M60E4 Fat Gas System - Port Size Question

[jbntex]

I bought a used 17" M60E4 barrel with the "Fat" ratchet gas system off this board a couple years ago. 

This barrel is the ratchet retained "Fat" gas system that was developed after the narrow E3 / E4 "narrow" gas system style system (with the expansion chamber that is held on via the wrench flat lock ring) but predates the modern E6 ratchet gas system where the ratchet teeth are forged into the gas block housing .    I am pretty sure the Fat E4 gas system barrel in question is a General Dynamics made barrel as it has a cage code of MFR26978, which appears to be  General Dynamics cage code.

I never got around to shooting this barrel until a month back or so but immediately noticed that it had some significant reliability issues. Symptoms were  small runaways where you would let go of the trigger and the gun would fire ~2 to 4 more rounds before the sear would catch the op rod.  I initially wasn't sure if it was ammo related (New Igman M80 Ball), temp related (low 30s), op rod or sear wear related, etc.  However I took the gun out again when it was warner and ran factory Fed Lake City M80 Ball  through the same barrel and  I would still get the burst runaway condition.  

Upon popping the top cover after each  burst using the Fat gas system E4 barrel I noticed that the op rod was always locked onto the sear via the secondary notch. I swapped over to another E4 barrel (US Ord made E3/E4 narrow gas system barrel) and everything would run just fine and the op rod was always held back on the primary notch as it should be.

This seemed to confirm that there was a gassing issue with this specific E4 Fat gas system barrel  that was essentially resulting in a short stroke condition of the operating group leading to small runaways.

Once I got the barrel all apart everything appears to be in working order, piston seems fine and slides back and forth with no issue, the rear part of the fat gas extension that is held in with the roll pin looked good, etc.   

I then got out the bore scope to make sure there were both no issues with the gas port (alignment with the gas block, debris, erosion, etc.) and all appeared to be good to go.

I then measured the gas port on this General Dynamics fat gas system E4 barrel and it measures out to right about ~0.98". I then disassembled two brand new US Ord E6 barrel and the gas port gauges out to ~0.140". I also checked one of my  used US Ord made E4 "narrow" gas barrel and it gauged out around ~0.144" but also has a decent number of round down the tube.  Even my original 22" M60 barrel gas port gauged out at ~0.125"

While the diameter between the US Ord E6/E4 narrow compared to the General Dynamic made Fat E4 is only like ~40% increase in diameter,  in terms of surface area of a circular port that is like 2X the volume for gas to flow which could clearly explain the short stroke issues on this barrel.

My first thought was, well just drill the General Dynamics E4 fat gas system barrel gas port out 0.14" to match the US Ord made E4/E6 barrels.  However, I don't know if there is any other factors on the General Dynamics made E4 fat gas system  barrel I need to take into consideration as to why it may have such a smaller diameter gas port. 

I just wanted to see if anybody with more knowledge than me can confirm why there there may need to be such a smaller gas port on the General Dynamics made Fat gas system E4 barrel, if it needs a different spec than 0.14", etc. before I drill it out with a #28 (0.0.14") drill bit since I can't easily "un-drill" it either. 

Edited January 11 by jbntex

[mattnh]

I have quite a few barrels with fat gas systems, but never tried swapping gas system styles on a barrel before.

I think that when you chop down an E3 long barrel to commando length, you might have to open up the gas port slightly.

I'm def interested to hear Aaron's take on this...

 

Edited January 10 by mattnh

[jbntex]

I guess its also possible this barrel started off life as a 22" long barrel that got cut down, but I have never seen an E4 fat gas system long 22" barrel before (but they may exist). 

I guess it would be possible to take a long E3 barrel, cut the barrel,  move and re-pin/stake the front sight, and then source both the E4 fat gas system ratchet locking collar and fat expansion chamber components and convert it,  but that seems like a lot of work and those E4 fat gas system parts seem to be pretty rare.

From everything I can tell this problematic barrel appears to be a factory General Dynamics made 17" E4 fat gas system barrel....it just doesn't run right and the gas port size appear to be the culprit as its the only aspect that is different from all the other barrels I compared it to.  I just don't have another 17" E4 fat gas system barrel that runs properly to check the port diameter on for reference.

Worst case I can slowly drill it up in steps until it works properly with a range trip each time to test fire, but that is a lot more time/effort vs. drilling it the right size the first time.

