Gunner

Gunners are a specialized type of force on any battlefield, their jobs are to lay down lead onto the enemy. But, don't mistake these gunners for the dramatic 'gunslinger'. Gunners focus on less on being dexterous with their firearms, and more about changing the tide of a battle with a volley of bullets. Or, a single, ear-deafening strike. Gunners are seen not on the direct frontlines, but close enough so that their bullets reach their enemies. Their usual weaponry, not surprising, are large machine guns or torso-blowing antimaterial rifles. But usually, they are proficient in all types of firearms, so don't underestimate them, even if you get past their deadly fire.

Level 3: Gun of Might

Gunners need a certain amount of strength and resilience to operate their weaponry. When using firearms, you can use your Strength instead of your Dexterity for your attack and damage rolls. Additionally, you ignore the loading property on firearms. Pick a Gunner Path to dictate what type of Gunner you want to be.

As you level up, you will gain more benefits with the given Gunner Path you have chosen.

Artillerist

When you hit a creature with a ranged attack and its with a firearm, you can add an additional 1d12 damage of your weapon's damage type, and push them back 5 feet (limited to once per turn).

Crowd Clearer

If another creature is within 5 feet of the creature you just attacked with a firearm, you can attack that creature for free (limited to once per turn).

Level 7: Gunnery Position

Gunners often pair their proficiency with devastating firepower with a good, and fortified position. If you did not move at all during your last turn, and you continue to remain stationary, you can use your bonus action to enter a Gunnery Position. In this, all of your ranged firearm attacks are made at Advantage. However, your speed is reduced to 0, have Disadvantage with Dexterity Saving Throws, and all melee attacks made against you are made at Advantage. Use a bonus action if you wish to exit Gunnery Position. Select the 'Gunnery Position' option when in it, and select 'Normal' when not.

Also, your Gunner Path has gained an additional benefit:

Crowd Clearer - You gain Volley Fire.

Artillerist - You gain Firebomb Strike.

Firebomb Strike

While in Gunnery Position, you can fire an explosive round onto your enemies. As an action, you make a one ranged firearm attack against a creature or point. Each creature within 5 feet of that creature/point must make a Dexterity saving throw or take 3d12 fire damage on a failed save, and half as much on a successful one.

Once doing this, you take 1d6 Fire damage, as the explosive round generates heat when being fired.

Gunnery Position

You are in a Gunnery Position.

Normal

You act as normal.

Volley Fire

While in Gunnery Position, you can let loose a cone of bullets onto your enemies. Using an action, each creature in a cone (with the size of the cone equal to your firearm's normal range) makes a Dexterity saving throw. On a failed save, a creature takes 3d12 damage of your weapon's damage type, or half as much damage on a successful one.

Once doing this, you cannot make ranged attack with the firearm, until the end of your next turn. As, your gun must cool down before being used again.

Level 10: Rapid Fortifications

Gunners, if in need of cover in a short amount of time, have found ways to make do with little. Protecting themselves from enemy attacks, even with the most impromptu defenses. You can make a Survival check, and if that check beats the DMs intended DC, then after 5 minutes, you can make a temporary structure that will fit a single medium-sized creature. When in this structure, you have advantage on Stealth checks (unless the structure is in an area where it cannot be hidden), and gain the benefits of Three-Quarters Cover. All ranged attacks that fail to hit you, hit the structure, taking the damage instead. The structure has 40 HP and once it reaches 20 HP, you lose your advantage on Stealth checks and only gain the benefits of Half Cover. If the structure reaches 0 HP, it is destroyed and you lose all benefits.

You can only do this equal to your proficiency bonus, recharging all uses on a long rest.

Level 15: Destroy and Clear

Gunners of this experience and skill are able to act with enough firepower to match entire units of soldiers. Being able to prioritize targets, or sweep large areas of ground if needed. Starting at this level, your firearm attacks now ignore resistances, and you gain an additional benefit from your Gunner Path. 

Crowd Clearer - You gain Never Ending Fire.

Artillerist - You gain Antimaterial Shot.

You must be in Gunnery Position to do either of these. 

Antimaterial Shot

Once per turn, you can replace one of your attacks with a powerful shot. If you hit your target, the attack is treated as a Critical Hit. Non-magical structures and objects take double damage as well. If a creature is hit, it must make a Strength Saving Throw or be pushed back 20 feet and knocked Prone. On a successful save, it is pushed back 10 feet and does not get knocked Prone. If that creature is pushed into another object or creature, they will also suffer an additional 3d6 Bludgeoning damage. 

Once doing this, you take 2d6 Bludgeoning damage, knocked 5 feet back, and you and any creature within 5 feet of you are Deafened until the start of the their turn.

Never Ending Fire

After using all of your ranged attacks with a firearm (including your 2 additional free attacks, if applicable), you can choose to attack again with a -2 penalty to your attack roll. You can keep attacking for free, but each subsequent attack gets another -2 penalty. These additional attacks end once you miss. The roll-to-hit penalty resets at the beginning of your next turn.

Once doing this, you cannot make ranged attacks with the firearm, until the end of your next turn.

Level 18: God of Destruction

At this point, Gunners reach the pinnacle of what they can do. Gunners of this level can operate with a level of precision and destruction to instill fear into even the most brave of soldiers. Starting this level, you can now move up to half of your movement when in your Gunnery Position. Additionally, your ranged firearm attacks now ignore immunities, and all damage that come from your Gunner Path abilities increases by 2 additional damage dice. This also includes the damage you take from your abilities, as well.