Gary I use 1/8" steel for the buttplates. This how I was taught, However I think that you could use 1/16" just as well. I forged one with that thin of material is it turned out just fine. I actually think that it was easier to work with the thinner stuff. It would also be more authentic to use the thinner material. Some rifle buttplates were so thin, they have nearly rusted away over the years. AS far as triggerguards go, it really depends on the style. I have used 1/4 x 3/8 to 1/8x1/2 all the way to a railroad spike. I use what ever I have on hand depending on the style. I suppose you could even use 1/16 stuff for a real simple barn gun strap type guard. Really depends on what you have and the style you are going for.
Good luck forging,
Marc