True but it's not the same M16 that first entered service some 50 years ago, we're on the A4(?) now and is largely superseded by the M4 in Army service. The 1911 did receive some minor changes over the years but I'll agree that it remained largely unchanged and some of the 1911s in service were quite old. However, there's a world of difference between a gun, which is a purely mechanical device, and a digital/electronic device like a motion tracker.
The closest real life analogue to the motion tracker is the modern field radio like the PRC77 which entered service in the late 60s but was largely being replaced by SINCGARS during the 90s. So chances are very unlikely that the Colonial Marine Corps issue motion tracker is 30+ year old unit, basic tech wise it may be that old but I highly doubt that the unit itself is that old.
Perhaps.
But, tear apart an M16, M16A1, M16A2, M16A3, M16A4, M4, M4A1, and M4A2, then mix up the parts.
Except for the selector and the presence/absence of a burst cam (M16A3, M16A4, M4A1, M4A2,) the internals are all but identical - and swap freely.
Most of the M4/M16 internals can even be used in the AR-15 - earlier AR receivers were machined to the same specs as the M16/M4 receivers, but later ones had parts of the inside "filled in" to deny the use of the FA issue parts.
The M1911 was revised ca 1981 (70 years after being accepted, 75-76 years after development) to make the M1911A1, which added a "drop safety" to the workings. This is a small plunger that interrupts travel of the firing pin until it is pressed by a lever that is actuated by the trigger. This plunger may be removed (reverting it to the operation of the M1911,) the lever may be removed (allowing installation of an M1911 slide on an M1911A1 frame,) or remove the plunger to put the M1911A1 slide on the M1911 receiver. A number of internals swap freely, save the "plunger safety" used in the M1911A1 and the associated lever.
Firearms have been used in some manner or another military for over 150 years, machine guns for over a century.
Given that technology has improved (I'm certain) between now and the first ALIEN movie, I don't consider it outside the realm of possibility that a competent engineer couldn't rig some variety of motion tracker, working on "micro-changes in air density." Given that he'd be working with "native material," it would of necessity be somewhat limited in capability - kinda like making a zip gun (cf: General Motors "Liberator" pistol, WWII, airdropped over France by the thousands.)
Give me access to a decent electronic shop, and I could build a RADAR set - although ultrasound would be easier. But, it wouldn't have anywhere near the capability of a properly-produced RADAR set - likely unidirectional, ranging only (speed discrimination would be limited, if present at all,) and no CRT/LCD display - audio feedback is easier to do, and takes less equipment (but isn't quite as easy to understand, until you're used to it.)
Make sense? Even if the principle were known then, there's still a world of difference between something built in a factory from a well-developed plan, probably using purpose-made components, and something stomped up in a workshop by a ship's engineer in a hurry because it would be a help...
(It may already have been general-issue, which would be how Brett got the idea - he just had to figure out how to make it work with what he had.)