text: If you want to know the true difference between the two, it's a simple answer, there is none. No I'm not crazy, velocity and accuracy is determined in the last  2 1/2" of your barrel. It's called the choke, you can purchase a shotgun with a number of different chokes as in ( Full , Modified, semi Modified, to Improved cylinder) I have five different interchangeable chokes for my Browning over/under and each have there purpose.\n\nBeing that both barrels have rifling the only difference you will notice between the two barrels would be the the recoil will be some what greater on the 18" barrel, and the 24" will get you 6"closer to your target.\n\nRange of the gun is determined by the slug and the drams of powder in the slug. You will notice that when you purchase slugs they are already state Magnum load on the box as well as size 2 3/4 or 3" shells.\n\nThis isn' t brag cause I"ve never done it but I watched my Dad with his model 1100 Remington with a 26" barrel with modified choke and 2 3/4" slug split penny post cards at 100 yds. So that 3" should get you that No problem \n\nGood luck on your hunt.