I will probably stay with historic military rifles. M1 Garand, M14, AR15.
The M1 and M14 are not much more than a semi-automatic "03 Springfield, so I will do others first.
I have the blueprints for a M1918 BAR, which may be a project soon.
The next one is already in the early stages. It is a M1911 Colt pistol. I will be doing it in 6:1 scale to match all the others. I plan on making it fully functional (except for firing actual ammo) in all details that are possible. I have decided to make the hammer, sear, disrupter, and trigger from titanium to give strength in very thin pieces. The rest will likely be sterling and gold.
Of course, it would be simpler in steel, but where's the fun in that?
When I was a boy, a neighbor gave me an antique (probably late 1800's) toy revolver pistol that shot .12 calibre ammo. It fired these tiny cartridges that looked like half size .22 cal BB ammo. It was basically a tiny percussion cap with a small lead ball pressed into it. The pistol was about 3" long. My mom was not happy when she found me playing with it, and told me to get rid of it. I hid it in my secret place in a false bottom I had built into a part of Grandpa's garage workbench. No one knew about it. I should go by there someday and knock on the door. Maybe it is still there. Probably worth a couple hundred bucks today. I don't think anyone would find the secret compartment unless they tore down the workbench.
Anyway, that ammo would be just perfect for a 1/4 scale .45 cal pistol that would fire real ammo.