Beth, accuracy consists of two different things.
The gun's ability to put multiple bullets in the same place.
The shooter's ability to make use of the gun's accuracy.
With iron sights, you line up the front sight with the rear sight. If the two sights aren't lined up exactly the same each time, the bullet will not go to exactly the same place each time, no matter how accurate the gun actually is.
The further the distance is between the front and the rear sight, the easier it is to see if you are slightly off. If you are shooting a revolver with a 8 inch barrel compared to one with a 2 inch barrel, it's easier to keep the 8 inch one pointed correctly.
It's the same with a rifle. If your sights are 12 inches apart, which is about normal on a 16" barreled carbine, it is easier to be just a little bit off, than if your sights are 24 inches apart, like on a 30" barreled rifle.
That's one of the reasons why "receiver sights" are more accurate than regular leaf sights. Check out these three Winchesters of mine. Notice the top two have the same length barrel, but since the top one has a receiver sight, the rear sight is 6" farther away from the front sight than the barrel sight on the middle gun.
Now look at these two Winchesters. The top one has a "tang sight". It is even farther back than the receiver sight in the other pic. The sight radius difference, on these two guns with the exact same length barrel, is about 12 inches.
With a scope, you see the same thing through your sight, no matter how long or short the barrel of the gun is. That's why the question.
John, there is going to be a noise difference. The 18" barrel is going to be louder and will have more muzzle blast than, say, a 24" one. That may or may not be a consideration for you.
But accuracy - no. No need.