Brahma
Obi-Bro Kenobi-X
FYI
Heavy artillery does not affect your damage. It only affects the cannons and ballistas found in the arena and zorah fights. The decoration you want is called Artillery, but it is very rare (about same as another attack decoration). However, you are already using the charm so you are covered there. Probably best off swapping those out with anything else.
If you enjoy shelling, get ready to farm Kulve Taroth when it comes to PC. It drops Wide4 gunlances that are almost purely shelling. Also note that shelling is only affected by two things: artillery skill and felyne bombardier food buff. It can not crit and attack does nothing. This will become more important later on when those Wide4 lances are available.
Thank You!