10-1000 USD depending on specifics
Blue Book of Gun Values can assist you.
On the top of the barrel near the flash suppressor it should be stamped with the caliber and rate of rifling. Example - 5.56 Nato 1/7.
a slug is normally use in a shot gun and they are not rifled a gun with a rifled barrel should shoot a Shell that is the same caliber as the gun.
The "rate of twist on rifling" is caliber specific. To give you an answer we would need to know which cartridge the rifle is chambered for.
The rifling is there to spin the bullet, which gives greater accuracy.
It is the rifling in a mossberg rifle barrel.
Inside the barrel.
Look through the barrel and when you see helical (spiral like) grooves, then you have a rifling barrel. These grooves make the bullet spin, which stabilizes it (gyroscopically) during flight.
The lands and grooves in the barrel that impart spin to the projectile
Rifling. The barrel is not smooth on the inside. There are small grooves spiraling down the barrel which makes the bullet spin. Nearly all shotguns do not have rifling in the barrel.
A process of putting rifling in a barrel.
There are several processes for rifling. Typically, a machine tool is inserted down the barrel and pulled out, while several gouges cut shallow grooves in the barrel.