Heavy stubbers.

[Path Walker]

A Heavy Bolter, like any Bolt weapon, is not easily or freely manufactured. Its a scarce and difficult weapon to supply in any numbers, a lot of Regiments and Worlds simply can't keep as many Heavy Bolters as they'd want in service. Speaking in Fluff terms, not rules-wise, the Heavy Stubber also has a much higher rate of fire, meaning it needs a lot less training so are more suitable for the rapid turnover of soldiers some regiments experience.

Heavy Stubbers are used when Heavy Bolters aren't available or if their ammunition is too scarce to assign to everyone who wants one. Heavy Stubbers are simple and direct weapons than can be made by anyone with a decent workshop and can be manufactured to fire whatever ammo you have on hand, thus why they're so popular amongst gangers.

Bolters and the Bolt shells they fire is seen by many in the Imperium to have religious significance. Each Bolt Shell is blessed and sanctified individually, the belt fed, mass produced ammunition for a Heavy Stubber is not. On some worlds, possession of a single bolt shell can be punishable by death if the local law catch you. The Bolter is so tied to the Astartes within the psyche of the average Imperial Citizen that most wouldn't deem themselves worth of such a weapon. A Heavy Bolter Team in the Militarum are lucky and while they tend to become more practical about these things than the average Josiah Agri-worlder, most would still be honoured with their task.

If you're looking at the army list and using this to try and justify the existence of the Heavy Stubber based on that, you're not going to get the correct answer.

[Da Gargoyle]

Isn't the stubber the one where the target slaps at his neck and comments that the mossies are biting hard?