I could be wrong but I have always thought that a Relic was somehow tied to a religion, church, etc. and an artifact was the everyday object like a CW bullet but could still have historical significance. I also consider them to be the same.
The way I read my dictionary an artifact needs to be made by human hands. A relic is essentially a corpse. Like a dinosaur bone, or this old carpenter's back saw I got with no teeth left on it. You know, a remnant is a relic.
an object surviving from an earlier time, especially one of historical or sentimental interest.
[TABLE="class: vk_tbl vk_gy"]
[TR]
[TD="class: lr_dct_nyms_ttl"]synonyms:[/TD]
[TD]artifact, historical object, ancient object, antiquity, antique"a Viking relic"
ar·ti·fact
ˈärdəfakt/ noun
1.
an object made by a human being, typically an item of cultural or historical interest.
"gold and silver artifacts"
[TABLE="class: vk_tbl vk_gy"]
[TR]
[TD="class: lr_dct_nyms_ttl"]synonyms:[/TD]
[TD]relic, article; handiwork
"hundreds of unidentified artifacts are stored in numerous rooms beneath the museum"