I have the opportunity to purchase this piece, but I have no idea of how to research it other than to ask here.
Could be a US 106 (105)mm recoilless rifle round.
Some info here: 105mm or 106mm?
Could you show us the side with the markings please and the headstamp?
It is not with me but as soon as I can meet up with the owner I will take a picture.
It is a 106 mm Recoilless Rifle Squash Head Round.
Being not an expert in this field, I know the 106 mm recoilless gun from hands-on experience and in my memory the case had a visibly larger diameter than the projectile.
Could it be the cartridge shown in the first messsage is a 90 mm?
Jochem, maybe you are having the 75mm on your mind which has a bottlenecked case?
The 105mm and 106mm in general appearance are both like above.
it is quite possible that my memory fails me, as it did in the case of Alarmleuchtkörper, if you remember.
Alas, I have no illustration of the 106 mm in my library.
On a side note, contrary to the impression I make, I must add that I am not old enough to have had hands on experience on a 75 mm recoilless rifle (which by the way was never used by Bundeswehr as far as I know). :-)))