Military Issue M4 Question

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  • Rating - 100%
    22   0   0
    Dec 29, 2008
    3,745
    113
    Danville
    The rage right now is over mid-length gas systems for ARs. My question is this: Do military issue M4 carbines have mid-length or carbine-length gas systems?

    I've read that mid length offers advantages in wear and tear on the ejectors to lower recoil. I've got an AR with a carbine length gas system and have never had an issue with it, and the recoil is negligible. I really can't say I feel any more recoil than with my rifle-length gas system in my Bushmaster A2. I'm just wondering because I'd imagine that if the M4s are carbine-length, then there is probably no durability issue.

    I just purchased a Spike's carbine length upper for my hog hunter and SHTF gun. My other carbine is a Model 1 on a Double-Star lower. It is my primary plinker, so I'll see how it holds up to cheap Russian steel cased ammo.

    Personally, I like the lighter weight of the carbine system. I like the light weight for hog hunting, but also because in a SHTF scenario, I figure I'll be carrying plenty of weight in gear, food, and ammo, making every pound count.
     

    Mordred

    Marksman
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    1   0   0
    Sep 8, 2010
    189
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    The only group i know about in the military that use the gas piston rifles are Delta Force. They went to them for heating issues. HKs i believe...and you can't buy on civilian market. Other than that, the carbine i had in Iraq was issued and it was not gas piston. Never saw a unit that had them either. Just some security pukes.
     

    esrice

    Certified Regular Guy
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    20   0   0
    Jan 16, 2008
    24,095
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    Indy
    An M4, but definition, has a carbine-length gas system.

    The differences in wear 'n tear, reliability, reduced recoil, and overall weight are negligible. If you already have an M4 don't sweat it. It's not like mid-lengths are awesome and carbine-lengths are suddenly crap.
     

    223 Gunner

    Master
    Rating - 100%
    199   0   0
    Jan 7, 2009
    4,410
    47
    Red Sector A
    An M4, but definition, has a carbine-length gas system.

    The differences in wear 'n tear, reliability, reduced recoil, and overall weight are negligible. If you already have an M4 don't sweat it. It's not like mid-lengths are awesome and carbine-lengths are suddenly crap.

    Agreed! But if it were the case, the military has an unlimited budget to repair any and all issues with M-16's/M-4's. :twocents:
     

    M4Madness

    Sharpshooter
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    3   0   0
    May 28, 2008
    743
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    Springville
    As mentioned, the M4A1 has a carbine gas system. They also have the SOCOM barrel, which is heavier under the handguards -- this is to handle sustained full-auto fire.

    I just sold one a few days ago:

    Colt_M4A1_SOCOM_Upper_Right.jpg


    Military 14.5" M4A1 SOCOM upper (top) and LE6921 upper (bottom):

    M4_Barrels1.jpg


    Carbine gas system:

    Colt_M4A1_SOCOM_Barrel.jpg
     

    kwood

    Sharpshooter
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    17   0   0
    Aug 27, 2010
    561
    18
    lizton/brownsburg
    if i am way off on this someone please correct me

    to my understanding the militaries M4 has a 14.5 barrel
    now....
    the 14.5 barrel with carbine gas system
    has the equivalent dwell time as a
    16 in middy
    which has the equivalent dwell as a
    20 in with rifle
    they each have 4 inches after the gas tube

    the reason a 16in barrel with a carbine length gas system has more "problems" is because the powder has more time to burn in the barrel and produces more pressure

    so since we can only get 16 in barrel for our civilian "M4"s, someone figured out that fixing the dwell time was the solution

    or getting a 14.5 in barrel with a permanently attached flash hider would make it nice and legal as well
     
    Rating - 100%
    22   0   0
    Dec 29, 2008
    3,745
    113
    Danville
    if i am way off on this someone please correct me

    to my understanding the militaries M4 has a 14.5 barrel
    now....
    the 14.5 barrel with carbine gas system
    has the equivalent dwell time as a
    16 in middy
    which has the equivalent dwell as a
    20 in with rifle
    they each have 4 inches after the gas tube

    the reason a 16in barrel with a carbine length gas system has more "problems" is because the powder has more time to burn in the barrel and produces more pressure

    so since we can only get 16 in barrel for our civilian "M4"s, someone figured out that fixing the dwell time was the solution

    or getting a 14.5 in barrel with a permanently attached flash hider would make it nice and legal as well

    The dwell time issue was going to be my next question. Is this something that would ever be an issue in a semi-automatic AR?

    I've been trying to wrap my mind around dwell time. My best guess is that dwell time has to do with the time the bolt and carrier are compressing the recoil spring. Since it takes longer for the bullet to get out the end of the barrel on a 16", the bcg would be under pressure longer, thus it would stay back longer. Wouldn't the gas just escape through the open chamber area? I'm not sure how it would keep the bcg back any longer, since the gas has someplace to escape. Is the whole idea of dwell time just theoretical, or is it proven?

