Omaha Beach find - need help on identification

Hi all.. new to this site so take it easy on me
I was recently in Normandy touring the beachheads and on Omaha Beach my 12 year old son found what I think is possibly a large bore (extremely rusted) barrel along the base of the cliff. Extending what I have it seems that it would be approx. 60mm wide with a 40mm bore with 10mm wall thickness. It has large cracks running through it and has been smashed up. It has distinctive lines running around the opening as the pictures show below... anyone have any ideas.. hard to get proper pictures but see my best attempts below. What we have is about 17cm long but it was obviously much longer.
Thanks in advance.. he would love to take this to his history class so any information would be greatly appreciated.

 

Yeah..I was thinking AA gun as well.

 

Soirry to dampen the mood, but I don't think your son should take this artifact to history class.

I assume you wish your son to grow up as a law abiding citizen? You have broken French law by removing relics from a battlefield. See here Clarity on WW2/WW1 relics in France - MDF Metal Detecting

By removing a historic artifact you make it harder for the archaeologists to carry out their "CSI Normandy" work and develop our understanding of what happened on that battlefield. Metal detectorists have been destroying historic evidence for the last decades. Many collectors of personal artifacts have also destroyed the opportunity for a family to know of the fate of a relative. The law is there to preserve the past for the future.

If its OK to take a piece of a gun barrel would you want your son to pick up some German's dog tags oer buttons?