CCar - the task ahead of you is a big one & commendable. I hope it is successful. The ability to buy a newly made DR of large bore would indeed be wonderful.

Doubles are a definite problem. There is no 'easy' method of mass production regulation - which you probably know - which some companies/people will not acknowledge or refuse to understand.

Best method is to pick an ammunition source and load, then build a gun to regulate to that. Unfortunately, as far as I know, there is no facory ammo in 20 bore that is suitable for heavy or big game. If so, I am not aware of it - entirely possible - just that I am ignorant of it's existance. I hope you have a choice in suitable ammo. Must pick one and regulated a gun for it as per The-Old-Way. Make any and all measurements possible, then duplicate them to inside a thousanth of an inch and see what happens. That's about the only advice I can think of, for a mass or semi-mass-produced gun.

Foster slugs are poor at best. Undersized pistol bullets in plastic sabots is not a suitable bullet either for shooting game - in my opinon. Full sized bullets of decent weight, ie: 460gr. or slightly heavier, as a round ball that size weighs around 320gr., the slug should be about 1 1/2 the RB's weight. The twist should be fairly slow - ie: about 36" for the short slug - or about 80" for round ball at high velocity. Of course, these handloads.

Unfortunately, there are few of us prepared to handload - it seems - the majority of large bore shooters - or perhaps large bore owners, shoot factory ammo - so they shoot very little due to costs & are not prone to developing loads for their guns, themselves. They want to buy a box of ammo that is suitable and shoots well - they were bore 160 yards too late, unfortunately. They'd have been about 40 years old in the 80's or 90's when such ammo was available.

There is some ammo out there, ie: H&H Paradox but I don't know if it's available in 20.

You are probably aware of this. Sorry if I wasted your time.