Breech loading or Mortar ?

[fernando]

Certainly a mortar. A breech loading chamber would not have such 'undulating' exterior, as it shoulf fit and rotate smoothly into the chamber. And also as Richard wisely suggests, its front shoud have a tapering section, to lock well into the barrel.
If you put it in the upright position, you will see it makes all sense that it belngs in the mortar typology, with its wider base and all.