Nothing wrong with hunting plains game with slugs.
Ginger van Zyl shot n Cape Buffalo with a Brenneke slug that was harassing locals and eating their crops. He had filled his licence and was close to a village doing some bird hunting, when some locals aproached him and asked if he could sort out a problem buffalo.(They had a permit to shoot it but no rifle). Not wanting to go all the way back to camp he decided to give the slug a go in his shotgun. He got close(actually stalked it from behind one of the hut's) and put the slug just behind the front leg, slightly quartering away. The bull gave a few steps backward and fell over, dead.
Would I recommend it for buffalo, NO. But it has been done.
Brenneke slugs offer the best penetration of all in 12ga.
When I started my hunting career I bought a 375 H&H on a Brno XKK 602 action. I shot everything with it and used it for back-up on many hunts. As things progressed to more dangerous game hunting, I stepped up to a 500 Jeff.
On one leopard hunt we had to follow up as the cat was wounded. The result was one of the follow up party got nailed by the cat and looked like he had been playing with a chainsaw. That day I decided a bolt action is not the thing for following up leopard. There is no time to reload, so with a bolt you mostly have one shot.
I decided a double was needed as you had an instant second shot. Only problem was I could not afford one. I did have a Brno sidelock sxs 12ga shotgun. So I started experimenting with it using slugs. All different kinds I tried. The best performing on game animals and giving the best penetration were the Brenneke slugs. (They actually over penetrate when used on bushpigs and when hunting bushpigs with hounds in maize crops I use cheaper spanish slugs that do not exit on bush pigs-for fear of hitting another hunter or a dog).
These slugs proved to be so effective and after shooting Gemsbuck, Bluewildebeest, quite a few impala, many warthogs and bushpigs. Longest shot was 87 yards on a one horned impala ram.
I took my double project one step further.
I shortened the barrels from 28 inches to 24 inches, fitted a front site and fitted a ghost ring site at the back.
I now had my leopard back up gun and I carried it on a few follow ups and it is a dream to handle. I still use it for hunting bushpigs in the the thick stuff.
The slugs being so effective I bought two combination guns that became available from above mentioned Uncle Ginger. One in 9.3 x 74R/12ga for myself and one in 7 x 65R/12ga for my 12 year old son. We use these all the time on our bushpig/warthog hunts with excellent results.
Forget about buckshot for hunting/back-up on leopard or bushpigs. They lack the penetration needed, only use slugs. For smaller thin skinned game you can use buckshot(bush buck), AAA duiker size, No.1 smaller duiker.
With slugs, I will use them and have in the past, and had no issues using them. No problem on Leopard or lioness, big male lion may be a different story on a charge situation.
When hunting with slugs you need to remember you are limited on distance and in my case to open sights. If you are patient, enjoy the challenge and practice beforehand and you can use slugs with very good effect.
Not the best option for a African safari, but if you have another primary rifle and want to pack a slug gun for fun and some hunting, no problems pack it in and have fun, you may well be surprised.
Many manufacturers use (and have used for years - see Foster slugs) rear attachments to give the projectile a weight forward profile. It is more akin to a sand-filled bag than a rifle bullet.
