A 12 gauge slug will put the hammer down on an elk. We've used them in the past, they're very accurate to 100yds. We normally have a rifled barrel but not always! Here's a cow I called in for my 12 year old nephew on a cow only tag. The shot was 40 yds & a complete passthrough, the elk went 10yds! Here's a picture of him & his dad!
http://www.elknut.com/files/Trevors_cow.jpg