In its most general sense, a
fair use is any copying of
copyrighted material done for a limited and "transformative" purpose, such as to comment upon, criticize, or parody a
copyrighted work. Such uses can be done without permission from the
copyright owner.
https://fairuse.stanford.edu/overview/fair-use/what-is-fair-use/
It would be flat uncool to copy someone else's design and call it your own. Similar is not the same. If you make a 1° change in shoulder angle and call it something different, the designed has been transformed. If you go into commercial production and cause confusion in the market and safety issues the original copyright holder may have cause for enforcement.
If you make up something exactly the same as and never "sell" anything having to do with it, the original copyright holder would have to show what harm was done. Since you probably would not purchase her/his stuff, she/he is at no loss or harm.
I make my own 257 Roberts brass from 30-06 or 7x57. Is it 257 Roberts brass, technically no. Have I violated any copyrights no. Have I caused any harm in which enforcement would prevail no. If I took my re-manufactured 30-06 into 257 Roberts and marketed it would there be problems, oh yeah. I am quite certain of that. Even though we can't get Remington to make any 257 Roberts, it would cause "material harm" in that I am not aware of Remington manufacturing and marketing plans for their property.
BTW: My 257 Roberts BPO, for Black Powder Only, is just that a low pressure Black Powder or BP substitute only cartridge that is super cheap to shoot. Much easier on the ancient 1884 Model Springfield Trapdoors than the original 45-70. Besides, I already had 2 45-70. One a 1886 manufactured in 1888 and one High Wall manufactured in 1913. Neither is a "collectible" they are shooters.