+1.

Pretty much every part is LMG-specific, and either expensive or made from unobtainium. Even the stripped upper is different, because it's an A2 with no forward assist (there are a couple of stripped uppers on GB now for $550). I've never seen gas tubes anywhere, though I have instructions on how to make them from brake-line stock tubing. Hand guards are also unobtainium.

I've had mine for more than a decade and love it. I run it closed-bolt, on my Colt M16A1. Just today, I finally located the correct Colt hydraulic rifle buffer and its green spring on GB, after looking for so long .... parts just are not out there.

I've seen factory originals go on various sales boards for $1,200-$1,600. You can't build one for that, if you even find the parts.

Here's a pic (not mine):

Yup. They are out there at reasonable prices -- you just have to be patient, and watch all the sales boards.

I have two 9mm uppers for my Colt M16A1 -- a 7" Colt DOE upper, and an LRM M169 integrally suppressed upper. Once upon a time, I shot them a lot (and loved them). Still own them, but haven't shot them in years.

Why? Shortly after I got them, I was shooting one day and noticed my RPM in FA was speeding up, slowing down, speeding up, etc. I popped the upper and everything looked fine ... until I touched the hammer, and it wiggled from side to side.

I had broken the hammer pin dead-center. The gun still ran, but the two steel stumps were wiggling in the alloy receiver the way you wiggle a nail to pull it out of a board.

I had caught it early enough that there wasn't damage, but it scared the daylights out of me. And yes, you can buy stronger pins -- I now run KNS double-locking pins -- but even those can break ... and do damage to your $25k receiver.

IMHO, the 9mm is beyond the design limits for the AR -- at least, in repeated FA fire, which is why we own M16s, right?

I am having a Glock-mag AR SBR built for my 9mm uppers;  for 9mm FA, I stick with my boringly reliable Uzi.

Your Mileage May Vary.

And because threads are useless without pix, here's da DOE:

Same thing happened to me shooting a mix of 9mm and 7.62x39 in full auto.

 
I used the KNS pins below which are solid SS.  I switched the orientation so the hammer pin with the middle groove is used for the trigger pin and it has never broken.




 

I have both so ... My M4A1 SOCOM upper is my go-to upper. It balances just right, and on an RR it is the epitome of an assault rifle the way it was supposed to be.

The Colt Light Machine Gun is a totally different creature, designed to lay down suppressive fire, primarily from a fixed position. The ROF is reduced and the added weight, from a fixed position, lets you walk the rounds to the target, and slowly but deliberately decimate them. It is my fave prone-position MG.

I had a Shrike and, even with the 16" barrel and a Grippod, it still bounced around too much to be truly accurate in FA at 300 yards, while the LMG is a very different beast in that situation.

There is no one M16 configuration for all needs. As I said, a M4A1 SOCOM is my fave overall ... but for long-range suppressive fire, the LMG is the hands-down winner.

YMMV.

I'm not Tony but I've been down the road of playing with various 'piston' setups as well.

First of all, as mentioned in Eugene Stoner's patent, the standard AR gas system is a 'stationary piston'. : https://www.google.com/patents/US2951424

'Piston's' are not new and existed at the time Stoner came up with the AR as we know it today.



Have you seen the Battlefield Vegas thread?: http://www.ar15.com/forums/t_3_118/677135_High_round_count_AR_M4_s__over_100_000_rounds__and_how_they_have_handled_on_our_range.html

"- We no longer use ANY piston conversions or factory pistons guns with the exception of the HK-416 "knock-off" TDI upper."



The fact is that 'piston setups' are:

1.  Heavier

2.  Never as smooth as the standard AR gas system since the op rod and piston are off the centerline of the bore

3.  use proprietary parts

4.  The bolt 'unlocks' differently - in the standard AR gas system while gas does go into the carrier it provides forward force on the bolt while the carrier is pushed away from the bolt (stationary piston as Stoner mentions in his patent) excess gas is expelled in the gas port holes of the carrier.  The military has done testing with the M16 in a sand box and high speed cameras and when the AR is fired the excess gas from the firing keeps sand from entering the action.  On a 'piston' system, this doesn't happen and sand is sucked into the action.  Think of quickly opening the door of your house with leaves in front and see how they get sucked inside the house.

5.  You have to be careful on how heavy a buffer you use on a 'piston' system, the heavier you go the more stress you are putting on the op rod.  With a standard AR, if you go too heavy it just won't cycle....it won't break anything.



How many top competition shooters run 'piston' setups unless they are getting paid to do so?  Very few.



In regards to handling a high volume of full auto fire, I don't believe Stoner designed the AR to be SAW....but these days with anyone having access to a bumpfire stock and a beta mag, this does get brought up.

The current SOCOM firing table endurance standard of 240 rounds is a mixture of slow fire and some bursts and (1) 30 round mag dump at the end....not doing 100 round mag dumps.



I used to the think the Nitrided gas tubes were snake oil until I tried it myself and they really do work.

I was blowing out standard non-treated gas tube regularly doing full auto testing of our gas block.

Then I tried the Nitrided and have yet to have one blow out.





Note that this testing is really kinda ridiculous.

Look at my friend Dr Dater from Gemtech's article on the pressures here: http://www.sadefensejournal.com/wp/?p=1093

The data in the chart below is right before muzzle where the gas port is closer back.  So in the case of a pistol length gas it may be in the 25K range for 5.56 based on that chart.





Even in the testing shown below with a suppressor, you can see the suppressor is glowing and the gas tube isn't glowing yet.





