Quoted:
2) Barrel
The more thermal mass, the better.  My fantasy barrel would be 18-20" CHF chromelined bull barrel made by FN.  Unfortunately, no such thing exists. Probably the closest would be taking a bull barrel and sending it in for after-market nitriding.

Quoted:
That's a .750 OD barrel - not really the beefiest you can get, the bull barrels are 0.927.  Fluting probably makes it cool a bit faster, though.

Quoted:
I think a nice HBAR Chrome lined barrel would work well. 18-20" sounds like a good length. Good call on a better gas tube.

I kind of want to do the same thing but it is impossible in CA.

Plus I run into realistic use issues. I just can't see myself burning through $200 of ammo in sustained fire. Machine guns rock.....when someone else is paying.

Quoted:

1) Feeding system.  
Belt fed would of course be ideal, but I don't know of any AR belt fed system that is bug free.  
2) Barrel
The more thermal mass, the better.

3) Gas system
Rifle gas.  Inconel gas tube.

4) Odds and ends
A quick change barrel system from DRD or Dolos would be great,

Quoted:
With the bump fire stocks and Fostech/Franklin binary trigger, I think we're getting pretty close to being able to produce select fire performance without shelling out for NFA registered machine guns.  I've been jonesing after a M249 or M240 but my chances of getting one were pretty slim. However - I see real possibility of getting within striking distance using either bump fire stocks (as shared in Bump-saw thread ) or the binary triggers.  

I would avoid the bump fire stock for serious use. They are fun and will do a decent job replicating full auto but hard to control. One of the keys of firing full auto is using your support hand to "pull" the rifle into your shoulder to help control it. But on the bump fire stock you do the opposite, you "push" the rifle away to make it cycle, making for poor accuracy. The binary triggers are def a step in the right direction, but you have to make sure you have lightweight BCG and buffers to get fast cycle rates and prevent hammer follow. Plus they allow for select fire so this is def the way to go.

Now I'm starting to think what else is needed to get rapid rate of fire on the level of M249/M240 from AR system.  Note - I'm going after rapid rate of fire,i.e ~ 200 RPM, not the cyclic.  

If you want high volume of fire, IE 100+ rds per min for multiple mines you will need a quick change barrel with a handle is a must.

1) Feeding system.  
Belt fed would of course be ideal, but I don't know of any AR belt fed system that is bug free.  Shrikes seem hit and miss, Freedom Ordinance 9mm upper hasn't been widely used and is of course 9mm.  That leaves more conventional magazines: Beta C-mags, Magpul D-60, Surefire 60/100 and X-50 drum.  Magpul seems the most reliable of the lot.

From what I have read the latest Gen 5 Shrike is reliable and they seem to have got the bugs worked out of it. Surefire 60/100 mags def have feeding issues as do the beta mags. Not sure on the magpul or x-50. Honestly Drums mags of all kinds for all types of kinds have issues which is why most militaries have preferred to use belt-feds

2) Barrel
The more thermal mass, the better.  My fantasy barrel would be 18-20" CHF chromelined bull barrel made by FN.  Unfortunately, no such thing exists. Probably the closest would be taking a bull barrel and sending it in for after-market nitriding.

A bull barrel would make for an insanely heavy gun if youre talking a 18-20" barrel. Just a regular medium/gov profile should be sufficient

M249 Stats rates of fire...
Sustained rate of fire: 100 RPM
Rapid rate of fire: 200 RPM
Cyclic rate of fire: 800 RPM

It does not use a heavy barrel in fact if you look closely it looks more like a lightweight barrel.

3) Gas system
Rifle gas.  Inconel gas tube.

If youre looking at high volume of fire you should consider a piston system, especially if you ever want to shoot suppressed. The action in my SBR starts to have malfunctions after a few hundred rounds due to the DI, which causes the bolt to dry and and gum up. A pistons system will keep the action, clean and cool for hundreds if not thousands of rounds. The trade off is the additional weight of the piston system, not something most people want on a SBR but no big deal when youre talking a LMG.

4) Odds and ends
A quick change barrel system from DRD or Dolos would be great, but I doubt they work with hbar/bull barrel profiles.  
There used to be a MG42 upper that went on top of AR-15 lowers.  The rationale was that people with registered lowers would be able to shoot MG42s, but ATF put a stop to that.  This could maybe potentially work with binary triggers, and obviate most the issues above - but they are out of production.

The DRD and Dolos Quick change systems are not designed to change out hot barrels, they are for making the gun more compact and transportable. They dont have handles so trying to remove the hot barrel is very dangerous. This is where a Shrike real shines, the handle and quick change setup is great. When the bolt locks back on the last round, you can toss the barrel and insert a new one, also you are just replacing the barrel, not handguards, bipods, etc...so it keeps the weight down when carrying spare barrels (if youre thinking from a tactical point of view)

The XMG34 is what you are referencing. It was registered as a title 1 firearm itself so it wasent just an "upper" like the shrike is. So it is illegal to put it on a RR M16 unless you are a FFL/SOT. They are hard to find these days as the demand dropped after the ATF changed their ruling. Maybe they will start production again with the new binary triggers coming out.