The long the barrle the more FPS and Ft/lbs,take a look at your Factory ammo,lets say a 357 Rem Mag out of a pistol.
4 in Tube 180 gr Bullet
M/FPS M/Ft lbs 100 yds 100 yds
1250 625 1080/fps 465/ft lbs

Rifle 18 in Tube 180 gr bullet
M/FPS M/Ft lbs 100yds 100yds
1550 965 1210/fps 590/ft lbs
Now there a bit of diffrence between a rifle and pistol,both in velocity and Ft/lbs,not to metion the accracy you will gain with a large sight radius.