The photo above is from the collection of Alex Young. It’s a nice look at some of the LMG options Colt made over the years. At top of the newest, the IAR. This is the gun that was submitted for the project that eventually became the M-27.
Below the IAR is the Colt LMG. Iconic and instantly recognizable by its hand guards, vertical fore grip, carry handle and bipods. Fires from an open bolt and was meant to be fill the light machine gun role. The barrel is the much heavier HBAR for sustained full auto. You will all notice that while it is a A2 upper, it has no forward assist. Not needed when the gun operates as an open bolt. A slightly different version of this gun is still in service with Canada and a few other countries as the LSW.
Above is the modern LMG/LSW. Everyone realized the flat top was superior 3 seconds after it was thought up. And so the flat top upper has made its way on to all modern AR15 variants.
Bottom two uppers are the M16A2 HBAR. The bipods used are the same as used on the M60 machine gun. This is the same basic idea as the M16A1 HBAR.
The HBAR and LMG variants never had widespread use. A few countries use versions of the LMG/LSW but most stick to belt fed LMGs like the M249. No doubt the limit of 30 round mags didn’t make the idea very attractive for the role it was intended for in infantry combat. Even with the Beta C-mag. Now we have the H&K IAR/M27 that is a carbine version of this concept that the USMC apparently loves so much they are going to give one to everyone. The average gun enthusiast has no idea that decades ago Colt already had this concept ready to go as well as a piston version of the AR15. Bad timing or ahead of its time. Go figure.