I doubt your factory load bullets are .426. Probably .427 as that was the standard in the beginning and for many years. Look on the box, as they often will tell you the bullet size. As Pettifogger mentioned, the Ubertis (and many other modern guns) have standardized on .429 for ease of manufacturing.
Anyways, here is what you will likely find. If you want to use .429 bullets, you will probably find that you need to limit your brass to Starline or Winchester, as those two have the thinnest necks. Uberti chambers are tight. .430 bullets get really iffy on chambering. A lot of folks use .428, and I've even in the last couple years gone from .429 to .427 in my Ubertis, and for CAS they are just fine. If you are trying for maximum accuracy, then the larger bullets may help. Loading and shooting 'em are the only way to know for sure. There are a lot of threads on this topic (what size bullets to use in Uberti 44-40) that you can peruse if you wish.