I think the upgrade path for the AR-55 is one of the more satisfying and rewarding paths of all of the weapons. Starting as a standard 3 round burst assault rifle and ending up with basically a 5-round burst assault rifle with very little recoil and a highly increased zoom.
The AR-55 is the standard-issue assault rifle of Steelport's National Guard and SWAT teams. Unlike the K-8 Krukov, the AR-55 is a military-assault rifle, boasting a burst function as opposed to firing on full-auto. This saves ammunition and makes the AR-55 much more accurate than its predecessor, the AR-40 Xtnd. It is easily acquired by salvaging one from SWAT or National Guard units, or it can purchased from Friendly Fire for $10,000 after The Belgian Problem. By default, the assault rifle is fitted with two scopes, a vertical foregrip, weaponlight and fixed carbine stock.
The AR-55's burst-fire makes it more suited towards engaging medium and long-distance targets, in contrast to the full-auto K-8 Krukov, which is more suited towards close-quarters combat. It also boasts the highest per-shot damage of the assault rifles (not including the Viper Laser Rifle when upgraded), making it extremely useful as a marksman rifle when upgraded with a zoom scope.
Every time the AR-55 fires, it fires an entire burst, which is inefficient when dealing with multiple targets. It also has a short downtime between bursts, during which the AR-55 cannot fire at all.
Excellent accuracy and range, even with no upgrades
Firepower is quite high
Zoom Scope when upgraded
Conserves Ammo With Burst fire
Fires only in bursts, cannot single-fire or auto-fire
Lag between bursts
Multiple bursts required for a single kill
Poor against multiple targets
The burst fire mode requires tactical thinking when fighting multiple enemies.
The AR-55 resembles a highly customized AR-15, with a duck bill flash suppressor, tan Magpul furniture including a CTR stock, PMag, and MIAD pistol grip.
The AR-55's dust cover opens and closes after each case has been extracted, while firing the AR-15 in real life does open the duct cover when fired, it does not close automatically after each round fired and needs to be closed manually.
The AR-55 has a similar name to the BR55 Service Rifle in Halo 2 and 3, they both fire in short bursts rather than being fully automatic, and have a scope, but are otherwise completely different, aesthetically.
The level 2 upgrade does not visually alter the rifle.
Although it has a scope attached by default, it cannot be used until it's upgraded to level 4.