I really wanted to find a configuration to use a 'piston' system for as I have several.  I thought the 5.45 running corrosive ammo would be good and left it for one day without cleaning and the piston and op-rod were rusted together and the gun was in-operable....However to be fair so was the bolt, any exposed metal in the carrier, engagement surfaces of the trigger, hammer disconnector, extension....it is a mess.  So I just kept lubing the piston assembly and kept shooting it and it didn't make it to the 1K mark until the op rod get bent from the full auto fire and this was an 8" 5.45 setup.  I went back to the standard AR gas system and as long as it is cleaned it is fine.



Just my 2 cents....



 

Yes, my bolts are ramped.  I just wish someone would go into production with a MP5 (delayed roller blowback) upper for the M16.

Like this:





 

I've tried piston uppers and don't like the way they feel when firing. I don't own any at the moment, and unless something special comes out, I don't plan on getting any.

amphibian is the authority, so I'll just second everything he posted above.

That appears to be it, though I wish I could see the profile of the barrel under the rail. That profile is the main difference in SOCOMs.

Yes, there definitely is a difference in how the SOCOM handles heat. But I also like the way it balances, and the extra weight IMHO helps with accuracy in FA.

YMMV.

Regarding LMG gas tubes, as noted, they are long out of production in the U.S., and good luck finding old stock.

Back in '04, a regular on subguns.com by the handle Shoepop posted this great, useful description of how he made replacement tubes:

Went to local Napa and went through their stainless brake lines with a dial caliper until I found one that was about 0.200 diameter.

Anyway I installed a small piece of drill bit in end hit it with a weld then dressed it up. Note: only stick bit in enough to be able to weld up or you will run into piece of drill you just installed when drilling new tube retainer hole.

Next drill your front sight gas tube hole to new gas tube diameter. Measure and drill new tube for retainer pin and gas hole. Bend to shape of old tube, cut to length.

Now either chuck up new tube in drill and sand or chuck it in a lathe (before you bend) that is what I do and take it down so it will fit stock carrier key same diameter as old ( this is what they do on LMG tubes) no problem because gas is a lot cooler when it get's to key so you are not defeating you purpose of installing heavier tube. They will not fail there. I have also drilled the key to new tube diameter in lieu of stock diameter but turning down to fit stock is easier in my opinion.

I hope this all make's sense. It's really a lot easier to do than explain. The actual diameter of a factory LMG tube is 0.2530 you can't drill out a stock front sight hole that large but you can get a after market gas block and drill as large as you want, install a removable front sight assembly and you will have as large/heavier as that of a LMG tube. It will now take MANY full beta dumps if so inclined.

I have a bunch of old 20" pencil barrels and when they are worn out I just move the assembly to next barrel.

Finally, in your search for correct diameter look @ tubes for hydraulic clutches etc. Also race car parts catalogs are a good source.

OK, a little history of the LMG off the top of my head (that means you nit-pickers will finds errors in this):

The Colt Light Machine Gun was developed by Diemaco, a Canadian defense company licensed by Colt, to fulfill a Canadian military contract. The Canadian military wanted (and today still happily use) an open-bolt, heavy-duty, tough-as-nails C7 variant. When the Canadian military went to it, Colt had Diemaco send down a batch of LMG kits to Hartford, so that Colt could offer the LMG to U.S. military and LE buyers, without having to deal with the whole import issue.

AFAIK, Colt never did get a military contract for the Light Machine Gun, and I doubt they sold more than a handful to LE. From the Colt Match Target listed in an ad above, Colt may have indeed sold a few as semis, but I had never heard of one before seeing that ad, so I am skeptical.

Then, in the late '90s, when Colt was going through one of its many bankruptcies, it sold off a huge amount of inventory including uppers for guns that were not popular.

The 9mm DOE upper I showed above is one of those. Colt developed them for the Department of Energy as in-cab SMGs for transporting nuclear materials. Colt made IIRC 2,000 uppers, the DOE contract was for 800, and the rest got sold off in a bankruptcy sale for around $1k each. (One clue to whether a DOE upper is a bankruptcy selloff upper is that most of the selloff uppers did not have sling swivels.)

The LMG upper was the same: Colt sold off about 1,000 of them in the late '90s, with semi BCs, again for about $1k each. They function fine in semi, but many have been snatched up by FA RR owners because of their qualities for handling a whole lot of heavy fire without melting gas tubes. Of course, RR owners switch to an FA B/C. Tracking down the original Colt LMG hydraulic buffer and green spring makes it run very much like its open-bolt counterpart.

Also note that almost invariably, an LMG upper will NOT run reliably in full auto with a collapsing stock. Trying to run it without the hydraulic buffer usually works in a rifle-length receiver extension, but with the shorter carbine extension and the resultant different cycling pulse, you're gonna have problems. I went to Dennis Todd, SAW and even Colt, and the answer was that this was an unsupported configuration and I was on my own.

FWIW, Diemaco -- which Colt bought in 2005 -- continues to produce the Light Machine Gun, and today the Canadian Military uses improved versions that have flattop receivers and some sort of collapsible stock ... but (a) they are in Canada, (b) under Canadian law they cannot sell components to individuals, and (c) Colt refuses to import Diemaco LMG parts and components ... so I have yet to find the solution. (Colt LMGs do run fine with Vltor A5 collapsible stocks with the correct Vltor buffer).

Hope these ramblings help somewhat. But I really do love my LMG with Betamags. In fact, since I just got the buffer and spring today, the collapsible stock comes off my M16A1 this weekend and the rifle stock goes back on. I been waiting for this for awhile.....