Seems I hit a nerve there.
With the "not listening" crack? Yes. Look in a mirror.

If I didn't know you better I would have suspected trolling. But, as I said, like most internets arguments I think we're just not communicating well.
No, the M27 is replacing and LMG, which was shoehorned into a tactical structure built in WW2 based on the BAR as the SAW. The M27 is to function as a modern BAR.
Automatic rifles are weapons capable of sustaining burst fire longer than a regular assault rifle, have a longer/heavier barrel for greater range/accuracy and sustained fire ability, and heavier parts in general to take the additional heat from high volumes of fire...which means they are heavier than regular rifles and thus more able to handle automatic fire accurately by absorbing recoil. They are not meant nor are able to put out the kind of suppressive/sustained fire of true belt fed LMG, but rather to have an automatic weapon that is light enough to keep up with the rest of the fireteam/squad and provide heavier fire than a regular rifle.
See? Yes, we're using different definitions. I'm talking about the "automatic rifleman" concept as discussed in military theory, and most importantly
how the USMC defines it in this context. (Unless I'm mistaken.) To whit: a rifle used as a
support weapon, which decidedly has as one of it's functions being prolonged suppressive fire. I.e., an LMG. And as I mentioned, experience has shown (more than once) that a closed-bolt rifle without a swappable barrel is not suitable for this function.
If you just want to say "the M27 adds an automatic fire and consequent short-term suppression capability that the USMC rifle squad has lacked because the M16A4 was not fully auto", well, to some degree I'll agree with you. Issue every Marine an M27 and I'd be all for it. (Actually, I think they might be trying to do this.) But then use them as what they are- rifles that can go fully automatic when required. And I would continue to take umbrage with any claim that they are suitable for suppression a la an LMG. (Maybe the entire squad
collectively could?) It would be cheaper to upgrade the M16A4s with floated barrels and an autosear. The only argument for the M27 over an upgraded M16A4 in a sustained-fire role is that the short-stroke piston (as opposed to the "direct impingement" gas system of the M16) helps a bit with weapon overheating.
Or, to put my entire argument
much more succinctly, I would say that reducing the number of LMGs in the marine rifle company to six is ill-advised.
Incidentally, the M27 does not have a "longer, heavier barrel," really. Longer than an M4, yes, but it has a 16.5-inch barrel as compared to the 20-inch barrel of the M16A4.
Of course the Marines are organized differently than the Army in their squad organization and their extra fireteam gives them great volume of fire than an Army squad, which changes the frame; as in WW2 with the BAR when they innovated the modern fireteam layout, they said 2 was not enough they needed 3 to have enough volume of fire for a squad. The M249 was shoehorned into that structure, which was not designed have a heavy belt fed weapon at it's core.
Are... you... defending the BAR? Because we're going to have another argument.
If by "automatic rifle" you mean "what the BAR was intended to be when it was designed", well, I don't have much to say beyond that's crazy. But I don't think that's what you are saying. Then, in the fireteam model you mentioned, it was the BAR that was attempted to be shoehorned into the LMG role, not the other way around! It
wasn't the LMG later being shoehorned into the BAR role! The US just didn't have anything approaching a functional LMG at the time, so they had to use the BAR! The realization that the BAR was insufficient was why they developed the M1919A6 (which had it's own problems, the primary one being that it was too damned heavy), but was at least capable of prolonged automatic fire, with a swappable barrel and an open bolt, etc., etc.- I'll not repeat myself.
What is an actual LMG if not a belt fed weapon?
Granted, it's hard to define, and a lot of it is merely how the weapon is mounted and employed; I would say that belt-fed is highly desirable but not required. The MG42
functioned as both a light and a heavy machinegun, frex, depending upon the mount, and despite being
practically speaking too heavy for the LMG role. But really that's a problem with any weapon that's meant to be a GPMG- it sucks equally at all roles.

Which is sort of why the US eventually adopted a dedicated LMG (M249) and MMG (M240)instead of continuing to field a GPMG (M60). Which of course hasn't kept people from trying to fire the M240 from a bipod, anyway...
