From my experience using the slug shotgun on Zealot and Arbitrator(mostly in Havoc 20-30 and Aurics)the main problems are ammo capacity and reload behavior.
The core issue is that this shotgun has a 13 rounds baseline. The long-barreled Executioner shotgun has only 4, and the double-barrel has 2. How did we go from 4 rounds to almost four times that amount, while the character still loads shells one by one? This problem isnāt limited to just one mark either - the other variants have 9 and 11 rounds iirc, and they suffer from the same issue.
This leads to a feeling I think everyone who uses these weapons has experienced: āWhy do I reload so much?ā
Thatās how I felt constantly. Not only do these shotguns consume ammo almost as fast as a bolter, but they also have a similar magazine size, while still reloading shell by shell. On top of that, they are worse in terms of damage. Their only real upside compared to the bolter is that theyāre faster to pull out, use, and move with. Overall, there are still more downsides to these shotguns than to the bolter.
Thatās probably why you see far more bolters than combat shotguns. When you use a bolter, the feeling is: āOh wow, this thing is cool. Big damage. Daka-daka.ā
When you use a shotgun, the feeling is more like: āWhy do I reload so much? Why did this guy need four shots to die - heās just a gunner? Where is my ammo? This feels useless.ā
I think a good solution would be to reduce magazine size(while the ammo reserve would stay the same) and give proportionally higher damage in return(from 14 to 8, from 11 to 6 and from 9 to 4 probably). Alternatively, the shotguns could get some kind of fast loader that loads the entire magazine at once.
Special shells
Another issue is the special ammo. It still loads one shell at a time, and I honestly donāt understand the purpose of one of the specials in particular.
ā¢ Wide spread stun shell
I never use this and find it overall useless. A shell that loads one at a time just to stun enemies in a āwideā spread or kill a bunch of poxwalkers? You know what already does that? Heavy 1 from the relic blade - or any other horizontal heavy or push attack from a heavier melee weapon.
Yes, the shell has a lot of forward cleave, but most situations where stagger is needed are right around the player, and using a melee weapon is simply much better than doing it with a shotgun.
And that raises the question: what is the purpose of this variant at all?
The fire shell deals better with elites(even armoured ones)because it has a DoT. It also performs similarly against hordes. The slug shell is better for specials and light elites. This leaves the stun shell shotgun with a redundant niche. This poor state could open a window for a rework of the special. For example, the special could apply Brittleness - perhaps weaker than the Executioner shotgunās version but usable more often, or equal in strength but compensated by the fact that the base shot of this shotgun is worse than the Executionerās in terms of damage. This would give it an anti-armoured elite niche, where the special both staggers and applies Brittleness, instead of larping as horizontal melee heavy attack.
ā¢ Fire shell
This one is probably fine as is, but it could benefit from a slightly wider spread so itās easier to catch more enemies in the fire effect.
ā¢ Slug shell
Iāve used this one a lot, but I still have questions. Does it one-shot any non-armored, non-unyielding enemy with a headshot at any distance in Havoc 40, when used with a melee-focused Zealot, Scum, or Arbitrator build? Probably not.
For example, on auric, my Martyrdom Zealot does not one-shot a Scab Gunner with a slug headshot unless it crits - it deals around ~1670 out of 1700 HP from a base state with no respite, full bore and +25% flak. In real gameplay this is often mitigated by team damage and buffs, but it would be nice if the slug was consistent and could always one-shot non-carapace, non-unyielding enemies on a headshot, without needing ranged buffs beyond its own perks and blessings. Considering the reload time for the special, it really should function as a reliable gunner / special killer at any distance, at least with headshots or crit body shots. Normal body shots might be too much, especially since the weapon effectively has an almost unlimited supply of special shells.
Overall take and conclusions
A good suggestion to fix the combat shotguns could be to:
Or
- Reduce the need to reload constantly by adding some sort of fast loader
Make the slug and fire(not really needed) specials more consistent, and rework the stun shell -perhaps by giving it a Brittleness effect like the Executioner shotgun, or by increasing the stun strength on the first targets hit so itās more effective against Crushers, for example.
Their role as a sidearm for melee builds would be fine, if not for the fact that they arenāt particularly good sidearms. Youāre almost always better off taking a revolver, bolt pistol, or executioner shotgun - those weapons feel great 
(except the fast-firing revolverā¦). These combat shotguns should be comparable to them in both feel and performance.
while sprinting forward