There is a ton of info out there on 12 gauge, however, there is very little on 20 gauge barrels. For some reason I was under the impression a 20 gauge rifled barrel groove diameter would be around .615". I recently lucked into (practically stole) a pair of NEF Tracker II's, both a 12 and a 20 gauge. I have not slugged the barrel, but the 12 gauge seems to be just like every other H&R or NEF at .731" groove diameter. The 20 gauge though, is much larger. I slugged the barrel multiple times, and the groove diameter is right about .6265".
The 12 gauge shoots great. The 20 gauge, not so much. I've only tried Winchester rifled slugs, and Hornady SST's, which historically have not been all that impressive for me. Still, they both shot around 8" at 50 yards. Oddly enough, all but one of my reloads have proven much better. The best so far has been a Lee 575-492 in a Fed 20S1 wad. Specifically, it has shot best with only a single card wad, then the slug, then another card on top to take up the space. This combo measures about .625". This same load in a Mossberg 500 rifled barrel shot even better, although with some fliers. These fliers appear to be the ones that shear the petals off.
Long story short, I would like to try some large round balls, maybe .630". My concern is that they may unintentionally find their way into another 20 gauge. My brother has the Mossberg 500, I can't measure that one until he stops by again. I only own the Tracker II. In fact, this is the only 20 gauge I own. I'm hoping some of you out there can provide some measurements of your 20 gauge rifled barrels. Even if it's just a rough measurement with calipers, it would help me tremendously.