No, sorry for the confusion. The 12 guage uses a 10 gauge blank and the 20 guage uses a 12 guage blank. Sorry about that. That is another reason the 20 gauge is a good choice in the ultra slug is because you would have better interchangability (is that a word?) with that frame. At least that is the way I understand it. You would be able to put any 12 or 20 guage barrel on it most likely. Your gun store should be able to tell you for sure. Or if you want I can ask or search on the other forum. Might take a few days though.
On the H&R's website they list the youth model Ultra slug at 7 lbs, and the others as 8-9 lbs. So I would again assume (probably not the best idea) that the 20 gauge weighs 8 lbs and the 12 weighs 9 lbs. A 1 pound weight difference. And the steel bar in the stock can be removed if you want. I just weighed it on our bathroom digital scale and is says it is 1.5 lbs. I do not know how accurate that is though because I am sure the scale is calibrated for heavier weights and it only reads in .5 lb increments. So it weighs between a pound and 2lbs. I am sure some searching would give you the correct weight. I saw somewhere before, just don't remember where.
The tracker weighs 5.25 lbs irregardless of guage. So after installing a scope and quality rings the Ultra could easily weigh twice what the tracker does.
JCC, it almost makes me want to call H&R again just to see what the response would be this time? That might not be a bad idea to do if you plan on buying one. I was going to email them, but could find no email option on thier website.
Really what it boils down to is what ever the rifling is in these rifles they are proven to be pretty accurate, way more accurate than they should be for the price according to my gunshop

. It really just seems to be a technicality, and barrel rifling is not really an exact science from what I can tell. Again I will use ML's as a comparison. I shot rifles with 1 in 36, 32, 28 and 24 inch barrels. They could all be made to shoot pretty well. You just need to find the right bullet and charge to get it there. And I shot the even slower side locks for quite a while when I was younger. They were pretty accurate with a light ball and slow twist. Precision Rifle's web page has suggestions for which bullets work well in what barrels for muzzle loaders. The results are sort of interesting. I don't see why the same things would not apply to a slug gun considering they are shooting a very simular bullet at simular speeds. H&R also told me all thier guns are tested with plain old Winchester saboted bullets, not the new fancy bullets.
If I had to guess the barrels with the tighter twist rate would work a bit better with the fast light bullets at longer distances. Really the weight differences are not the much though. A one ounce slug is 437 grns, and faster saboted bullets are around 400 grns. There is probably a difference in the BC because of the shape though.
Good luck,
Paul