    I can understand there being more pressure and thus a harder slam on the bcg, causing a more violent extraction of the spent cartridge. I'm just not sure how exactly dwell time could occur. I'm also wondering if it would be more of an issue with 5.56 ammo vs. .223, with 5.56 being loaded to higher pressure.

    I'm no engineer, so I may be way off on my thinking of what is really happening inside this interesting machine. Forgive me if that is the case.
     

    kwood

    Sharpshooter
    Rating - 100%
    17   0   0
    Aug 27, 2010
    561
    18
    lizton/brownsburg
    The dwell time issue was going to be my next question. Is this something that would ever be an issue in a semi-automatic AR?

    I've been trying to wrap my mind around dwell time. My best guess is that dwell time has to do with the time the bolt and carrier are compressing the recoil spring. Since it takes longer for the bullet to get out the end of the barrel on a 16", the bcg would be under pressure longer, thus it would stay back longer. Wouldn't the gas just escape through the open chamber area? I'm not sure how it would keep the bcg back any longer, since the gas has someplace to escape. Is the whole idea of dwell time just theoretical, or is it proven?

    I can understand there being more pressure and thus a harder slam on the bcg, causing a more violent extraction of the spent cartridge. I'm just not sure how exactly dwell time could occur. I'm also wondering if it would be more of an issue with 5.56 ammo vs. .223, with 5.56 being loaded to higher pressure.

    I'm no engineer, so I may be way off on my thinking of what is really happening inside this interesting machine. Forgive me if that is the case.

    i think you are partially right
    check this out

    Carbine vs. Mid-Length Gas System on a 16" Barrel [2010-01-10] - 03DESIGNGROUP

    i was wrong on the barrel length after the gas port
    this ought to help you out a bit
     

    sporter

    Master
    Rating - 100%
    30   0   0
    Mar 9, 2009
    2,394
    48
    Southern, Indiana
    if i am way off on this someone please correct me

    to my understanding the militaries M4 has a 14.5 barrel
    now....
    the 14.5 barrel with carbine gas system
    has the equivalent dwell time as a
    16 in middy
    which has the equivalent dwell as a
    20 in with rifle
    they each have 4 inches after the gas tube

    the reason a 16in barrel with a carbine length gas system has more "problems" is because the powder has more time to burn in the barrel and produces more pressure

    so since we can only get 16 in barrel for our civilian "M4"s, someone figured out that fixing the dwell time was the solution

    or getting a 14.5 in barrel with a permanently attached flash hider would make it nice and legal as well

    Seriously?
    There are no problems with 16" carbine barrels or carbine gas systems in AR15's.
    There have been literally hundreds of thousands made in carbine configuration over the last 40 years.

    Can you point to any credible sources as to the failure or issues with the carbine system? (in 16")
     
    Rating - 100%
    22   0   0
    Dec 29, 2008
    3,745
    113
    Danville
    i think you are partially right
    check this out

    Carbine vs. Mid-Length Gas System on a 16" Barrel [2010-01-10] - 03DESIGNGROUP

    i was wrong on the barrel length after the gas port
    this ought to help you out a bit

    I wonder how much the carbine length gas system on a 16 inch barrel affects the bullet velocity, if any?

    I'm really not that worried about durability. The extractor spring in the Spike's is rated to maintain its spec strength after 90,000 cycles. I guess one could keep a bolt and firing pin in a MOE grip if it caused much worry.

    Thanks for the link. It was fascinating, although indicated more preference than actual performance difference. I'd like to see actual data on velocity, and dwell time.
     

    kwood

    Sharpshooter
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    17   0   0
    Aug 27, 2010
    561
    18
    lizton/brownsburg
    Seriously?
    There are no problems with 16" carbine barrels or carbine gas systems in AR15's.
    There have been literally hundreds of thousands made in carbine configuration over the last 40 years.

    Can you point to any credible sources as to the failure or issues with the carbine system? (in 16")

    wow, it may just be the internet talking but that came across harsh...
    i thought i left my post very objective and open to correction
    im still doing my research as well
    i was hoping some more credible sources (members) would chime in and answer those questions for us
     

    ryknoll3

    Master
    Rating - 75%
    3   1   0
    Sep 7, 2009
    2,719
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    From my reading on ar15.com....
    There's not necessarily a problem with the carbine length gas system in a 16" barrel. Some people just have more issues with certain ammo.... carbine gas systems in a 16" barrel MAY be more picky about ammo.

