Epic

For the guys to have pre planed positions does make sense when you take into account they have spend weeks training for this mission in particular, so they probably knew the layout before arriving on site. Tho they didn't know they were gonna fight aliens so it didn't do much in the end.

In Half life Opposing Force  we see the HECU use the m249 SAW. Also thanks for the breakdown video.

If you want, JoAcoNS also released a video showing behind the scenes and listing the references made in the video.

The headcrab zombies ARE ALIVE, Basically the headcrabs kinda break the head and stuff but not enough for the person to die, they need a live host, and in half life 2 if you reverse their audio you can hear some stuff like "God help! Help me" and "why why!"

Really cool analysis man, cool to hear it from someone who understands both the Half-Life side of it and actual tactics side of it! Joacons has two other really good Half-Life animations, they're called "Clean-up and Dispute," and "Hostiles," if you liked this one you'll definitely like them too.

Given that Half-Life 1 is mentioned to be taking place somewhere in the early 2000's, it's entirely possible that the HECU are fielding experimental gear given their role as a "government clean-up" squad. With them using MP5's, possibly M16A5 concept rifles it's entirely possible this unit is fielding more experimental equipment than your average Marine.

Also, at 12:51 it's not a health pack they throw to each other but a power-cell. All the marines wear PCV's or Powered Combat Vests, in universe these act as a dumb downed Hazard suit, able to provide ballistic and hazardous protection as long it's powered. One of the known features was an onboard medical delivery system, typically when critical injured the suit would provide a shot of morphine to combat the pain.

If it was set in 2000's, most marines of the period would normally be wearing Interceptor Body Armor with Ceramic SAPI plates. Much of the gear (pack, webbing) the marines wear is based off ALICE gear from the late 60's and 70's.

These marines are screwed regardless if they got on the Osprey or not. Even those who escape will likely either die by the combine’s invasion or be converted into Soldiers. Losing their memories and individuality only thing remaining their combat experience and training.

The SAWs failed due to high round counts, combined with their stamped steel receivers.  The rails the bolt rides on stretched and warped, shifting the bolt just enough out of alignment with the chamber to cause the gun to make an embarrassing “CHUNK”, instead of a “RATA-TAT-TAT”.

The M240, also known as the last of the Browning machine guns, was made just like the M1917, M1919, and Ma Deuce, with a receiver milled from forged steel.  As it turns out, John Moses Browning knew the truth.  Heavy is good.  Heavy means reliable.

In lore it takes a few hours for the headcrab to mutate the host and turn their blood into headcrab blood, it also depends on what species the host is and what is already on the host's head due to it taking longer for the headcrab to break through.

Videos like LZ Sparrow are why I'm a huge tactical horror enthusiast because it makes you wonder how our world's technology and military would fare and adapt against the extraterrestrial or supernatural where phase lines, logistics, and mobility are either irrelevant or decided through alternate means that cannot be countered conventionally.

33:22
or
- get crowbar'd by Gordon Freeman
- get killed by Black Ops (as seen in Opposing Force)

PEAK has arrived ladies and gentleman

14:01 Wait, that ain't wood, that's tape.

11:07 M16s with M4 stocks would technically made it into a Canadian C7A2 rifle which is just an M16 but none of the BS the army and marines wanted. But given the context that the HECU might be using experimental stuff they are probably fielding M16A5s which is real or rather a concept of putting the C7A2 into Marine Corps service with improvements carried over from the A4 but obviously it didn't went through as the Marines adopted the M4A1 which had already seen service amongst recon battalions and CAAT (combined anti-armor team) and of course MARSOC.

Also if you can, I can suggest watching a channel named Flawless War Gamer and his Arma 3 WW3 series latest part just dropped 7 months ago but he also covered historical battles that you can take a look as well.

You should react to Hostiles and Clean up and Dispute

You bring up a really good point at 9:25 if you played black mesa, throught the game the evacuation orders you hear on the radios that they continuously expand the evacuation area, basically as the game is going on the area of teleportation is expanding further and further. Its impossible to create a line of defense when the enemy keeps teleporting behind your lines no matter how well entrenched you are.

The HECU is weird in terms of gear. The infantry is mainly Marines, riding in V-22s but they use Bradleys and Apaches (Army vehicles) and receive air support from Air Force jets. I think the most common "head-canon' is that the HECU is a joint unit from all the branches of the U.S Military.

32:07 you are correct the headcrab zombie host are still ver much alive and aware of what’s happening to them hosts need to be alive for the headcrab to control the body

28:30, yes, more specifically an Alien: Covenant reference.