Is the "LightField" slug a flat faced/nosed slug? ie; about full diameter for flat nosed? I'm wondering how it would do with round balls.

A .615" ball weighs 350gr. in pure lead - that is exactly 20 bore in size. The grooves can be taken up by the wads, with the cup under the ball holding it centred. of course, if using black powder, then a lubed cloth patch will soften fouling and not allow it to build up.

A normal 2 3/4" hull should allow that 350gr. ball to achieve at least 1,800fps - about the same as a heavier 130gr. 2F charge in a muzzleloading rifle of .62 cal. - nothing to sneeze at - and powerful enough for anything here, that's for sure. As to range, if you can get them regulating and accurate enough, they'll give you a good 200yard effective range on deer, elk and moose. My bro's new Hawken, .62 cal,shooting a .615" ball with 128gr.(happens to be what his hand made measure threw at the largest setting) is now making in the 4" to 5" 5 shot groups at 200yards off a rest.

I would not trust a factory slug that far, but do-so with the round balls - if you've achieved the accuracy. I surely do like that rifle!