You speak of the G3 with full wood kit as been custom. I wouldn't call it custom at all, none of the parts that go into one are user specific. A better description is period specific.
Same thing with an M16, You can have the modern RIS look, but you might go for a solid stock, large front hand-guard, chrome bolt and make it 1960's period specific. It's still not a custom.
Now if I were to take the G3 to a wood worker, and have a full walnut stock in a different pattern, with brass additions and file work. That to me is custom. It's made just for me, no one else has one, and if they do it will be because they copied mine, the term bespoke could also apply.
Picking off the shelf parts will never equal a custom/bespoke gun. It'll end up as a modified, or upgraded.
Not custom.