Last rifle for the time being, I present;
Name: Smith & Wesson Model 1973 "Sully" Automatic Rifle
Designer: Smith & Wesson Firearms
Type: Direct-impingement automatic rifle
Caliber: .243 Liberty
Feed System: 30, 60, 100-round detachable box/coffin/drum
Adopted: 1973
Notes: By 1970, the U.S. Army under the MacArthur Administration had decided to develop a light-weight service rifle that would replace machine pistols and select side-arms while also increasing squad firepower. Research into light-weight polymers and aluminum would help bring down weapon weight while also permitting high volume fire. Ultimately, a design by aviation engineer J. Guillaume Sullivan and armorer Rick de la Vega would result in a design licensed to Smith & Wesson being the U.S. Military's new service rifle, the M1973 Automatic Rifle, or the "Sully Gun" in common parlance.
As expected from the example set by its predecessors, the M1973 used an unusual action; direct gas impingement, a system developed originally by Prussian inventor Heinrich Freiburg, wherein gas from a fired shot would act directly on the bolt and cycle it (and later improved by Century Armaments Factory in the United States by changing the impingement point to a bolt carrier group as an ersatz "micro gas piston"). While this theoretically would make for more fouling in the receiver and lead to severe maintenance issues, Sullivan and de la Vega made it clear to the Army that cleaning requirements would be higher than that of the M1956 Vicario [1]. However, the benefits of direct impingement more than made up for such characteristics.
The Sully Gun was America's primary service weapon in the Asia-Pacific War, and gave quite an excellent showing in the field. Not only was it less than eight pounds loaded and more accurate than the Vicario Rifle even at its expected range limit of 500 yards, it could lay down ferocious firepower (firing around 750rpm from a variety of magazines[2]). While Japanese troops had a deserved reputation for bloody-mindedness and a predilection for bayonet charges, the Sully Gun was discovered to inflict grievous wounds despite being considered low-powered compared to either the PQ-45 or PQ-66[3] due to its barrel twist causing high rates of bullet deformation upon hitting soft tissue, giving the rifle considerable stopping power.
As it stands, the M1973 still remains the primary U.S. Military service arm in its E3 format[4], a testament to its inherent versatility and proven combat effectiveness.
[1] Unlike OTL, there was no stick powder-ball powder issuing f**k-up that led to chronic jamming and malfunctions. Chalk it up to butterflies, S&W not promoting the gun as self-cleaning like Colt did IOTL, more thorough/less hectic ammo development, and (most satisfyingly IMO) no Robert Strange McNamara running the show.
[2] Box, coffin, and drum mags respectively. Of course the last option isn't terribly field-rugged by time the Asia-Pacific War goes down, but the other two certainly did the trick.
[3] Closer to an Italian BM59 in the paratrooper format.
[4] Think an M-4 lower receiver mated to an M16A1 upper receiver, only with modern accessory rails and optics. Better yet, like a pencil-weight barrel version of the Diemaco C7A2.
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