Wikipedia says:
The DC-15A blaster rifle that the majority of the Clone Troopers carry in Attack of the Clones and Revenge of the Sith is based off parts from the German MG-34 General Purpose Machine Gun from World War 2, which was used as the basis weapon for the BlasTech DLT-19 Heavy Blaster Rifle in the Original Trilogy, and components from the BlasTech E-11 Blaster Rifle that were carried by the Imperial Stormtroopers from the original Star Wars trilogy.
The DC-15A blaster rifle that was seen and used in Attack of the Clones and Revenge of the Sith appears to be a hybrid of the parts from the E-11 blaster rifle and the DLT-19 heavy blaster rifle of the Original Trilogy as well as showing that the designs for both the E-11 and the DLT-19 originated from the DC-15A blaster rifle.
The DC-15B blaster carbine that the Republic Clone Troopers carried in Attack of the Clones and Revenge of the Sith appears to look like a "de-evolved" version of the BlasTech E-11 blaster rifle the Imperial Stormtroopers carried in the Original Star Wars trilogy with the DC-15B carbine having features like foresight protectors, a blaster power cell located on the side of the blaster's receiver, and a folding rifle stock. The features that were on the DC-15B would later become standard on the E-11.
In Revenge of the Sith, the DC-15B blaster carbine was used by many regular Clone Troopers on a larger scale. The compact look and handling of the DC-15B carbine was preferred by many Clone Troopers in Revenge of the Sith over the bulky DC-15A blaster rifle the Clone Troopers carried in Attack of the Clones.
The wider use of the DC-15B blaster carbine by the Clone Troopers in Revenge of the Sith also provides more hints to the Republic Clone Troopers' evolution into the Imperial Stormtroopers of the Original Trilogy.