Couldn't find the old posts but this has been a work in progress for a long time. Right now all of the slugs will go into that group. The low shot from the right barrel is from a rifled tube that I had cut by Les Bauska. When I was testing it on a scoped bolt gun it would group into 2" at 100.
It really is a 100yd gun at best by power, so the left tube is smooth and I threaded it for choke tubes. Thought I would have a combo gun that would be good for deer as well as reasonable with shot. The 70 twist in the right barrel should be ok with shot loads.
I am making a rib for it that looks more shotgun than rifle. Set up with a bead front (or flip) and a folding leaf. At this point I am just trying to keep the weight down. If I can beat 7# I'll be happy.
Probably installing scope mounts flush with the rib.

I may have solved the left issue. The cheekpiece is a bit too fat so taking that down should solve the problem.
The right barrel is larger diameter than the left. I will file the rib in so that it is pointed in the right direction and hopefully that will solve the patterning problem.

For regulation I went to the Brownell's book and the sight adjustment page. The barrels originally crossed by 7"
The distance between the center spacer and muzzle was around 11".
I treated that as my sight distance and had to drive the muzzles apart by .120 to adjust for 28" at 100 yds.
It worked, but I will probably add another .010or a bit less for a 50yd regulation.

Tom