 

Edited January 10 by jbntex

[johnsonlmg41]

which lends some credence to the rumor back 20+ years ago when presented with the option of buying the E3 barrel or the fat E4 barrel the better choice was the E3.   I have not shot any of the E4 barrels I have, but if so I would expect lackluster performance based on what "older" dudes told me at the creek years ago.  The E4 barrel was for display, and the E3's were the shooters.   I've never taken any of those apart....probably the only thing I haven't taken apart?

I'd agree that they weren't likely cut down.....that is a good amount of work.

[Aaron in Mohnton Pa]

 Before you drill it out,  Take the gas system off and Ream the gas port and holes in the rear of the cyclinder with a combo tool… The reamer tool will protrude into the barrel if it’s the correct size …Put a e3 gas system on and try it…. They are interchangeable…… The fat e4 gas systems replaced the e3 .. The outer cover on the e4 systems were prone to cracking … they went back to the e3 due to this … Which is why US Ordnance M60e4s had the e3 type gas system.

    We sell brand new old stock e3 systems if that ends up being your issue. 
 

 

 

this was a factory long barrel … can see how small the port is. 

 

Edited January 11 by Aaron in Mohnton Pa

[Aaron in Mohnton Pa]

Also - factory General Dynamics short barrels have some differences from the Saco ones…. Look at the photos below. Note the difference in steps on the machining …. If your General Dynamics barrel is machined in the same way it is likely a factory short barrel. I have not seen any long General Dynamics Barrels. The newer long barrels I have seen (other mfgs) all have a thicker profile there… 

   Usually you can tell if they are cut down by the pins for the front sight and gas cyclinder being smashed as opposed to a clean factory set. 
 

general dynamics on the left. Saco on the right … both are factory barrels 

 

Edited January 11 by Aaron in Mohnton Pa

[jbntex]

This is the actual E4 fat gas system barrel in question below.  Not sure if there are any clues as to its origin in these pics.

I know I have one of those combo wrenches somewhere but I have not used or see it in years can't seem to find it this evening.  My honest guess is that the reamer won't fit into the barrel gas port as the barrel gas port given its diminutive size and the tip of the reamer isn't all that much smaller that the base below the step.  The hole in the gas block leading to the barrel gas port hold appears to be normal size around ~0.190" and my gas port hole in the barrel is like half that size.

I do have a spare new E3 gas system I bought from you that I could swap over and test and see if it makes any difference before drilling the barrel out.  That is a pretty easy swap and test.

 

[Aaron in Mohnton Pa]

General dynamics barrel made in 1987 …. So somebody added the e4 gas system at some point ….. Pins look clean on it from the side you posted …… Those ports can get clogged hard with carbon and serious fouling... If you are able to see with your bore scope that it isn’t clogged and is just undersized then carbon obviously isn’t the issue …. If it were my barrel, I would just want to rule out any blockage of the port and go through process of elimination on other potential issues before permanently modifying the port…Just because that is not something you can undo easily ….
   If you don’t have a reamer tool use a drill bit that is the just under the size of the port.. Make sure it’s free and clear.. If there is fouling in there you’ll see it breaking apart and falling out etc…. If it doesn’t run after that and the gas system swap, then I’d say you have pin pointed the port size as being the definitive issue …. It’s just highly unusual. 

Aaron - Mohnton, Pa

Sales@BeltFeds.Com

Edited January 11 by Aaron in Mohnton Pa

[Texdave]

Jbntex,  I have what appears to be the same E4 Fat Gas system barrel with the exact manufacture 26978 Cage code only with a manufacture date of 11/02.  Before I go any further I wanted to tell you that I try and read every thing you post because of just how detailed you are and the fact that you are so very unselfish and try and help others in this community with your extensive knowledge just as Aaron does as well.  I greatly appreciate you both !!  So I pulled this E4 barrel out this am after I read your post. Also to note this is still an unfired barrel so cannot comment on performance.  My go to commando barrel is the E4 barrel with the slim E3 gas system. Because I am short of time this am I did not post any photos but did take some.  I can tell you that my gas port on this barrel swallowed a 0.125 (1/8”) drill bit shank without issue and it also took the E4 combo tool’s reamer as Aaron suggested to fully inserted status.   I also tried a 0.140 but it did not go but was just a used an old drill bit. The port size in my barrel does seem larger than the 0.125 but not sure yet on the 0.140?  I need to order a #28,29,30 drill rods so I can closely determine the port size. I know I only have limited info just yet but wanted you to know if your barrel’s gas port is sized at only 0.098 then it seems way undersized compared to the gas port in my barrel dated 11/02 ?  I wonder if G/D changed the port size between 1987 and 2002? Please let me know if you have any questions.  I will get an accurate port size measurement and let you know what I find.  I hope some of the info helped in this quick reply.   Take my M60 Brother !!

