The "HK Gun" is the MP5 from Tomb Raider 3 (HK stands for Heckler & Koch, the manufacturers of the MP5 series submachine guns. It is smaller than the M16 gun and comes equipped with laser sight. It has 3 modes of operation: sniper, burst and rapid. In sniper mode, only one shot is fired when the trigger is pulled. Burst mode shoots 5 rounds and rapid mode fires continuous shots.
In Chronicles, the weapon has an optical sight and can be aimed manually by the player. Enemies do not see the red dot that indicates where the bullet with land. Single shot mode is useful if you want to conserve ammo by going for head shots. Burst fire is largely useless because 5 rounds is not enough to kill anything in the VCI levels, but 10 rounds is overkill. As with most weapons in Tomb Raider 1-5, reloading is not done in real time.
The weapon needs no time for reloading and is very effective at long range.