12ga barrels

Question
Would a 12ga 28" smoothbore shotgun barrel fire a slug as straight as a 24" rifled barrel? which one would be better for hunting hogs?

Answer
The purpose of rifling, as you know, is to impart spin with increases range and stability.

A rifled bore on a shotgun becomes ammunition specific - you will damage the rifling if you try to run shotgun shells through it.

The reason for the shotgun's existence is its versatility. It can fire a single projectile or multiple projectiles. Getting a rifled barrel gun diminishes its capability.

Fortunately, there is more than one way to skin the cat. You can get rifled slugs. These have "rifling" cut on the projectile to impart spin, and they do a good job at it.

Assuming you will take hogs in the brush, either avenue will work for you, and the smooth bore will allow you versatility.

If the question is not what to buy, but what to use (because you already have both), I would use the 24" rifled barrel, not only because it will likely be a bit more accurate with slugs, but it will be faster into action, which (depending on where you're hunting the hogs) can be a safety issue.