What caliber is the Lancer from Gears of War

Judging from a blueprint of the lancer I saw with a see through magazine, is say the closest modern day round would be the FN 5.7x28mm. The round itself is slightly larger than the M16s 5.56mm , but on a shorter, fatter casing. In modern times this round is used in Submachine guns and the FN 5-7 handgun. It is somewhat underpowered to be considered a full power assault rifle round, but in the Gears universe who knows what manner of propellant the rounds actually use. It could be just as powerful as a full size assault rifle round with less propellant needed and therefore a shorter casing. And on a side note, the magazine is shown as being double quad-stacked, meaning there are 4 rows of 14 rounds each in the magazine.