Complex sentence: The other six agents involved in the stakeout in five vehicles, who did not reach the shootout in time to participate, did have additional weaponry including Remington shotguns, Heckler & Koch MP5 submachine guns, and M16 rifles.
Split into simpler sentences: The other six agents involved in the stakeout in five vehicles, who did not reach the shootout in time to participate, did have additional weaponry. Aside from Remington shotguns, there were also Heckler & Koch MP5 submachine guns, and M16 rifles.