Path of the space marine

Space marines are genetically engineered super soldiers, who can shoot guns, clobber people in heavy armour, chew through metal, and other 40K lore based things you regular D&D player doesn't need to know about.

You were born, then taken in to be a super soldier, where you were tested, then had extra organs added and some other DNA meddling shenanigans. You may want to talk to your DM on if Space marine Barbarians exist or not, or how it can fit in with the story. Maybe you're a half humanoid/construct from the astral plane or far realm, maybe a celestial blessed you after watching you overcome a hard task, or maybe you got used to killing demons in the abyss, for example.

Marine training

At 3rd level, your training allows you to shoot guns and preform better whilst wearing full plate, even tapping into your rage, you gain proficiency in firearms and heavy armour. Additionally, you can gain the benefits of rage whilst wearing heavy armour.

Augmented body

At level 6, The process of becoming a space marine greatly impacts your being, you gain the following benifits:

• You're proficient in the perception skill
• You can eat poisoned or otherwise inedible food
• You can breathe through toxic smokes or underwater
• You grow an additional 3d4 inches
• Your Unarmed strikes deal 1d10 + your strength modifier

Explosive ammunition

At 10th level, Your firearm shots deal an additional d6 worth of fire damage equal to your proficiency bonus. you can gain the damage if the damage is with a firearm, you have advantage or a friendly creature is within 5ft and you don't have disadvantage.

Martyrdom

If you die you take the enemy with you. starting at level 14, you can use your reaction when dropping to 0 HP to a melee attack to deal the same amount of damage back at your attacker. You can do this once per dawn.