Howdy Again
From what I've heard, most Uberti 44-40 rifles seem to be coming through with .429 rifling groove diameters these days, but you will only know for sure after you slug it. My '73 is an older one, it slugged out at .427. But it was made back in the 1980s. The Henry that I bought a couple of years ago slugged out at .429.
You can probably use those Big Lube bullets as your slug, I used a bunch of J/P 45-200 Big Lube bullets to slug some 45 Colt pistols the other day. As long as they are reasonably soft, they will work fine. The lead will probably collapse a bit into the crimp groove and the Big Lube groove, but I had enough bearing surface to slug some barrels with the Big Lube bullets the other day.
Make sure when you measure your slug that there are actual drag marks on the raised portion of the slug. You will see drag marks running the length of the slug on the low points, representing the lands of the rifling. Make sure you see similar drak marks on the high spots, representing the grooves. If you do not see drag marks, the lead may not have completely filled the grooves, and your slug measurement will not be an accurate representation of your rifling grooves. Hopefully, enough lead will be displaced from the land part of the rifling to smoosh over into the grooves and completely fill them up.
If you don't see drag marks, you cannot be sure the slug is an accurate representation of your rifling.
Here are a couple of photos to illustrate. This photo shows the J/P 45-200 bullets I was using to slug some 45 barrels. At the top is a J/P 45-200 as cast, then you can see how it gets deformed as it is forced through the barrel.
In this photo, the arrow on the slug at the top points to good drag marks on the high points of the slug, indicating that the rifling was completely filled with lead, and measuring the slug will give a good measurement of the diameter of the rifling. On the slug at the bottom of the photo the arrow points to the same spot on the slug, but there are no drag marks, indicating that the lead did not completely fill the groove. So that particular spot on the slug will not give an accurate measurement of the rifling groove diameter.
Now, it must be said there is more than one way to rifle a barrel. These barrels were rifled by dragging a tool through the barrel after the original hole was bored. Dragging the tool left tool marks running the length of the grooves, and the drag marks on the slug were created by the tool marks in the rifling. Some methods will not produce tool marks like this, or they may have been lapped out. But most barrels I have peered down have tool marks that will leave behind drag marks on the slug, once the lead has completely filled the rifling. If you peer down the bore with a bright light, you will see these tooling marks.