So, the definition of an LMG is largely
doctrinal- more about how it is employed than anything else- and a decent try might be "an air-cooled rifle-caliber automatic weapon, carried by an individual soldier as opposed to it and it's mount being being broken into components to be carried by a team for movement, fired from either a rest or a bipod as opposed to a tripod, capable of long-term sustained fire including suppressive fire, intended to directly support an infantry squad, and not really suited for the heavy machinegun functions such as indirect fire or sustained defensive grazing fire." There then are, of course, terms in there that need to be defined, like 'long-term sustained fire.' The Marines claim to eke 39rpm average from the M27 (including reload time) and further claim that this lets it serve as an LMG. The M249 could sustain 50rpm, so the Marines are already admitting to a 20% reduction in firepower. But then the elephant in the room is that the M27 can only do that for a comparatively short while before the barrel melts (figuratively) whereas the M249 can swap the barrel and drive on.
"LMG" and "squad automatic weapon" are essentially synonymous. In the modern day the term also
generally implies an intermediate cartridge rather than a full-powered battle rifle cartridges (or larger).
The RPK was drum fed too. Open bolt and swappable barrels can be added to a rifle, but that doesn't make it an LMG.
Yeah, again, that's sort of my point. You're making my argument for me. The RPK is not suitable as an LMG because among other things it lacks a swappable barrel and fires from a closed bolt. It's more what you're talking about: a heavier rifle capable of fully automatic fire, but not really capable of long-term suppression. Adding a swappable barrel and open bolt to a rifle does in fact probably make it into an LMG, since that's
clearly the role you're designing it for. It'll just probably be a poor one.
The sustained suppressive fire ability comes from not having to repeatedly change out mags or drums and deal with those sorts of malfunctions. Seems we have different definitions, which is a serious issue. The Ultimax is closer to the M27 in weight and role than a M249.
Oh, yes, we have issues. Being able to sustain fire for only a few minutes until the barrel melts is absolutely
not what is meant by "sustained fire" in this context. The swappable barrel is rather key.
The Ultimax is close in weight to the M27, yes, but
most definitely not in role. Ultimax is designed for sustained automatic fire.
BTW the Swedes and Belgians created a quick change barrel version (all versions were open bolt) of the BAR (still magazine fed), does that make it a full LMG?
It makes it a pathetic one, yes, IMHO. The quick-change barrel indicates that it was designed for the sustained-fire role, and IIRC it was also open bolt. The very small magazine was a severe limitation, though. The later belt-fed BAR variants were better in that role.
Suppression is always useful, I was saying sustained belt fed suppression is not necessarily as useful as mobility on the offensive and in mobile engagements. The Germans apparently loathed the MG34/42 for that very reason and sought to replace it entirely with Sturmgewehrs, but had to compromise and keep it in a special weapons squad at the platoon level when integrating the STG44 (story about that is in the book "Sturmgewehr!")
No doubt, the MG34/42 was too heavy to be a practical LMG. Do you think I was disputing that point? I seriously think that when you read my posts you're reading what you
expect me to say as opposed to what I'm
actually saying. That seems to have happened at least twice so far.
The if M249 wasn't too heavy for the roles it was used in why did the Marines drop it for an automatic rifle?
That's... a tautology- a circular argument. I won't respond to it. But I will of course acknowledge that load is always a concern for the poor bloody infantryman.
My point is that the Marines have
not traded a heavier LMG (M249) for a lighter one (M27). Rather, they have just gotten rid of the squad LMGs and replaced them with
rifles, which IMHO is a mistake. It's yet another case of "learning to fight the
last war, rather then the
next one."
But, to synopsize:
Yes, we're using somewhat different definitions. The capability for
sustained automatic fire including suppressive fire is, to me, the sine qua non that differentiates a machinegun from a rifle. If you concur that the M27 is not capable of this- which you seem to- then we should just stop arguing, because we don't really disagree. We're just getting bent about
semantics, and maybe about the utility of the LMG in a rifle squad.
OTOH if you do disagree and think that the M27 can manage sustained automatic fire just fine, well, I would want to see the data. Even the USMC's testing was basically "fire 90 rounds, let the barrel cool, repeat." This is one reason that a lot of folks suspected (and still do) that the USMC wasn't really looking for a new support weapon even though that function was in the RFP, but rather they were trying to "back-door" a new service rifle. Which suspicion seemed to have been validated when the USMC started talking about issuing one to every Marine, of course. But then they backtracked and issued an RFI for a new service rifle. RFI, though, not RFP, so it is not terribly likely to ever happen.