I know it can be done in a saved gamed game.
{Inventory 0x8255
{box
{clear}
{item "bar" filling "bullet155" "fg" "ammo" 999999 {cell 0 0}{user "hand_right"}}
{item "bandage" 5 {cell 4 2}}
{item "helmet1_usa" {cell 0 2}{user "head"}}
{item "body_armor" {cell 2 2}{user "body"}}
{item "bandage" 5 {cell 4 3}}
{item "m24x5" "grenade" {cell 4 4}}
{item "m24x5" "grenade" {cell 0 4}}
{item "morphine" {cell 4 7}}
{item "morphine" {cell 4 8}}
{item "mk1" "grenade" {cell 5 0}}
{item "mk1" "grenade" {cell 5 1}}
{item "mk1" "grenade" {cell 0 5}}
{item "mk1" "grenade" {cell 1 5}}
{item "mk1" "grenade" {cell 0 6}}
{item "mk1" "grenade" {cell 1 6}}
Now the BAR is firing 155mm fg rounds. Here is an example of this,