    Because of the length of the gas system, the gas port hole is larger in the carbine than in the midlength, which is larger the gas port on the rifle length gasy system. The larger hole causes the gas impulse to be sharper, with higher pressures into the bolt carrier. This MAY cause the rifle to have extraction and/or ejection problems, because the bolt is trying to extract the cartridge while it is still expanded against the chamber walls. The longer gas systems with their longer dwell time allow for more volume of gas, so the impulse pressure needed to get the bolt carrier moving is lower. This allows for a "gentler" extraction, and can be more reliable with a broader range of ammunition and conditions. It's just basically more forgiving. If someone is having issues with extraction/ejection in their carbine length system, there are ways to address them. Heavier buffers may help retard the cycling, or a new or heavier recoil spring may help as well.

    Remember, the M4 and it's gas system was designed around 1 cartridge, the M855 with it's 62-gr projectile. Of course the military also uses heavier rounds, like the 77gr, but it was designed with this one load in mind.
     
    Rating - 100%
    22   0   0
    Dec 29, 2008
    3,745
    113
    Danville
    From my reading on ar15.com....
    There's not necessarily a problem with the carbine length gas system in a 16" barrel. Some people just have more issues with certain ammo.... carbine gas systems in a 16" barrel MAY be more picky about ammo.

    Because of the length of the gas system, the gas port hole is larger in the carbine than in the midlength, which is larger the gas port on the rifle length gasy system. The larger hole causes the gas impulse to be sharper, with higher pressures into the bolt carrier. This MAY cause the rifle to have extraction and/or ejection problems, because the bolt is trying to extract the cartridge while it is still expanded against the chamber walls. The longer gas systems with their longer dwell time allow for more volume of gas, so the impulse pressure needed to get the bolt carrier moving is lower. This allows for a "gentler" extraction, and can be more reliable with a broader range of ammunition and conditions. It's just basically more forgiving. If someone is having issues with extraction/ejection in their carbine length system, there are ways to address them. Heavier buffers may help retard the cycling, or a new or heavier recoil spring may help as well.

    Remember, the M4 and it's gas system was designed around 1 cartridge, the M855 with it's 62-gr projectile. Of course the military also uses heavier rounds, like the 77gr, but it was designed with this one load in mind.

    This makes sense. Thanks for the excellent description!

    What twist rate was specified with the 62 grain bullet?
     

    kwood

    Sharpshooter
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    17   0   0
    Aug 27, 2010
    561
    18
    lizton/brownsburg
    From my reading on ar15.com....
    There's not necessarily a problem with the carbine length gas system in a 16" barrel. Some people just have more issues with certain ammo.... carbine gas systems in a 16" barrel MAY be more picky about ammo.

    Because of the length of the gas system, the gas port hole is larger in the carbine than in the midlength, which is larger the gas port on the rifle length gasy system. The larger hole causes the gas impulse to be sharper, with higher pressures into the bolt carrier. This MAY cause the rifle to have extraction and/or ejection problems, because the bolt is trying to extract the cartridge while it is still expanded against the chamber walls. The longer gas systems with their longer dwell time allow for more volume of gas, so the impulse pressure needed to get the bolt carrier moving is lower. This allows for a "gentler" extraction, and can be more reliable with a broader range of ammunition and conditions. It's just basically more forgiving. If someone is having issues with extraction/ejection in their carbine length system, there are ways to address them. Heavier buffers may help retard the cycling, or a new or heavier recoil spring may help as well.

    Remember, the M4 and it's gas system was designed around 1 cartridge, the M855 with it's 62-gr projectile. Of course the military also uses heavier rounds, like the 77gr, but it was designed with this one load in mind.


    sporter
    this is what i was referring to
     
    Rating - 100%
    22   0   0
    Dec 29, 2008
    3,745
    113
    Danville

    Wow. I thought 1/7 was more for 70+ grain bullets. I know my 1/9 rifles handle 62 grain bullets quite well, with excellent accuracy.

    I'm looking forward to seeing how the 1/7 does at the range. I've got a bit of 62 grain ammo, but most is 55 grain. I'm hoping it does fine with it, too.
     

    ryknoll3

    Master
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    3   1   0
    Sep 7, 2009
    2,719
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    When the M16 was introduced with its 55gr bullet, it was originally equipped with a 1:14 twist bullet. This was later changed to 1:12 twist to stabilize bullets in cold climate.

    When the M16A2 was developed, and the M855 62gr bullet was introduced the optimal twist for this bullet is 1:9. The reason the military went with 1:7 was to stabilize the long tracer bullet fired mostly by the M249 SAW. One of the objectives is to have ammo compatibility across weapons systems, and amongst NATO allies. That's why the M855/SS109 cartridge is the NATO standard. The 1:7 twist allows stabilization of both the standard 62gr FMJ round and the tracer round. Also, this twist will stabilize the 77gr OTM round that is preferred for CQB, though this round was developed later and wasn't considered when the barrel twist was decided on.

    If you have a rifle with a 1:9 twist, it can USUALLY reliably stabilize up to a 69gr bullet. The 1:8 twist is actually optimal for most of the heavier bullets and is often found in 5.56/.223 match barrels.
     
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