It is actually a semi-automatic firearm (even acknowledged in its description).
Assuming that this is the ammunition used in-game that gives it its Assault Rifle power, the magazine should be at least triple the length of one it currently has. 45 ACP, with only comes in an 8 round magazine flushed within the pistol grip.
The strongest caliber the CX4 Storm can fire is the.
The CX4 Storm is actually a pistol-caliber carbine, but is (for whatever reason) portrayed as an Assault Rifle in-game, with a magazine magazine capacity far greater than what it can actually hold in real life.