[Aaron in Mohnton Pa]

@Texdave thanks for the kind words Dave … My General dynamics barrels are also 2002 manufacture dates. 
 

Aaron 

[jbntex]

Texdave, thank you for checking the gas port on your general dynamics E4 fat gas system barrel and it sounds like your gas port is way north of 0.125" but maybe not quite 0.140"

I went back today and rescoped the barrel and after some twisting/cleaning with the cutting end of a #39 drill bit (0.995") the back barrel end of the #39 drill bit finally pushed all the way through into the barrel itself.  Granted this particular #39 drill bit barrel end measures out about a thousands less than the hole its supposed to drill and measures out to ~0.985".

This is a pic from the borescope with the #39 drill bit barrel end (~0.985").  Its a tight fit and there is little to no carbon or fouling left as the bit comes pretty much edge to edge with the chrome lining of the bore.

 

This is pics from the borescope comparing the E4 and E6.  Same borescope and  you can easily see the significant gas port size difference.

 

I think my steps are going to be try an E3 narrow gas system expansion front end as its easy and can't hurt anything.  My suspicion given the small gas port hole size is that its going to have the same issue but you never know.

If that doesn't work I will step it up to a 1/8" (0.125") and retest with both the E3 narrow and E4 fat gas system using a couple different types of NATO M80 ball ammo and see if I have any success.

If the barrel still doesn't run with a 0.125" gas port I will step it up to 0.140" like my other E4 and E6 barrels appear to be and retest with both the E3 and E4 gas system front ends.

It may take me some time as this is going to be a couple range visits assuming I have to complete all these steps but will report back what eventually worked and where the barrel port ended up.

Thanks again for all the replies from everyone its been very helpful before potentially fucking up a couple thousand dollar barrel.  There isn't a ton of information out there on M60 gas systems specs and I didn't want to do something dumb by drilling it out and then finding out there was something different or special about the E4 fat gas system  that I wasn't aware of.

[Texdave]

jbntex, I was getting back to you with some info.  I did get a #29 (.1360) and a #28 (.1405) drill rods. I verified their size with calipers.  I also wanted to relay the complete info that are stamped on the barrels that I gauged and will be discussing. Both barrels are short light commando contour and stamped MFR26978-MF3604.  So I guess they are made by General Dynamics as have been stated. The dates are different.  One barrel that has the E4 Fat Gas system has the date of 11/02.  The other barrel has the slim E3 / E4 gas system and it is dated 08/90.  I have shot this barrel with the slim gas system over the years and it always runs 100% with LC ammunition. So I made sure I cleaned out both barrel’s gas ports with first the factory combo tool / reamer and then followed by a close sized drill bit as Aaron suggested. Both barrel’s ports took the #29 (.1360) and it did seem close to fit.  I of course did try the #28 (.1405) and it would not fit into either barrel’s  gas port.  So I am going to see if the port is slightly larger than the #29 (.1360) by slowing turning down a #28 (.1405) and of course keep on checking the size with calipers and see if it is somewhere between the #28 and #29.  I know I have still incomplete info for you but closer than before and also on two G/D barrels instead of just one. I am almost positive the port is closer to the (.1360) than the (.1405) but we will see.  And I wanted to ensure you that both ports did seem the same and so are surely larger than (.1250) so I would at least say #29 (.1360) would be close and so also quite a bit larger than what is the current port size you stated that is in your G/D barrel.  It is also interesting that you say your new US Ord barrels are ported at (.140). and if the US Ord barrels and gas systems are the same otherwise I wonder if they increased the port size to increase the gas and maybe the reliability?  I will report back when I get the chance to turn down the #28 and see what I find. I hope helps a little more. Take care, Texdave

 

[jbntex]

Thanks again for taking the time to do some more measurements, this is really helpful. 

It sounds like I can probably just skip the 1/8th port size and just start with a #29 and then to a #28 if needed.  My guess is that a 28 and 29 are so close that either will probably work and US Ord may have just gone up one size to get a bit more reliability under adverse conditions.  

Its been too cold here (even in central Texas) to do much of any range visits but hopefully in the next month there is a warm weekend to test with.