Would a 12 Ga. shotgun slug shoot safely/properly from this DR .72 Cal M/L if you pulled it from it's shell?

Or can one buy conical bullets for 12 bore M/Ls?

Jack: Probably not. To fit inside the 12-gauge shell, the slug is likely undersized and considerably so if it fits in a sabot.
If you cast your own, check the NEI Handtools site, as they make a number of conical molds for this gauge. If you don't cast, do a Google search for Wayne Doudna (hope I spelled it right).

The bores on a 12-bore is nominally .730, give or take. Slugs for a 12 gauge shotgun are usually cast to .715, hence the reason for the foster slug to obterate close to bore diameter while the rifling is theoretically supposed to empart a spin on the projectile.
Although I haven't tried it, a Foster slug probably wouldn't give you the ballistics nor the accuracy you are seeking with your 12-bore.
I'm having a brain fart right now...but there's a certain slug maker who makes hard-cast slugs that would work perfectly for this. It's a slug that looks like a Cast-Performance bullet on steriods. I'll get back to you on this as soon as I can remember the manufactures' name.

Hah, I'm a retard. Okay it's the Dixie Terminator slug, don't exactly recall the weight but I would suggest using one of these for a conical.
They have all sorts of conicals out there, but avoid using a solid such as a Bridger. Your rifle will take a serious pounding from using one of these, such discussions on this are on the Double Rifle forum. Being the Terminator is a solid, it is lead so it's easier on the rifle than a monolithic brass bullet, for example.

Be sure to check your rifling twist rate. Be a shame to find out you have a roundball rifle after you buy a custom mold for a .730 conical.

Pop over to www.shootersforum.com and talk with the guy from Bearthooth bullets,, he has come up with a slug for this gun and may be casting them now