To answer your original question, the 18.5" barrel is quite sufficient to get the full velocity and spin out of the sabot, so extra length will not help much with a cantilever and scope/red dot. If it has iron sights, extra length will help some with with the extra sight distance. 'Follower' has a good point in that the H&R is a great gun, and I have never used more than one shot anyway.

My 14yo son uses the 870 20ga with a deer barrel and bird barrel, and it works great for switching between deer and bird season while maintaining full familiarity with the gun. I do the same thing with a Benelli Nova with a bird barrel and cantilever slug barrel. That said, just today I was coveting an H&R Ultra Slughunter in thumbhole laminate on Gunbroker for $300....

12 ga.
http://www.gunbroker.com/Auction/Vie...?Item=77331915
20 ga.
http://www.gunbroker.com/Auction/Vie...?Item=77331869

These are great guns and much lighter to carry.
I have an H&R topper in 20ga that I used for pheasant hunting 28 years ago in college here in Iowa (paid $54.99 at KMart). It's still a fun shooter.