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Thread: M-4 / Carbine Gas Port Modification

  1. #11
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    Quote Originally Posted by opsoff1 View Post
    Well, to clear up some of the confusion - yes, I am cutting down 20" barrels from the chamber end. The breech end is re-threaded, rechambered, a new barrel extension is installed and it is timed exactly to the exisiting gas port. It is actually very easy - so I am not sure what he fuss is about.
    I have been doing this for years and have done probably dozens of M-4 bbls - every one has hammered and there are no issues. Each extension and corresponding bolt is individually headspaced.
    Attached is a pic of one of them. If you look closely - the index pin in the bbl extension has not been installed yet - but I can assure you - the port, and the pin are in EXACT alignment - I use a fixture that is set up on my milling machine's table.
    Open to questions.

    Edited to add - a new Krieger 20" bbl is up in the $400 range - so when faced with using them as a tomato stake or reutizing them as tack driving M4 bbls - I choose the later - start to finish, I can have one completed in under 4 hours, with the majority of that time spent chambering. And I do it because I can...
    Interesting...

    I will have to noodle this some more.

    ETA: I concede it can be done but it isn't worth the time or money. Four hours chasing headspace is just short of crazy.
    Last edited by MarkG; 09-30-13 at 09:54.

  2. #12
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    How many rounds is one of these reconditioned barrels good for?
    John

    If you spend much time around the guys who really, really know their craft, and who truly live this stuff, you tend to find that they are very soft-spoken and modest -- almost to the extreme. To my mind, that is a model worthy of emulation

    AC

  3. #13
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    I must admit I am a bit taken back at the level of "can't be done, not worth it, just short of crazy" etc remarks.
    It's somewhat ironic given the amount of hand wringing and obsessing over which buttstock or buffer is going to provide someone more first round zombie hits...
    Thr reality is that it isn't 4 hours of chasing headspace. It is 4 hours from start to finish of machining my own barrels - the majority of the time is spent on chambering - not headspacing. My chambering method is slow, true and precise. I also like my reamers and prefer to not abuse them. The headspacing cuts are usually the last 2, maybe 3 cuts that are at best 2-4 thou. each and take maybe 5 mins.
    Bear in mind - this in't a shop where I earn my living - this is my personal shop where I build all my own rifles, reload and hang out in mantown. I have no overhead, no hours. It is important to fully understand that this isn't geared toward someone with a hacksaw and a file.
    I own all my own equipmet, lathes, millers, digital measuring equipment etc and when I want to stop - I do. I may do 2 barrels on a rainy Sat. I may do a barrel over the course of two weeks. I may not do any and spend my time reloading, or building a new bolt rifle. I started the post to query folks here who may have been down this road before re: gas ports, specifically if anyone has successfully used a bushing to reduce it inside the front sight housing. All of the M-4's I've assembled function fine - albeit stout. So in in effort to calm the rifle down a bit, and not have to play with buffers and springs and orings etc - I asked the question.
    As far as cost benefit - I have a rack of used barrels - all high dollar match barrels that have been technically "shot out" Which really means that the throats have been eroded to a point where I can not load a bullet (80SMK) to be within the required distance to the lands. At that point in the barrels life - it come off the rifle, a new one goes on and the old barrel goes on the rack or on my son's blaster rifle, or a practice backup rifle used only at 2 or 300yds. I want X ring accuracy on the long line. Borescope inspection shows the bore itself is in pristine condition - the problem area is the original throat.
    So - what to do? Throw them away? I'd prefer to repurpose them and have fun.
    These repurposed M-4 bbls will shoot MOA all day long w/ good ammo. Some of the 20" barrels came off the rifles at 2500 rds, some at 5000. Really depands on the powder, the rapid fire frequency and the bullets. Some last longer than others. Some shoot better than others. I have a 5R Obermyer on my M24 clone that started life as a 28" prone gun. It was cut / rethreaded / rechambered to the 24" M-24 length. The original tube had over 7k rds thorough it - set back & rechambered, that rifle shoots better than the 28". (200-16x at 1K)
    So - in the end, it's worth it for me - maybe not for others, thats their call. It costs me nothing, I gain a tremendous amout of satisfaction doing it and it is fun.
    Tough crowd!!

  4. #14
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    Quote Originally Posted by Tzed250 View Post
    How many rounds is one of these reconditioned barrels good for?
    Really depends - what kind of shooting? What is your accuracy cutoff? At what point do you say - "I can no longer consistently hit XXX at XXX range"?
    Basic rule of thumb though - it would not be unusual to get 10K through a barrel based on "combat" accuracy. So one of these barrels could easily get another 5000 - 7500rds.
    Personally - I shoot steel at 2/300yds w/ M4s for fun. So when I can't get consistent hits and the rifle starts printing high flyers, then that's an indicator that the bbl is starting to give up the ghost. On a match rifle - if the rifle will not hold 1.25 MOA at 600, then it becomes a candidate for repalcement. (Bear in mind - some match bbls won't do that out of the box.)
    If you want to shoot minute of barn door - you could probably shoot 50K through it, and to the other extreme if you want MOA or better - it may be on the short side of 5K.
    Hope that helps.

