20 gauge rifled slug (chosen at random) launches a 3/4 ounce (328 grain) slug at 1600 fps. At 100 yards it's still carrying 831 fpe, but (and this is a big But), what we're talking about is a .61 caliber ball of lead.
That's pure muzzleloading velocities and folks have been killing animals with those ballistics for hundreds of years. When a 61 caliber lead slug hits something, it's going to leave a mark. It will probably punch all the way through, leaving a big hole that lets out lots of blood.
If you step up to a sabot slug, the ballistics will improve somewhat.
Back in the '80s I hunted and competed with muzzleloading rifles and when I'd show up at the camp with my front-stuffer, I'd get ribbed about it. I reminded them that I didn't care if my bullet expanded, that I was starting out with a ball about twice the size of theirs. Their .27 caliber whiz-bang bullet had to expand to twice its size to get to the point where my .54 caliber ball started out. I didn't need expansion, I just needed to hit the deer.
Yeah, that 20 gauge slug will do fine in the deer woods. Buy a couple of boxes of your preferred ammo and get some practice time. When deer season rolls around, you'll be fine.
Now I have to seriously consider my lack of funds and responsible spending against my desire to buy a new rifle.
We're in a tough economy and we're all pinching pennies. I remember as a young man starting out, making a family, trying to keep kids in groceries and tenny shoes. You know the right thing to do. Take care of what you need to take care of. New toys will come later.
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