I agree with badger. We have to remember that these artillery “collector items” were laying around in the tens of thousands during the period most of the “Trench Art,” whether actually done by soldiers in the field to pass the time away or by factories selling “war souvenirs,” was made.
I personally prefer the ones that look to have been actually made by amateur artists, but most of the few examples I have were likely done in small factories.
My personal opinion is that this is an absolutely true, and interesting, art form. These souvenir items are not limited just to images worked into the brass. There are many examples of small knives made, for example, from 7 x 57 mm cases and bullets, and then engraved with various “captions” on the side of the case. I have a stamp for impressing the wax seal on documents or envelopes using a cartridge case for the handle, and a cigarette lighter made from a rifle case. I have a couple of 7.9 Mauser cases made into pencils - actually, I should say they are made to look like 7.9 Mauser cartridges, but actually made purposed for that use, one of which commemorates the German - Austro-Hungarian Alliance in WWI, as I recall. Those might not qualify as “trench art,” but rather as “commemorative cartridges,” but who cares? They are along the same lines.
Interesting stuff as a side line to any cartridge collecting!