A 40mm grenade is a slug, they just soar their way to the target like a football, maybe even a little slower than an NFL quarterback throws a football. Bullet means aerodynamic and fast, slug means, heavy and slow.
Meaning it fires bullets rather than slugs, it's key feature is RANGE. When they refer to something as a rifle they do so because that's the most important feature as well as it's intended purpose, the barrel is long and rifled. With that designation people should know you mean a shot shell for the grenade launcher or cannon, not a conventional shotgun shot shell. Also you can load it with shot shells usually known as beehive rounds because they fire so many pellets it's like and looks like a swarm of bees. Yeah, the launching tube is a smooth bore because it's similar to a cannon or a musket, it's a shotgun, it just lobs a heavy weight.
