The following is the description of the Light Assault Gun from Book 4: Mercenary.
Light Assault Gun (LAG): Eaentrarly a heavy rifle, the LAG fires a 20 mm. 30 gram bullet at weloctles of 400 to 5oo meters per second.
Now, 30 grams is a fraction over an ounce, 1.0582189 ounces to be more precise, so about the weight and size of a 12-gauge shotgun slug. The velocity of 400 to 500 meters per second equates to 1,312 to 1,640 feet per second, or again in the velocity range of a 12 gauge shotgun. An explosive round of that size is going to have very little fragmentation effect, but would be quite fatal if it detonated inside of a person's body. The 12 gauge Paradox gun, with the final 12 inches or so of the shotgun barrel rifled, fired a 750 grain/49 gram solid lead slug at around 1300 feet per second. Twelve gauge shotgun loads typically weight more than 1 ounce of pellets per round.
Since the weapon weights 4000 grams, or about 8.8 pounds, I would not even call it a heavy rifle. While heavier than the M-16, it would be lighter than the M-1 Garand or M-14, and on par with the 1903 Springfield.
Basically, it is a 12 gauge shotgun firing an explosive round of very limited fragmentation effect. At for Tech Level, explosive bullets for the .58 caliber Springfield rifled-musket were used in the U.S. Civil War, albeit in a single-shot muzzle-loader. However, a semi-automatic 12 gauge shotgun was developed by John Browning in the early 1900s and was marketed by Browning Firearms in 1905, with a 5 shot magazine. Basically, this is a Tech Level 5, and maybe even a Tech Level 4 weapon.