Vinicius sent us this photo of troopers belonging to the military Police of São Paulo State, Brazil, GATE (Grupo de Ações Táticas Especiais) squad. GATE is modeled after American SWAT. The barrel on the foreground AR-15 looks to me to be about 24″ in length (or 22″ + flash suppressor/brake). The rifle has no optics on it, just carry handle iron sights.

UPDATE: I generally get accused of overestimating barrel lengths, but this time readers are commenting that it looks more like 26″!

Thanks Vinicius.