Visible crew/equipment: When the woman is performing an autopsy on one of the dead "creatures" the lights supposedly flicker and turn off. There is a hand visible flicking the switches on the lamp to the right side of the screen. (00:45:40)
Continuity mistake: There is blood on Samantha's gloved hand when she presses the keypad, but there is no blood on the keypad after she removes her hand.
Continuity mistake: When Goat dies, after the last attempt at defibrillation is made and the others give up, you can see the supposedly "dead" Goat take a big breath just before Reaper says "He's gone". In addition, in this shot Goat's eyes are wide open when the camera is on him. When the shot goes from Reaper back to Goat, his eyes are now closed. (00:41:05)
Continuity mistake: There are lines in Dr. Carmack's message when Sarge shows it to his team that were not when he first recorded it. (00:02:20 - 00:08:20)
Continuity mistake: When the female, running after Dr. Carmack, gets her hand stuck in the door, her hand is first in a horizontal position. When it cuts the arm is suddenly in a vertical position. (00:02:00)
Other mistake: The chaingun was actually a heavily dressed up Browning Browning Model 1919 machine gun with a fake rotating barrel cluster surrounding the real firing barrel - in several firing scenes a careful eye will notice the barrels are not even rotating.
Continuity mistake: When the woman's arm gets cut off in Dr. Carmack's door, a lot of blood spills on the door. In the following shots the blood is gone. (00:02:05)
Continuity mistake: When the Rock is wrapping the wire around his hand at the end, the first shot of him wrapping the wire has one lap done on the top of his hand, in the next shot he has 3.
Visible crew/equipment: When they first go through the door to the labs from the Ark, you can see the camera's shadow moving in on the door as Portman walks through it. (00:15:45)
Continuity mistake: When the Sarge wraps the wire around his arm, it coils down his forearm once, like a spring, with just enough to wrap around his hand and make the spike. In the next shots of him, the wire criss-crosses the Sarge's forearm, indicating a longer wire was used during the "fighting." Also, the wire sits comfortably around his arm, when in actuality it should grip the forearm hard enough to leave marks and cut off the blood circulation to the hand, with only the ends of the wire relaxing.
Revealing mistake: In the first person shooter sequence, when Reaper gets the chainsaw, there are many parts where he is moving it around near Pinky, now the beast thing, and it does not have a shadow, and at other times when it is at the same angle, it does.
Answer: From what I know, that gun is now basically a working prototype, it would a nice thing to have. Also, it could be adapted to the RRTS (Rapid Response Tactical Squad) needs. Or just as likely, the UAC wanted it back.