adding horn to a military stock would be an option

here on an older austrian Frühwirth rifle

and here on an M 88 sporter

you find here many sporters with octagonal barrels
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=188673&an=0&page=0#Post188673
and I would not hesitate to copy this design for a Mosin sporting rifle.

this Mosin is clearly inspired by such old Suhl made M 88 sporting rifles

something like this with an octagonal barrel