12G shot shell

I have a live shotshell with hs: FC 12 G HL. I hear BB’s inside and it feels much lighter than a regular 12G shell. Someone wrote “M80” on the side. Does it mean anything?

My guess would be that it’s a reload where someone tried to make a “bird bomb”-style “grenade” of sorts; the fuse on the M80 firecracker would be ignited by firing the cartridge, while the “designs” I’ve seen for these types of things usually had shot glued to the outside of the M80. These would travel out 70 or 80 yards and then blow up, spraying the shot as shrapnel (or so the theory goes).