The Atlas 10/12 inch lathes are not a smaller lathe with a riser block under the headstock. They are their own models of lathe in the Atlas line. The 6 inch Atlas was a small hobby lathe comparable to the Chinese 7x12 mini lathes. Zamac alloys are damned good alloys. They are strong and have excellent bearing qualities. Zamac alloys have a tensile strength of 41,000 to 61,000 psi, depending on the particular alloy, and a shear strength of 31,000 to 47,000 PSI. The zamac parts of a 10'12 inch Atlas were all non-structural parts such as change gears and covers. All the important parts were steel and cast iron. For airgun and most gun work, an Atlas 10 or 12 inch lathe is plenty. I have an old QC54 and am looking at a 12x24. I also have a 7B shaper. So far, they are both very good units. If you're going to chamber rifle barrels for a living, you'll want a heavy lathe, but air gun work is not the same as reaming rifle chambers.
You did well with your lathe purchase. If I had the room, I'd like to have an old British Harrison AA330, as it's a 13" swing by 25" between centers toolroom lathe that weighs 1400 pounds.
Zinc die casting alloys have gotten an undeserved bad rap because of of "pot metal" which is NOT Zamac. It is a cheap lead/zinc alloy that used to be used in toys and other older die casting.
Atlas used one of the best Zamac alloys available, though I don't remember exactly which one. There are something like 7 different ones.
