UH-1H
They initially got the UH-1B/C/E model, but since less then 200 of that model were built they also used a lot of the more numerous UH-1H (long fuselage, upgraded engine) versions as well (over 5000 built). Also after the introduction of the UH-1M the USMC upgraded all their UH-1E to the new engine, making them basically UH-1Hs. Rather then include three different models of the UH-1E I went for simplicity and called them UH-1Hs, which is essentially what they were.
GAU-19 50cal gattling
In an on-again/off-again discussion they've been talking of mounting a chin turret with the GAU-19 on the MV-22 Osprey. So they have some GAU-19s (they got about 100 of them in 2007).
Since they haven't gotten around to putting the chin turrets on the MV-22s they're occasionally used as a replacement for the M134 7.62mm minigun on UH-1's. So I stuck them on my "Light Attack Helicopter (Unit Class 221) variant which I've set up to be mainly "soft vehicle"/"crewed weapon" killer helo (the 50cal vs the 7.62mm, less HE and more AP rockets).
HTL Gunship (Unit 629)
(A.K.A. YR-13 / H-13 / OH-13 Sioux)
This was a field mod commonly made to observation helos during Korea. Some pilots also carried hand grenades to drop as "bombs". This was never an "official" modification but was very, very common.
The US Army formalized this with the "XM1 armament subsystem"
http://tri.army.mil/LC/CS/csa/xm1m37c.gif
HRS Gunship (Unit 630)
(A.K.A. Sikorsky S-55 "Chickasaw" / H-19 / HO4S)
These were another common field modification (they were the predecessor to the H-34 Stingers).
You mentioned the tests in 1951, yes,
officially the tests were discontinued.
Weapon #174 M23 Machinegun
Yup, exactly right ! I didn't want to use the standard MMGs for helo doorguns because of their range.
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This is probably a good time to explain the "why" of one of my weapons modifications.
You'll find that "fixed mount" (i.e. tripod, CMG, etc.) type MGs have a 20% greater range then "flex mount" (i.e. bipod, pintel, etc.) mounts. Also the "fixed mount" types are not AAMG capable.
Example :
Weapon #14 M60 7.62mm LMG (WC 2, Acc 21, Rng 16)
Weapon #36 M60 7.62mm MMG (WC 3, Acc 20, Rng 30)
Weapon #52 M60 7.62mm AAMG (WC 4, Acc 20, Rng 24)
Weapon #64 M60 7.62mm CMG (WC 5, Acc 20, Rng 30)
Weapon #174 M23 7.62mm MG (WC 3, Acc 20, Rng 24)
Yup, it's a LOT of variations on a simple 7.62mm MG, but each has different capabilities and applications, and, the USMC OOB had the extra space in the weapons tab to allow all these variations!