Ok so my grandmothers friend is French and as a young kid picked up a shell casing that I can't identify also it just happened to be from dday... or so I'm told!! The case is 25mm by 100m is straight and not bottle necked it anyway the headstamp on the casing says S3 in that order
Your description sounds like a 20mm round , but a picture would really help. 20mm (rather than 25) were used by MGs for example.
There would be little chance to prove it was expended on D-day, of course. It might be a German round. Pictures will be useful.
I hope You don't mind, mate, but knowing what a pain uploading here from a mobile device can be I'll drag your pics over from the 2T thread you also started. (The more ID-ing, the merrier!) My only immediate thought on there was 'Flare pistol'.
I think you're correct, Herr Von Poop. My vote is for a 26.5mm OR 1" flare cartridge case. As for specifics: It *could* be a German 26.5mm signal flare casing. The diameter and length (~102mm) match the those used in the standard German Leuchtpistole 42. The large primer is also consistent. But I believe the cases were aluminum but the one in the photo appears to be brass. I brass cased flares were made for the Webley brass flare guns (the model numbers escape me but they're unmistakably WW1-era British -- heavy, clunky, and meticulously machined from solid brass). I know next to nothing about flare guns. Hopefully someone can pick up from here.
