Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 24

Thread: Shortening a BCM 16" LIGHTWEIGHT barrel?

  1. #11
    Join Date
    Apr 2010
    Posts
    217
    Feedback Score
    13 (100%)
    as far as cheap ammo has'nt been such a thing for a loooooong time (think .10 a rnd for m193 real m193) so my solution is get to know someone with your local pd (depending how friendly they can be) find out when they qual for carbine and go ask for their brass. take along a bucket if it's a small department 5 or 6 totes for a big qual. your expierience may vary depending on who you deal with. try to get for the cost of a please if that don't work offer to pay 50.00 for a five gallon bucket full . but that would be the start of cheap ammo cuz then you only have to pay for bullets,powder,primers (when avaliable) and you get to shoot more

  2. #12
    Join Date
    Mar 2011
    Posts
    165
    Feedback Score
    0
    I would think the 16" LW middy would be a highly desirable barrel. I would hate to see it get molested. That was what I was looking for when I finally went with the .750 barrel. I'd do a swap when barrels come back and keep the 16" for a later time or for trade. kwg

  3. #13
    Join Date
    Mar 2013
    Posts
    85
    Feedback Score
    0

    Shortening a BCM 16" LIGHTWEIGHT barrel?

    I actually have 2

    Sounds like the general consensus is to keep the 16 LW middy as-is. I think I'll just trade one off for a 14.5" when the opportunity presents itself.

    I'm actually trying to unload one of them locally.

  4. #14
    Join Date
    Jan 2008
    Posts
    3,714
    Feedback Score
    4 (100%)
    Quote Originally Posted by websniper View Post
    There are pros to the 16, such as being able to swap muzzle devices or pull the fsb for a rail install etc.

    I'm curious why you are not a fan of 14.5 mid-lengths?
    a lot of problems crept up in a reliable system when we shortened a 20" barrel with a rifle lower, to 14" with collapsible carbine stock. the gas system was shortened to 7" to keep dwell time app. the same as a 20" rifle.

    there is a sweet spot for dwell time. overgassing decreases durability. undergassing decreases reliability as the gun gets dirty or less powerful ammo is used.

    midlength is the solution to the problem of overgassing a carbine gas system when .gov arbitrarily required all barrels to be 16" min. cutting a midlength back to 14.5 reduces dwell and sways it over to undergassed.

    it's true, many people have used 14.5 midlength with good reliability but not with all ammo/buffer/spring combinations. there is also variability in gas port sizes among different manufacturers and some vary according to barrel/gas length involved.

    the 14.5 midlength was born from the idea of looking for a "soft shooting" gun. that's nice and if I set up a gun just for competition or paper shooting I might look at that. but that's not the primary purpose of my guns...
    never push a wrench...

  5. #15
    Join Date
    Oct 2008
    Location
    Poquoson, VA
    Posts
    534
    Feedback Score
    6 (100%)
    Since the barrel is midlength (gas system), cutting it to 14.5" might create new issues. Keep it, sell it or best yet, trade it for the one you want.

    Mark

  6. #16
    Join Date
    Mar 2013
    Posts
    85
    Feedback Score
    0

    Shortening a BCM 16" LIGHTWEIGHT barrel?

    I always thought that the mid length gas system's purpose was to make a "softer" shooting gun. Plus, frankly, I like the way the middys look with less barrel past the FSB.

    Thanks for the education, ra2bach.

    To further the technical part of this discussion, can not the barrel length be compensated for by adjusting the gas port's size?


    Quote Originally Posted by ra2bach View Post
    a lot of problems crept up in a reliable system when we shortened a 20" barrel with a rifle lower, to 14" with collapsible carbine stock. the gas system was shortened to 7" to keep dwell time app. the same as a 20" rifle.

    there is a sweet spot for dwell time. overgassing decreases durability. undergassing decreases reliability as the gun gets dirty or less powerful ammo is used.

    midlength is the solution to the problem of overgassing a carbine gas system when .gov arbitrarily required all barrels to be 16" min. cutting a midlength back to 14.5 reduces dwell and sways it over to undergassed.

    it's true, many people have used 14.5 midlength with good reliability but not with all ammo/buffer/spring combinations. there is also variability in gas port sizes among different manufacturers and some vary according to barrel/gas length involved.

    the 14.5 midlength was born from the idea of looking for a "soft shooting" gun. that's nice and if I set up a gun just for competition or paper shooting I might look at that. but that's not the primary purpose of my guns...

  7. #17
    Join Date
    Jan 2008
    Posts
    3,714
    Feedback Score
    4 (100%)
    Quote Originally Posted by websniper View Post
    I always thought that the mid length gas system's purpose was to make a "softer" shooting gun. Plus, frankly, I like the way the middys look with less barrel past the FSB.

    Thanks for the education, ra2bach.

