Rook dressed up as a Rifle Battalion Commander, along with old (and well preserved) Auto Rifle and AA Infantry Shotgun.  
The idea here is that early in the Grate War (Fallout Equestria) Equestria’s military had to quickly modernize, using weapons and tactics they were not use to using. Among this was the Rifle Battalion’s, that focused on the uses of mobile and skilled sharpshooters, used powerful semi auto firearms. This was drastically different from the high volume, quickly trained solders using full auto smallguns that the war would later be known for.
 
Auto Rifle - practically a light machine gun, this large rifle was made be carried by solders, and hold the enemy down with covering fire. Introduced before Celestia stepped down, it was a highly effective rifle, but also exceedingly expensive to use because of the .308 ammo it fired. It would later be replaced by the gataling gun, which used a much smaller, and cheaper ammo, that could be used in a far higher volume before reloading.
 
AA Infantry Shotgun - another late Celestia fire arm, the Anti Air Infantry Shotgun fired 10 gage griffin rounds. Though Equestria had air domination thorough the war, the Zebras still employed several flying raced to counter the Pegasus, as well as made tech of their own to aid flight. this shotgun was made to counter any flyers when their were no pegusus to fight them, but would later be shelved in favor of full auto small guns.