Looks like they are firing blank shells in that film.
Normally the barrel assembly would slide rearwards on firing. I believe it's the first "recoil less" artillery piece (I believe that is the term), which obviously helps accuracy. It was designed in 1897.
The tank periscope looks like those used in the turret and drivers hatches of the Centurion. Not magnified, just for all-round vision. Some have a sacrificial optically ground lens ahead of the prism lens and prism. Not a lot of use because bullet fragments and shrapnel just take out the sacrificial lens, the prism lens AND the prism! But it does protect the prism lens and prism against those tankies who get out to clean them with anything that's handy. Usually something that's covered in grease and grit. A bit like the stuff that civvy shooters use to clean their No32 ocular and OG lenses..............
It was labeled up as coming out of a Chieftain tank but I took that with a pinch of salt when I purchased it. I assumed it to be a periscope from a British AFV, and, perhaps, made by Avimo. No it doesn't appear to be magnified and so out of a Centurion then?