Good luck with your project!
The state I live in (Oregon) has "No Centerfire Rifle" hunts for both elk and deer. They may also have shotgun only but I have never hunted in one. In these hunts shotguns, pistols and muzzleloading firearms are allowed. The muzzle loaders tend to be darn new.
www.dfw.state.or.us/ Above is the internet address for our Fish and Wildlife folks. It has the big game regulations and the firearms restrictions, definitions, etc. If you have trouble finding what you want in there PM me what you would like to read.
http://www.sos.state.or.us/archives/banners/rules.htm -This site has the Administrative rules and some guidelines on how they are prepared.
http://www.oregonhunters.org/ - This is a state hunting organization that is active in developing legislation. They may be able to give you some help as well.
As to slugs effectiveness on elk sized game I do not know by personal experience. I have killed deer and bear with both 20 and 12 gauge Foster slugs and they worked fine for that. I have only recovered one slug and it had penetrated a small bear, come out and lodged in the rear paw pad after breaking the bones.
Based on reports from others for elk I would likely use Brenneke's or another hardened slug in a smoothbore. Without those reports to influence me I would have just used regular slugs as they penetrate very well. I suspect they would work.
In my rifled barrel there are many choices but I believe I would use the Winchester, Hornady Federal or Remington Sabots that use a bullet shaped projectile with an 1800+ muzzle velocity. The Winchesters performed best in my Ithaca. They extend the usable range to around 200 yards.
The places here where they require shotguns tend to offer extremely short range shots so I have always used my smooth bore Deerslayer barrels with Scout scopes.
I'm not sure how to limit Grandpa.
