You might have some issues finding the 18.5 inch barrel. I am sure they have been gobbled up with the scare lately. This would work too....and give you a smaller field barrel option for general woods carrying. I have a 21 inch field barrel on my 20ga for my son and I enjoy using it for the woods. I saw that new 870s are doing the 30.00 rebate now. Find a place selling it on sale, get the rebate and this barrel below and your good to go.
Remington 12 Gauge 20" Barrel for Model 870 $125.00
Remington 12 Gauge 20" Barrel for Model 870 $125.00