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Anonymous asked in SportsOutdoor RecreationHunting · 7 years ago

How to covert full length AR-15 to carbine length?

I was thinking about converting my .223 Colt AR-15 from a full length barrel (20in) to a carbine length barrel (14.5in) to optimize maneuverability and reduce overall weight. Obviously I need a new barrel and handguards, but I assume I also need a new gas system. The gas system I've never messed with so any input would be appreciated.

It has a fixed front sight about 6.5in away from the end of the muzzle brake. I see the gas tube begin about 8.5in from the end of the muzzle brake (the front sight being 2in wide). The visible portion of the gas tube is 11.5in from the front sight to the delta ring.

I would just get a new carbine, but they're all currently sold out and being price gouged anyway.


Just for clarification, I'm not cutting the barrel or any parts. I would order a new barrel and gas system and replace them myself.

Update 2:

I'm currently under Colorado law, so no issues with this.

4 Answers

  • 7 years ago
    Favorite Answer

    You are very fortunate you posted this here first.

    "Carbine" length in the United States is 16 inches. Any shorter and your rifle must be registered under the NFA as an SBR (Short Barrelled Rifle) and now has many of the same legal characteristics as a machine gun.

    Not sure if those extra two inches are worth it to you.

    When they come back in stock, I recommend picking up an entire 16" upper, and a "field" headspace gauge, and, using your same bolt and bolt carrier, check to make sure it headspaces correctly (Closes on a live round, doesn't close on the field gauge), and call it a job well done.

    The gas system on the AR is very simple -- basically a stainless steel straw, allowing the gas to go from the barrel, through a hole in the barrel (gas port) into the front sight, down the stainless steel straw, into the gas key of the bolt carrier, at which point the bolt BECOMES the "piston" with the expansion chamber being the interior of the bolt carrier, causing the bolt to move forward and unlock, which in turn kicks the bolt carrier rearword, and the action cycles.

    There isn't a whole lot to it.

    However, that simplicity hides a whole lot of really close tolerances.

    Which is why I'd recommend getting a carbine-length upper and calling it a day. If you're really into learning about your rifle, you can buy a carbine-length barrel (16", not 14.5") with the gas port already drilled, put a sight or gas block on it, and then install the barrel on an upper of your choice, tightening it down, and then carefully installing the gas tube correctly without bending the hell out of it.

    This can be very helpful.

    Don't try to actually cut down your barrel -- first off, it will damage the chrome lining, and unless you really know what you're doing, it will be near-impossible for you to recrown it properly, which is one of the keys to mechanical accuracy.

    Second, you need to know where to drill the gas port, and if you don't, it doesn't take more than a few thousandths off for you to turn your barrel into useless junk.

    Third, if your barrel has been profiled for a RIFLE barrel, there may not be enough metal at the gas port point for a carbine once it's cut down.

    So, your EASIEST way to do this would be:

    1) Buy a carbine barrel.

    2) Learn how to replace your rifle barrel with the carbine barrel:

    a) Remove handguards

    b) Drive out gas tube pin with a punch

    c) Remove gas tube

    d) Using barrel nut wrench, unscrew barrel nut

    e) Remove barrel from receiver

    f) Note presence of barrel nut, front sight tower, and handguard cap on new carbine barrel.

    g) Remove the delta ring, spring weld, and snap ring from your old barrel nut, and put them on the new barrel nut prior to barrel installation on receiver. Be sure to line up the hole in the delta ring, the gap in the spring weld, and the gap in the snap ring, so the gas tube will be able to fit through it.

    h) Place carbine barrel in receiver, lining up the stud on the barrel extension with the notch in the receiver

    i) tighten down the barrel nut to TM spec, between 35 and 85 foot pounds, but most importantly so that one of the "spaces" in the barrel nut "sprocket" line up perfectly with the gas tube hole in the front of the receiver.

    j) line up the hole in the delta ring, the gap in the spring weld, and the gap in the snap ring so you can easily insert the gas tube through them and into the receiver.

    k) using a carbine-length gas tube, install the gas tube in the receiver, closed end with the holes on the sides towards the muzzle, the open end with the slight machined flange to the interior of the receiver.

    l) When you have the gas tube properly aligned, with the one large hole on the side of the closed end facing DOWN into the gas port area, and the two small holes on the sides lining up with the gas tube pin holes on the front sight, gently drive in the gas tube pin to finish the installation of the gas tube.

    m) Verify all steps have been taken properly.

    n) You have now just rebarrelled an AR-15 with a carbine barrel.

    3) Verify your installation has been done correctly.

    4) Headspace with field gage and live ammo.

    5) Test fire.

    All that being said, I accept ABSOLUTELY NO resposibility for your use of these instructions, as I have no way of being able to control the method by which you follow them or my own error or omission which could otherwise be adjusted by my presence or otherwise. They are merely step-by-step guidelines and as such, should be practiced only by people who are reasonably knowledgeable and willing to accept responsibility for their own actions.

    OR, you could just buy a completed upper with bolt and bolt carrier and call it a day.

    Your decision.

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  • Chris
    Lv 5
    7 years ago

    You are right that you need a new gas system, but then you will still be stuck with the rifle length gas hole. You need the new gas system because the dwell time on a rifle length gas system and a sub 16" barrel is not long enough to work the BCG. I have never heard of someone just plugging that hole and lathing the barrel down at the carbine length and clamping the gas block there. It may have happened, but whatever you do to plug the hole reliably is going to stress the barrel also. Why you would want a carbine length gas system on a 14.5" barrel is also beyond me. Midlength is much better for that length barrel.

    While barrels are hard to find, they are still available from some makers. I would suggest just buying a 14.5" barrel and swapping with what you have, rather than destroy a perfecly good 20" barrel.

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  • 3 years ago

    What Is Carbine Length

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  • 7 years ago

    Make sure to check your local laws first. I'm not sure what your local laws are but where I live I believe the barrel length has to be above 16 inches.

    If your flashhider is welded and pinned it counts as part of the overall barrel length. Don't want the ATF kicking down your door sooner then Obama orders them too. ;)

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