Assault Rifle is a Primary Power Set for Blasters and Corruptors and a Secondary Power Set for Defenders.
Maybe you're a trained weapons expert. Maybe you just know your own gun. Maybe you're a cyborg and your arm is a gun. Maybe you have the magical ability to pick up a branch and somehow shoot bullets from it. Whatever form it takes, you do what you do best with your trusty assault rifle in your hands.
Taking this Power Set provides proficiency with rifles of all types. However, you almost certainly have a specific weapon that is more than a mere over-the-counter hunting rifle, and even something beyond a modern military automatic weapon. Your rifle is likely unique, and certainly special to you in some way (even if only by virtue of its broad features). It may or may not be irreplaceable, but losing it is never inconsequential. Guard it closely. If you lose your assault rifle, you can make do with any other semi-auto rifle that you pick up off a fallen foe or otherwise acquire. However, you must complete a short rest studying any new weapon before the following features become available (assuming level requirements are met): Buckshot, M30 Grenade, Beanbag, Flamethrower, and Ignite. This time represents you constructing or adding the necessary components for those features, or unlocking and enabling them if they're already built into the weapon.
Regardless of its specific form, your assault rifle is a ranged attack weapon that deals 3d8 piercing damage and can burst fire two additional rounds. When a weapon burst fires, it shoots one round plus its burst value. In the case of your rifle with a burst value of 2, a burst is always three bullets. You do not make separate attack or damage rolls for each bullet, but instead you may re-roll any damage die in an attempt to get a higher value. With your burst value of 2, you roll 3d8 for your damage and may opt to re-roll up to two of the dice. You may not re-roll a single die more than once, and if you do opt to re-roll a die you must accept the new result. You expend all three bullet for each burst regardless of how many damage dice you re-roll, and you must choose to burst before initially rolling for damage. Note that until you gain the Full Auto feature, you burst shots still strike a single target.
Your rifle has a range of 200/600 feet and a capacity of 30 rounds. If you run out of ammunition, you may use your Action to swap out magazines (assuming you have a full one handy -- and knowing you, you do).
As with all ranged attack weapons that you are proficient with, you add your Dexterity Modifier and your Proficiency Bonus to your attack roll, and just your Dexterity modifier to the damage roll.
At 1st level, you gain proficiency with all rifles. You can also make a Sniper Shot at 4th level if Assault Rifle is your Primary Power Set or at 8th level if it is your Secondary Power Set.
You have modified your rifle so that it can quickly convert its standard ammunition into rounds with various special features, often at the cost of efficiency. Selecting a non-standard round is quick and does not cost an Action, but it must be done on your turn. If you make multiple shots on a single turn, you may switch between ammunition types in between shots. The exceptions are burst firing or using Sniper Shot or Full Auto. Those features may only use standard ammunition. You may use Slug and Buckshot starting at 1st level. You can use M30 Grenade at 2nd level if Assault Rifle is your Primary Power Set or at 4th level if it is your Secondary Power Set, and Beanbag at 3rd level if Assault Rifle is your Primary Power Set or at 6th level if it is your Secondary Power Set.
Note that you do not possess this special ammunition as a separate resource. Your Assault Rifle converts standard rounds into special bullets on the fly.
You have further modified your rifle so that it can generate a combustible liquid and direct for your use. This liquid fuel is separate from your rifle's ammunition. Your rifle has the capacity to make 20 Flamethrower shots, after which you will need to complete a short rest in order to produce more.
You may use Flamethrower at 7th level if Assault Rifle is your Primary Power Set or at 11th level if it is your Secondary Power Set. You may use Ignite at 10th level if Assault Rifle is your Primary Power Set or at 14th level if it is your Secondary Power Set.
Starting at 12th level if Assault Rifle is your Primary Power Set or at 15th level if it is your Secondary Power Set, when you use your weapon's burst fire, you can divide your three d8 rolls among up to three individual targets. In addition, if you are a Blaster, you can add your High Damage bonus to each shot. Each target must be within 80 feet of you and no more than 10 feet from one another.