The E-5 was a blaster rifle manufactured by Baktoid Armor Workshop

Baktoid Armor Workshop Organization Manufacturers

on order from the Trade Federation for their Droid Army and was the standard-issue armament of the OOM and B1-Series battle droids

B1 Battle Droid Droid

and the elite BX-series droid commando

BX-series Droid Commando Droid

. The E-5 blaster rifle was a powerful, light, but somewhat inaccurate weapon.
The weapon had a large gas chamber that allowed for powerful blasts. It was originally a BlasTech

BlasTech Industries Organization Manufacturers

design that was reverse-engineered by Baktoid Armor Workshop for use by its droid troopers. The fact that the weapon was intended for use by non-living wielders allowed BAW to skimp on some of the finer points - such as the fact that short barreled weapons are comparatively fragile and produce a great deal of heat over repeated firings.
During the Clone Wars, E-5 blaster rifles were still used by B1 battle droids, but they were rivaled by the B2 super battle droids

B2 Super Battle Droid Droid

, which had a wrist-mounted blaster instead of a rifle, something that proved to be a more effective and versatile weapon.