I've got a 20" open sighted barrel, plus a cantiliver barrel that I use on my 870, both are Hastings barrels; they are accurate and very effective. I've shot a bunch of different slugs on the range, and the most accurate in both of my Hastings Barrels are the Federal Premiums, I've settled on the 3" loads.
I've fired the cantliver barrel on the 200 yard line at our KD range, and I'm extremely impressed with the accuracy of the 870, Hastings barrel, Federal Premium's combination. The drop was about 14 inches with a 100 dead on zero. Nice round holes so the slugs were nice and stable.
Copper solids & Hastings barrels had a problem last century, but according to the web sites it was a short term problem.
I can't comment on any other after market barrel, but if I were to buy one tomorrow I'd buy the Hastings cantliver barrel and make up my mind to shoot it on the range and determine which slugs performed the best.
Invest in a good heavy duty scope. The combination does kick!
Just my $.02
Shotgunman