Strange crimp

it looks to be factory made
on the aluminium part is engraved the number of the lead shot
(italian shell, with Fiocchi hstp)
No idea?

here are better pictures


Possibly some experiment designed to improve shot pattern?

That’s what I’m thinking too; these sorts of things are sometimes used in “spreader” shells, to give as wide a pattern as possible when shooting at fast, close targets (when hunting woodcock, or shooting skeet, etc.).

shot pattern determined by choke

??? what does it mean ??

Mario has a very similar cartridge.Same crimp but his round is closed with a paper disk instead of your metal cap.He will give me some more info

Shot pattern is not 100% controlled by the choke. There are other factors that involve the wad or cup, the regularity of the shot in size and shape, etc. One of the reasons you should pattern a shotgun with the ammunition you intend to shoot in it for any given purpose.

simply put ‘choke’ is a restriction in the bore at the muzzle of the shot gun.smaller diameter usually equals tighter shot groups and vice versa.

“Usually” is the operative word there. Wads, shot quality, and even just different loads can effect a shot pattern easily by a half choke (IC becoming as full as full chokes normally are) and sometimes even a full choke (modified becoming as tight as full normally is). The point was, that while choke is the primary factor in determining shot pattern, it is not the only factor as your first note implied. A day with your favorite shotgun and a hand full of patterning targets, along with eight or ten different shotgun loads, will prove that out. They will not all pattern identically.