The ideal rifle depends to some degree on how you are hunting them and what sort of cover they're in. For pigs the size you are describing a .223 would do, with the right bullet. I've killed pigs that size with smaller, but shot placement is at a real premium. With a bigger calibre than .224" you don't want a bullet that expands slowly IMHO - a plain cup and core opens fast and drops them where a tough bullet may not open well and therefore not kill nearly as well unless you spine or brain them.
Walking up on pigs I like a light rifle that fits me well so I can get the shots away fast. If you bounce a mob of pigs you'll often be able to drop several of them if you get your shots away quickly, provided of course that you make them count. I have dropped as many as eight out of a single mob with a bolt action, maybe taking the odd step or two to keep them in view. You may do better with a self-loader, if you are disciplined enough to make each shot count.
The same rifle also does the job if you are spotlighting them.
Some of the shots can be close, if there's cover - sometimes pigs get a bit confused and run at you, or want to have a go at you - so for the shooting I do I like a scope with a low enough magnification to allow for this. Shooting them out in open paddocks though you might want to be able to reach out a bit, and may want a bit more magnification.
