There are a number of firearms that people make for themselves, such as the AR type, and the AK. Perfectly legal to make a non-full auto in the US for yourself- you can't sell it, but you can make it.
To make a Muaser 98, or an M1 Garand, you will have some VERY complex machining and heat treatment to perform. If you are a Jouneyman tool & die maker, with an entire machine shop, imagine you could build one. Should not cost more than $5-6000 to do it.