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  • whbonney26

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    Nov 2, 2008
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    I have built many AR rifles but recently built my first AR pistol a while back and did it a little different than most and went with a pretty short barrel. I went with 300 Blackout and a 6" barrel with a Kaw Valley Linear Compensator in order to try to gain the pressure needed to send the BCG back.

    Well I am not quite getting enough to send the BCG completely back when using subsonics unsuppressed. The bolt goes back far enough to eject the spent casing but not far enough back to pick up the next round and chamber it.

    The supers fired pretty well for the most part.

    I am thinking that possibly a light weight BCG may be enough to fix the problem and I am looking for recommendations from folks that have used whatever light weight bcg they suggest.

    I thought possibly about an adjustable gas block also but wondering if a lw bcg will be enough.

    Open to other ideas also if anyone has any that will help the pistol cycle and run well.
     

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    MTx11B

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    Carbine spring and h2 buffer? an adjustable gb tends to solve most problems gas wise if your port is under or oversized ect. Some AR builds require a break in period before they properly run well and some take a little more lube than usual during this process.
     

    whbonney26

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    Carbine spring and h2 buffer? an adjustable gb tends to solve most problems gas wise if your port is under or oversized ect. Some AR builds require a break in period before they properly run well and some take a little more lube than usual during this process.

    I have been thinking about putting the adjustable gas block and possibly the JP Captured Spring kit eventually.
     

    whbonney26

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    shrxfn

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    what length gas system does it have? For my 300BO build that I haven't put together yet I found online that pistol length is the preferred short barrel gas system you want to get the right amount of gas to the BCG. If all else fails you could try drilling out the port with the next size up drill bit and see if that helps with the undergassing.
     

    DAVE_M

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    Often times undergassing is misdiagnosed as overgassing. There is a good chance your bolt is outrunning the magazine. Disregard this bit, I noticed you said it only has this issue with subs.

    Are you using a carbine (H) buffer and spring?

    I also have to ask why you are shooting subs unsuppressed? I would not tune a gun to run subs unsuppressed, because it will be severely overgassed when running supers.
     
    Last edited:

    DAVE_M

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    what length gas system does it have? For my 300BO build that I haven't put together yet I found online that pistol length is the preferred short barrel gas system you want to get the right amount of gas to the BCG. If all else fails you could try drilling out the port with the next size up drill bit and see if that helps with the undergassing.

    Do not drill the gas port. That is a last resort when you know for certain that the port is under size from what it should be.
     

    Magdump

    Don’t troll me bro!
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    Dec 31, 2013
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    what length gas system does it have? For my 300BO build that I haven't put together yet I found online that pistol length is the preferred short barrel gas system you want to get the right amount of gas to the BCG. If all else fails you could try drilling out the port with the next size up drill bit and see if that helps with the undergassing.
    his barrel is 6 inches so the only option there is pistol gas. I’ve seen this issue and 2 things can be done. .08in bit (nothing bigger) to drill out the gas port and or reduced weight buffer and buffer spring. You May also get some benefit from an actual brake. Inline comps are great for short barrel thumpers but don’t really help to brake them, just channel everything out the front. Good luck.
     
    Last edited:

    whbonney26

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    Often times undergassing is misdiagnosed as overgassing. There is a good chance your bolt is outrunning the magazine. Disregard this bit, I noticed you said it only has this issue with subs.

    Are you using a carbine (H) buffer and spring?

    I also have to ask why you are shooting subs unsuppressed? I would not tune a gun to run subs unsuppressed, because it will be severely overgassed when running supers.

    Running pistol gas tube. I’ll look again but I’m pretty sure I have the lightest buffer I could find. Not sure if it’s the H or not. I’ll look again in the morning.

    I am not really trying to tune the gun for the subs but would like it to cycle with both. No plans to drill anything yet. I’m confident I can remedy the situation without drilling.
     

    Magdump

    Don’t troll me bro!
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    Running pistol gas tube. I’ll look again but I’m pretty sure I have the lightest buffer I could find. Not sure if it’s the H or not. I’ll look again in the morning.

