Howdy
I had the brass from an Uberti Model 1866 analyzed a few years ago. The analysis said it was 56% copper and 44% zinc. A true brass.
The original Model 1860 Henry rifle as well as the original Winchester Model 1866 rifles had frames made from a type of bronze known as gunmetal. The composition was 80-88% copper, 10-15% tin, and 2-5% zinc with a small amount of lead added to help fill the molds.
Uberti's replicas of the 1860 Henry and 1866 Yellowboy are currently only available chambered for 45 Colt or 44-40, although a friend does have an older 1866 chambered for 38 Special. You will not find any Uberti brass framed lever guns chambered for 357 Magnum, I suspect because the impulse of recoil might tend to stretch the brass. Don't forget, both of these rifles were originally chambered for the 44 Henry Rimfire cartridge, which was pretty anemic by modern standards. 45 Colt and 44-40 pack more punch than the old 44 Henry Rimfire round.
I seem to recall an incident Mike Venturino mentioned in one of his books. He was looking at buying a used replica of the Henry or 1866, the details escape me right now. Mike went off on a trip before buying the rifle, and while he was away the owner ran some ammo in that developed too much pressure, probably higher than SAAMI Max. When Mike got home the brass had stretched and the head spacing was ruined. He did not buy the rifle.
But with standard SAAMI spec ammo the brass framed rifles are fine.