For CAS ranges the .427 bullets work fine. I load all my wifes smokeless 44-40 with .427 since she also shoots an older navy arms with the .427 bore. They shoot fine in the newer .429 guns too.
Now for longer ranges the larger bullets sometimes will tighten up the groups. I use .429-.430 in my black powder loads except some that are specifically loaded for original Winchesters.
I've heard some say their guns won't chamber well with a .429 or larger bullet but even our older ones with .427 bores will chamber a round with .429 bullet.
Bore size doesn't make the big difference. The big difference is the size of the neck area of the chamber. Some are tighter or looser than others. If you shoot enough .44-40s, especially if they are NOT Italian, you will find variations. I have Merwin Hulberts, Colts, Original Winchesters, repro Winchesters from Uberti and the new "Winchester" 73, and several Lightning rifles both original and reproduction. After years of piddle fooling around with different loads for different guns and getting the ammo mixed up on occassion I have standardized my own .44-40 loads. (I.e., bullet size, brand of brass and dies used.) I found that even rounds that fit into a commercial case gauge would not fit into all my guns. I finally acquired a beat up old Colt cylinder and use that for a case gauge. I have found that if rounds drop into the cylinder with zero interference they will fit in all my guns. The added bonus is I can gauge six at a time with the cylinder as opposed to one at a time with a case gauge. When shooting my Lightnings I gauge every round as Lightnings have no camming action like a lever rifle so the rounds have to drop into the chamber.