  5. #15
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    I find it quite impressive and refreshing that you are willing and able to do this. I'll admit I'm a bit jealous that you have the setup to pull it off.
    "Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -Benjamin Franklin

  6. #16
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    OP- I'm personally very impressed. Totally legit, and a fantastic use of otherwise useless tubes.

    MK18Pilot seems to be one of the resident expert sour-pusses, and he's used to dealing with people who learned everything they know about AR15s from youtube tutorials... so someone who knows what he's talking about (and obviously knows MORE about something than him ) is something he probably can't even cope with. Please don't take him as a representative sample.

    As to port size, I think you're right on rack. You might consider, for future re-chamberings, welding the old port, timing the barrel extension to a different spot, and just drilling your own port to your own desired size.
    Last edited by Dead Man; 09-30-13 at 13:54.

  7. #17
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    Quote Originally Posted by Quiet-Matt View Post
    I'll admit I'm a bit jealous that you have the setup to pull it off.
    Indeed... My hobby shop full of fancy jigs and reamers suddenly feels inadequate.

  8. #18
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    Quote Originally Posted by Quiet-Matt View Post
    I find it quite impressive and refreshing that you are willing and able to do this. I'll admit I'm a bit jealous that you have the setup to pull it off.
    Appreciate it - like I said, it's fun. The equipment is the result of a lifetime of involvement in the shooting sports, both as a professional and as a hobby. I've been doing this - 'smithing for 35+ years. My game is / was NM Service Rifle & Precision LR (sniper & bullseye NRA type) shooting. My desire is to pass all of this, equipment, knowledge & toys to my son someday. (hopefully before I forget my own name..)

  9. #19
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    Quote Originally Posted by Dead Man View Post
    OP- I'm personally very impressed. Totally legit, and a fantastic use of otherwise useless tubes.

    MK18Pilot seems to be one of the resident expert sour-pusses, and he's used to dealing with people who learned everything they know about AR15s from youtube tutorials... so someone who knows what he's talking about (and obviously knows MORE about something than him ) is something he probably can't even cope with. Please don't take him as a representative sample.

    As to port size, I think you're right on rack. You might consider, for future re-chamberings, welding the old port, timing the barrel extension to a different spot, and just drilling your own port to your own desired size.
    Deadman,
    Thanks & understood. LOL. Way back when I started doing this - I briefly considered welding but quickly abandoned that idea - too much heat and I know my limits with my welder - also, bear in mind that this is 416R Stainless - it is a resulphurized stainless which aids greatly in machinablity and finish. I wouldn't want to end up with distortion or slag in the bore. I briefing thought about a plug cut with an endmill to create a "pocket" where I could insert a bushing in the barrel. One of the cardinal rules came into play at this point - always cut/grind/weld/drill the cheapest part. Easy - front sight base - the passage between the leg on the barrel and the gas tube tend to be about .125" - so it's an easy chase with a #29 drill (.136) and tap with an 8-32 set screw. I can predrill the set screws w/ a #52 (.0635) port. Easily replaceable and captured - the plug can't go in or out. I am setting up three this week and will test the port sizes to see how they function with std carbine buffer springs and H buffers.
    And ammo too. Obviously makes a big difference. Most of the ammo will be Mk262 & 70TSX and probably M855 as a baseline.
    More fun!

  10. #20
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    Nice work opsoff!

    You should see a night and day difference going from an .093" gas port to an .063" gas port with the 14.5 CAR.

    .093" is extremely overgassed. No amount of springs and buffers can get this adequately under control.


    Since you can make just about any configuration from that 20" profile, I urge you to explore the 2" longer midlength gas system using a gas port around .076"-.080".

    MID is a better configuration than CAR for barrels longer than 12.5".

    The difference is not as important as proper gas port sizing, but it is better.


    Also, a low profile gas block is an option depending on your choice of rail and sighting system.


    How do you get the extension to index with the gas port at the proper torque?


    Quote Originally Posted by opsoff1 View Post
    Well, to clear up some of the confusion - yes, I am cutting down 20" barrels from the chamber end. The breech end is re-threaded, rechambered, a new barrel extension is installed and it is timed exactly to the exisiting gas port. It is actually very easy - so I am not sure what the fuss is about.
    I have been doing this for years and have done dozens of M-4 bbls - every one has hammered and there are no issues. Each extension and corresponding bolt is individually headspaced.
    Attached is a pic of one of them. If you look closely - the index pin in the bbl extension has not been installed yet - but I can assure you - the port, and the pin are in EXACT alignment - I use a fixture that is set up on my milling machine's table.
    Open to questions.
    Quote Originally Posted by Clint View Post

    Also, when rechambering and chopping rifle barrels, I'd suggest a mid length gas system for 14-18" barrel lengths.

    CAR length gas is not the best choice for barrel lengths over 12.5"

    Cutting 3.375" off the breech end makes a 16.625" middy barrel.

    This would make a nice barrel, if the overall profile allows it.

    You always have the option of cutting and recrowning the muzzle if it needs to be shorter.
    Last edited by Clint; 09-30-13 at 17:59.
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