    I am not really trying to tune the gun for the subs but would like it to cycle with both. No plans to drill anything yet. I’m confident I can remedy the situation without drilling.
    If you had more barrel I would agree with you on no drilling, but it’s gonna be a challenge with just a tad of barrel out past the gas port. Most of the gas is going the path of least resistance because the bullet is out of the bore before the powder is all burned. I’ve never seen a .300 shorter than 10 inches not suffer from under gassing with subs and no can. Your barrel’s gas port may also already be drilled to the max recommended size which would rule out that fix anyway. I’ve got light weight buffers and springs if you wanna try some parts. I’ve also seen someone modify a standard buffer to reduce weight and it worked fine.
     

    DAVE_M

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    Running pistol gas tube. I’ll look again but I’m pretty sure I have the lightest buffer I could find. Not sure if it’s the H or not. I’ll look again in the morning.

    I am not really trying to tune the gun for the subs but would like it to cycle with both. No plans to drill anything yet. I’m confident I can remedy the situation without drilling.

    Do you have access to pin gauges?
     

    rcm192

    Sic semper tyrannis
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    I had the exact same problem with my Spikes 300 blackout build and I only run supersonics through it. Picked this spring at brownells and now it'll cycle everything I put through it. https://www.brownells.com/search/index.htm?k=nemo&ksubmit=y

    We use this option and recommend it to customers quite often. Cheap way to cycle subs without a can....the better option is to get a can though!
     

    MTx11B

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    Weigh your buffer on an oz scale, if you have dial calipers or access to some you can measure gas port also since you dont have gauges. Tuning for optimization can start with gun but usually comes down to load development also. There are known port sizes that work with pistol lengths and an adjustable gas block can help with both over and under gassed guns as it loosens or restricts gas flow, according to science and extensive testing that is
     

    DAVE_M

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    32   0   0
    Apr 17, 2009
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    Weigh your buffer on an oz scale, if you have dial calipers or access to some you can measure gas port also since you dont have gauges. Tuning for optimization can start with gun but usually comes down to load development also. There are known port sizes that work with pistol lengths and an adjustable gas block can help with both over and under gassed guns as it loosens or restricts gas flow, according to science and extensive testing that is

    Pretty much all of this. I don’t care for adjustable blocks, but it’s the only way to remedy an oversized gas port that can’t be treated with increased spring and buffer weights. Triple check the gas port size before you get crazy with a drill.

    I say only way, but BRT gas blocks are an option as well.
     

    shrxfn

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    If you have a complete drill bit set you can use the shank of a drill bit to measure the port size without pin gauges. I am talking the ones that have the smaller bits that are marked with numbers sometimes. You can use the number and find the actual decimal value.
     

    whbonney26

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    Pearl River, LA
    I had the exact same problem with my Spikes 300 blackout build and I only run supersonics through it. Picked this spring at brownells and now it'll cycle everything I put through it. https://www.brownells.com/search/index.htm?k=nemo&ksubmit=y

    Thanks for the info and link.

    - - - Updated - - -

    We use this option and recommend it to customers quite often. Cheap way to cycle subs without a can....the better option is to get a can though!


    I do hope to get a can soon. Or at least start the process soon.
     

    whbonney26

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    37   0   0
    Nov 2, 2008
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    Pearl River, LA
    If you have a complete drill bit set you can use the shank of a drill bit to measure the port size without pin gauges. I am talking the ones that have the smaller bits that are marked with numbers sometimes. You can use the number and find the actual decimal value.


    Excellent point and idea.

    - - - Updated - - -

    Pretty much all of this. I don’t care for adjustable blocks, but it’s the only way to remedy an oversized gas port that can’t be treated with increased spring and buffer weights. Triple check the gas port size before you get crazy with a drill.

    I say only way, but BRT gas blocks are an option as well.

    I will def. try some of the cheap ideas such as buffer spring and buffer before drilling.
     

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