The 13.5 inch G&G CM16 Wild Hog. Amped lists that length with a 363mm inner barrel, a Version 2 gearbox, and a 14mm-negative muzzle thread. PolarStar's chart lists the G&G CM-series M4 on F2 nozzle 1, the M4/M16 nozzle shipped in this player package. The $589.98 package is the listed kit: the F2 V2 conversion for M4/M16, a tank, and the included Amped SLP air rig. Optional regulator and line upgrades are not in that price. The Prometheus 363mm EG barrel is the M4A1 length, the same 363mm Amped lists for this rifle. The Prometheus chamber is sold for the G&G CM16 series. The Macaron bucking is sold for Marui-compatible AEG hop chambers. Example build by LoadoutLab. Every part is listed by its shop as made for this gun. Photo: "HPA setup.jpg" by Kimse84, CC BY-SA 4.0, via Wikimedia Commons (https://commons.wikimedia.org/wiki/File:HPA_setup.jpg). The photo shows an HPA M4-style rifle with its tank, regulator, and air line; the parts list is a suggested setup for the 13.5 inch rifle.