The AER9 was actually not the top laser rifle in service at the time of the Great War. The model line went up to the state-of-the-art AER12, which saw service in a handful of specialty units. The reason that the AER9 is much more commonly found is that it was much sturdier and more reliable than the models that followed.
The AER9 features a titanium housed crystal array which proved to withstand long years of exposure to the elements much better than the gold alloy housing of the later models. As a result, the crystal arrays stayed focused within operating parameters, rather than falling completely out of focus like the newer models. Power is drawn from microfusion cells and processed through a wave/particle diverter manufactured by General Atomics International. The diverters are protected by a carbon-fiber housing, preventing frequent malfunction, but when a diverter fails the weapon becomes unusable.
As with all energy weapons, the AER9 can suffer from poor performance if not properly maintained. The crystal arrays and non-mechanical components are delicate and if not properly serviced can lead to a loss of beam intensity, overheating, and energy regulation failure.
As with all energy weapons, the AER9 can suffer from poor performance if not properly maintained. The crystal arrays and non-mechanical components are delicate, and if not properly serviced can lead to a loss of beam intensity, overheating, and energy regulation failure. Given that laser weapons were still in their infancy as regular military weapons, parts can be hard to come by in the wasteland. However, its lack of moving parts (aside from the microfusion cell holding latch and trigger mechanism) means it is resilient against extended use. By nature of firing a beam projectile, the shots will instantly hit, negating any need to lead the shot.
In combat, the laser rifle is an exceptional weapon. It performs admirably at medium and long ranges, where its accurate semi-automatic fire combined with its large ammunition capacity makes short work of most opponents. However, its semi-automatic nature prevents it from being a viable close quarters combat weapon. As such, the laser rifle is a good weapon for traveling through the open wastelands where dispatching enemies at a distance is a frequent opportunity. Although its damage output is only comparable to the hunting rifle and lower than the sniper rifle, its rapid semi-automatic action, sizable magazine, minimum spread of 0 (meaning potentially perfect accuracy), extreme effective range, and exceptionally low AP cost allows the laser rifle to really stand out. A deadly critical hit from this weapon, as with other laser weapons, will not only kill enemies but also turn them into a pile of colored sparkling ashes, which can still be looted as normal.
Wazer Wifle - This unique variant of the AER9 has higher damage and durability, as well as a larger magazine. It is obtained from Biwwy in Little Lamplight for five hundred caps, for two hundred and fifty caps with a Barter skill check, or free with the Child at Heart perk.
Metal BlasterThe Pitt (add-on) - This unique variant of the AER9 fires nine beams at once, using one microfusion cell similar to the Protectron's Gaze laser pistol. This feature, coupled with its remarkably high damage-per-shot makes it a devastating short to close-range weapon, though its effectiveness greatly diminishes at a range with the "shotgun effect". It can be obtained by bringing fifty steel ingots to Everett in The Pitt.
Tri-beam laser rifleBroken Steel (add-on) - The tri-beam laser rifle is a variant of the AER9 that fires three high-powered beams per shot - similar to the Metal Blaster - though it does require three microfusion cells every time the trigger is pulled. This weapon is not unique, and is dropped by super mutant overlords but is not repairable with anything but other tri-beam laser rifles. It was first made available in Broken Steel.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
The Operation: Anchorageadd-on includes an unused DLC-specific version of the laser rifle. It is identical to the normal laser rifle in both statistics and appearance. Its intended purpose in the DLC is unknown and it was cut from the final release, rendering it accessible only through the use of console commands.
A "demo" version of the laser rifle is included in Fallout 3, presumably left from the game's E3 demo. It does one point less damage per shot and less than half the critical damage than the standard variant, but has a much higher critical chance percentage. It is not used anywhere in the game and is only accessible through the use of console commands.
The lightning gun is a version of the laser rifle which was cut from the Broken Steel add-on. Upon firing, it creates a natural gas cloud, then shoots a second laser, possibly igniting the gas. Due to this weapon being cut, it can only be obtained through console commands.
End of information based on Fallout 3 cut content.
If you explore Fort Independence, you will come across a terminal in the Weapon Research room near the Armory Room, which says the AER9 will overheat after numerous shots, and that it is only supposed to be able to fire 12 to 13 shots per microfusion cell when you can shoot 24 (or 30 with the Wazer Wifle) lasers per cell. A terminal in the Citadel laboratory likewise describes the model for being notorious for overheating.
One very useful trait about this weapon is that, with a high enough repair skill, you can repair them to 100%, giving them the sizable value of 999 caps. Meaning at 100 barter skill, you can usually sell one for around 890 to 950 caps, depending on the vendor.
On the barrel, there is a warning: "Caution: laser radiation". It's also rated as a "Class 6" laser, which may be a subtle reference to the internationally used laser class rating system, which currently rates the strongest industrial lasers as "Class 5".