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  1. #1
    Expert Slow Hand's Avatar
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    Problems cutting down a 20" AR barrel?

    Recently traded into a heavy barreled 20" AR-15. My main interest in this one is a 100 yard paper puncher and possibly some coyote hunting later on. Figure 3-400 yards max range for it. It's got a 1-9" barrel twist on it. My other big interest if it turns out to be a good shooter is to cut it down and thread for a can. My suppressor is a .30 cal SAS can that's 9" long, so I"d like to keep OAL down as short as possible. I'm wondering if I have the barrel cut back as far as possible with the FSB (or 16" if possible) will I have cycling problems? My can is threaded 5/8" so I can't just use a shorter barrel and gas system with the factory 1/2" threads.

    Also, out of curiosity, is there any way to figure out who made an upper? It's a parts gun, but is very well put together. I'm just curious who made the upper if I ever go to sell it.

    Thanks
    Doug K
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  2. #2
    Master 451_Detonics's Avatar
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    If there aren't any marking on the upper it would be hard to trace down the manufacturer...there were just so many that made identical uppers it would be a WAG at best.

    Cutting the barrel back to 16 1/4 shouldn't pose any problems as you will still be ahead of the gas port and the pressure drop from 4 inches of barrel isn't enough to matter. If you plan on creating an SBR that is a whole different matter. I chopped 2 inches off a 20 inch barrel on my last build and and it has not been an issue at all.
    Today is the tomorrow you worried about yesterday

  3. #3
    Grandmaster cosermann's Avatar
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    The gas system on the AR is a balancing act. All the variables; distance from the chamber to the gas port, size of the gas port, distance from the gas port to the muzzle are important. If you start cutting down your barrel, you are reducing the distance from the gas port to the muzzle and consequently the time/pressure the gas can work your action (since the pressure rapidly falls to zero after the bullet exits the muzzle). Cans have an effect also.

    So yeah, it can cause problems. You'd be removing FOUR INCHES from a 20" barrel to get to 16". I don't have the info handy, but yeah I think that will cause problems. I suggest you buy a 16" threaded barrel with a midlength gas system.

    To learn about the AR gas system see this page:

    AR15BARRELS.COM - Randall's description of AR gas operation and how everything works in harmony

  4. #4
    Master Fargo's Avatar
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    Quote Originally Posted by cosermann View Post
    The gas system on the AR is a balancing act. All the variables; distance from the chamber to the gas port, size of the gas port, distance from the gas port to the muzzle are important. If you start cutting down your barrel, you are reducing the distance from the gas port to the muzzle and consequently the time/pressure the gas can work your action (since the pressure rapidly falls to zero after the bullet exits the muzzle). Cans have an effect also.

    So yeah, it can cause problems. You'd be removing FOUR INCHES from a 20" barrel to get to 16". I don't have the info handy, but yeah I think that will cause problems. I suggest you buy a 16" threaded barrel with a midlength gas system.

    To learn about the AR gas system see this page:

    AR15BARRELS.COM - Randall's description of AR gas operation and how everything works in harmony
    I know Pat Sweeney routinely cuts 20 inch m16a1 barrells down to 16 inches for police departments and hasn't had any problems.

    IIRC, he described it in Vol 1. of the "Book of the AR-15".

    Best,

    Joe

  5. #5
    Master
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    I have a 14.5" middy and it runs fine.

    The same isn't always true of cutting down a rifle, though.

    I'd cut a rifle to 18" without hesitation, but I'm not sure I'd try it at 16". Someone who knows how to run the numbers might have a better response.

  6. #6
    Sharpshooter billyboyr6's Avatar
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    i had a 20" barrel that was cut down to 11.5" that i redrilled the gas port and used a carbine length gas tube and it cycled without a single issue. It cycled with rifle length stock and buffer and it cycled with carbine collapsable stock and buffer.
    "Every man dies, but not every man really lives".

  7. #7
    Grandmaster cosermann's Avatar
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    Quote Originally Posted by Fargo View Post
    I know Pat Sweeney routinely cuts 20 inch m16a1 barrells down to 16 inches for police departments and hasn't had any problems. ...
    Really? With no other mods, like say, redrilling the gas port?

    I didn't say it was impossible, it's certainly possible, just that all the variables need to be balanced within certain limits. Change one too much and you'll have to compensate by changing the others.

    If you know you want a 16" barrel from the get go, buy one. It's the simplest solution.

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