    To further the technical part of this discussion, can not the barrel length be compensated for by adjusting the gas port's size?
    it can and manufacturers do have different GP sizes for long and short barrels. I don't know if many or any do vary the size between midlength and carbine though.

    but dwell time is the time (in milliseconds) from when the bullet passes the gas port till it exits the muzzle. it has nothing to do with gas port size - that affects the volume of gas, not the time in which that pressure is vented to the carrier. it is important as it relates to where in the pressure cycle extraction occurs.

    there is a range that is the best balance between reliability and your gun beating itself to death.

    here is the best reference material to this I have seen, taken from ar15barrels.com -- http://www.ar15barrels.com/prod/operation.shtml
    never push a wrench...

  8. #18
    Join Date
    Mar 2013
    Posts
    85
    Feedback Score
    0

    Shortening a BCM 16" LIGHTWEIGHT barrel?

    Thanks again r2a, I'll read up on that link you posted.

    Slapped my bfh lw middy upper on a lower I've had stashed for a while tonight.

    Loving this lightweight profile.


    Going to keep this rifle lightweight and KISS.
    Last edited by websniper; 03-28-13 at 20:54. Reason: Updated for less-crummy pic resolution

  9. #19
    Join Date
    Dec 2009
    Location
    Utah
    Posts
    8,422
    Feedback Score
    3 (100%)
    Quote Originally Posted by ra2bach View Post
    a lot of problems crept up in a reliable system when we shortened a 20" barrel with a rifle lower, to 14" with collapsible carbine stock. the gas system was shortened to 7" to keep dwell time app. the same as a 20" rifle...

    ...midlength is the solution to the problem of overgassing a carbine gas system when .gov arbitrarily required all barrels to be 16" min. cutting a midlength back to 14.5 reduces dwell and sways it over to undergassed...
    Actually, the gas system length was all about the bayonet. The distance from muzzle to FSB on the M4 was set to match that of the 20" rifle so the issue bayonet could be used. In fact, When the M4 was first introduced, an article came out in one of the gun magazines stating that Colt picked the barrel length by finding what the minimum barrel length would allow the shortened rifle to function reliably while maintaining bayonet use, then added a half inch for insurance. I cannot vouch for the veracity of the statement, but that's what the author wrote.

    When M4 clones became popular on the civilian market, oft repeated criticism was that with the 16" barrel required federal regulation the distance from muzzle to lug was too great to allow the use of a bayonet. In response, Armalite lengthened the gas system to allow bayonet use with a 16" barrel. It was even stated so in their catalogs of the time.

    If dwell time is defined by the time it takes the bullet to travel from the gas port to muzzle, then it has little to no affect on the "gassing" of the system, as the actual time it takes a bullet to travel an extra inch and a half is measured in a small fraction of a millisecond. In reality, the bullet has left the barrel before the gases have a chance to charge the carrier to operating pressure. What that extra inch and a half of barrel length has an affect on is how long it takes the gasses to vent. The extra length increases the time needed for the gasses to vent and holds the pressure longer. This in turn fills the gas tube & expansion chamber quicker and the carrier is accelerated quicker. To slow down the rate at which the system pressurizes, a smaller diameter gas port should be used.

    This is why the addition of a suppressor causes an AR to be "over-gassed". The suppressor acts as a pressure reservoir and slows the velocity of the expanding gasses. This prolongs venting, keeping pressure higher longer after the bullet has exited. The gasses that fill the system are at a higher pressure than they'd be without the suppressor

    ETA- There are a couple of errors in the linked article-

    The gasses do not exit the muzzle at 5x the speed of a high velocity rifle bullet. If I recall correctly, the gasses of modern smokeless powders exit at about 5700 fps when the bullet uncorks the muzzle.

    The gasses do not push the bolt forward when they enter the expansion chamber. The pressure of the gas inside the expansion chamber of the carrier cannot exceed the pressure of the gas inside the bore & chamber. In fact, the pressure inside the expansion chamber is much less than it is in the bore when the carrier moves, separating the gas key from the gas tube, cutting off gas flow to the carrier
    Last edited by MistWolf; 03-28-13 at 01:13.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  10. #20
    Join Date
    Jan 2008
    Location
    AZ-Waging jihad against crappy AR's.
    Posts
    24,902
    Feedback Score
    104 (100%)
    All hocus pocus aside, do not cut a perfectly good barrel for some perceived notion that you are gaining anything by removing part of the barrel and pinning a muzzle device. The only time it makes any kind of sense is if you live somewhere like NY, CT or other weird state that requires a permed device.



    Owner/Instructor at Semper Paratus Arms

    Facebook: https://www.facebook.com/SemperParatusArms/

    Semper Paratus Arms AR15 Armorer Course http://www.semperparatusarms.com/cou...-registration/

    M4C Misc. Training and Course Announcements- http://www.m4carbine.net/forumdisplay.php?f=141

    Master Armorer/R&D at SIONICS Weapon Systems- http://sionicsweaponsystems.com

Page 2 of 3 FirstFirst 123 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •