Shooting the 16bore WR today with 5-1/2 drams and ball that's a few thou over groove size today seemed to work very nicely!

As the balls are pretty much never perfectly round (but can be very close plus and/or minus a couple or so thou all over the globe) and in the context of wanting to get a good gas seal behind the ball for consistent pressure and velocity, it's good to budget a few thou oversize any way you look at it.
The amount of effort it'll take the charge to swage the ball all the way into the rifling (compared to an undersized ball) is insignificant. The insurance against leading and excessive fouling is very valuable.

Today's shot string was the third time out with this new-to-me rifle.
Previous targets were shot with slightly undersize ball (as it was what I had) and the vertical difference on the target between right and left barrels was greater than today's shooting with oversize ball.
I got the same kind of results with the Purdey and with my other two sixteen bore double rifles.
With the other rifles I'd found that overall (aggregate) accuracy is always best with oversize ball.

This is shooting real black powder, with real wool felt wadding, lubed with a relatively wet lube recipe, over waxed board cup-shaped over-powder cards.

Cheers
Tinker