I always wondered if guns like CC.'s evans needed a faster twist because it was only a rifled choke (stripping issues?)

I checked the barrel specs on my 20ga. It was rifled with 8 narrow lands of .005. With full bore slugs it is exceptionally accurate and that was also the plan for the Martini. Round balls are the main focus as I think that they do a more consistent terminal job than virtually any drag stabilized slug and at much longer range.
It is nice to have a gun that shoots factory ammo accurately as well

I originally chose the slow twist in the 20 because many of those full bore slugs are drag stabilized and shoot well with a smoothbore. I wanted to see what "some" twist would do with them, knowing that their max range would probably be around 100yds anyway.
That 20ga barrel is currently on my cape gun and the smooth and rifled barrels shoot pretty well parallel with 3" Brennekes so I am limited to those. Neither the Hastings or Lightfields shoot in the smoothbore and round balls are probably out of the question although I haven't tried them.

As Daryl mentioned earlier, I too am not a fan of overstabilizing. ( I have two interesting stories about rifle bullets taking a 90deg course correction after entering an animal - both I am pretty sure were due to overstabilization.)

My area of the state is shotgun only so that has been my focus for the past few years.
Switched to Lightfields this year and so far I am confident, but one deer alone says little.
I didn't like that the rib/ lung hit at 80 yards was on the verge of staying inside the deer. If I held on bone, the results may have been different in a way that I wouldn't have been happy with. I found some 3"ammo after the season and that may add on some insurance.

I did take two deer this year with my .58 flint and the round ball performance bordered on perfect.
A doe at 15 yards and a buck a few minutes after at 70. RB, 100gr fff, shoulder shots, pass throughs easy recoveries. I took a doe a number of years back at double the distance of the buck with the same .58 and that was a pass through as well
Every deer shotgun should work as well terminally as the equivalent muzzleloader. If round balls work, then I am hoping that will be a simple solution.

Tom