There are Carlson's 18-1/2" barrels with removable chokes. I use a Mossberg-mfr. 18-1/2" cylinder bore barrel on my Remington 870 with a simple bead sight. At the last defensive shotgunning class I took, the patterns with Federal 00 buckshot were fantastic. Very tight and slow-opening. The bead made 50-yard slug shots suffer, and I suspect that the loose "sissy pad" I put on the butt stock [

] for the slug drill may have actually played a factor in decreasing slug accuracy.
If you can, get some butcher paper or some large CHL type targets--the Texas state CHL target is a menacing "bowling pin" shape--and pattern your shotgun with different defensive/tactical loads that you might be considering. I'd agree with the advice up post that it is far cheaper to find a good patterning ammo than to get a whole new barrel.