Opening-Up Gas Port

Dogtown

Ke = (mv^2)/2
Full Member
Minuteman
Jun 21, 2007
5,327
2,211
CONUS
I got an 18" SPR barrel from Craddock earlier this year that I had converted to an LMT quick-change barrel for use in an MLR. It shot really well but after a couple hundred rounds through it I realized that for this rifle application I would prefer it a bit shorter. So I had it cut back to 16" and now it's undergassed because, silly me, it's now a 16" barrel with a rifle length gas system and won't cycle reliably. I tried all of the obvious fixes like switching gas blocks, making sure everything was aligned properly, swapping gas tubes, played with an adjustable buffer, but it's never getting enough gas probably due to reduced dwell time.

Since this barrel shot lights out before getting cut down, I just don't want to give up on it, so now I'm looking at opening up the gas port a bit.

That's something I've never done and part of me thinks "how hard can it be?" Is this something I should be trying to do myself or should I send it out to someone who can do it correctly? Enquiring minds want to know.
 

TonyTheTiger

Like a Boss
Full Member
Minuteman
Feb 14, 2017
4,063
5,586
Done it quite a few times. Get a machinist drill bit set, go slow, maybe dribble some cutting oil if you care enough to. Start small too, if you work up to perfect function you can go without an AGB.
 
  • Like
Reactions: rpoL98

rpoL98

USA
Full Member
Minuteman
Sep 27, 2017
349
208
USA
i've done this a few times, here's what I've learned.

use the high-school math formula of pi-R-squared, get an accurate measure of your current gas port diameter. use the formula to determine gas-port cross-sectional-area. The size of the CSA determines gas flow, in a basic sense.

based on your under-gassed symptoms, you make a judgement call on whether to go with 5% increase, 10% increase, or 15% increase in CSA. it's better to go too small, because you can re-iterate the process. Although, if you go too big, an adjustable gas block can mitigate that, and also going from a std 3oz (carbine) buffer up through the range to H3 for finer resolution.

your under-gassed last-round-bolt-hold-open symptoms (with your lightest buffer) will range from:
empty case does not eject, stays in the chamber (BCG just does a little bounce)
empty case ejects, but BCG returns to battery on empty chamber
empty case ejects, but bolt carrier (not bolt) hangs up on bolt catch (may depend on shape of parts used)
empty case ejects, but bolt hangs up on mag follower (this is the case for when you drop the mag, the bolt slams forward)
empty case ejects, BCG intermittently, not consistently, locks back on bolt catch.

you need to have a complete drill bit set: numeric wire gauge, and fractionals. It helps if you have a hole gauge set to accurately gauge your initial gas port, to establish your basis. Thousandths make a difference.
 

the monk

Supporter
Supporter
Full Member
Minuteman
Supporter+
  • Jun 17, 2018
    141
    103
    If you ping SOLGW they will tell you what size gas port you should have for a given barrel length. You can also get a good idea by specking out a gas tube on black river tacticals site. It will tell you what size it should be based on what you want to do.
     

    S197

    Gunny Sergeant
    Full Member
    Minuteman
    Nov 22, 2019
    915
    789
    I got an 18" SPR barrel from Craddock earlier this year that I had converted to an LMT quick-change barrel for use in an MLR. It shot really well but after a couple hundred rounds through it I realized that for this rifle application I would prefer it a bit shorter. So I had it cut back to 16" and now it's undergassed because, silly me, it's now a 16" barrel with a rifle length gas system and won't cycle reliably. I tried all of the obvious fixes like switching gas blocks, making sure everything was aligned properly, swapping gas tubes, played with an adjustable buffer, but it's never getting enough gas probably due to reduced dwell time.

    Since this barrel shot lights out before getting cut down, I just don't want to give up on it, so now I'm looking at opening up the gas port a bit.

    That's something I've never done and part of me thinks "how hard can it be?" Is this something I should be trying to do myself or should I send it out to someone who can do it correctly? Enquiring minds want to know.
    What is the current gas size?
    Which direction are the cases being ejected?
    what buffer system are you running? Rifle, carbine, A5?

    Before you drill, start reading up on AR Dissipator set ups. The original dissipator was a 16” barrel with rifle gas, and these can be tricky to tune, but have been done.
     

    Dogtown

    Ke = (mv^2)/2
    Full Member
    Minuteman
    Jun 21, 2007
    5,327
    2,211
    CONUS
    It's a 0.093" port

    Cases are ejecting around 5 o'clock, not locking back on empty mag, and was running a carbine H2 buffer. I started with an adjustable gas block, but even with it all the way open it didn't want to cycle properly. I swapped the gas tube and eventually replaced the block with a non-adjustable version, but same results. Next I replaced the buffer spring and also tested with an adjustable buffer. As the weight was reduced, the ejection pattern improved but the bolt had difficulty returning to the locked position.

    I'm leaning towards trying to open it up to 0.096" and if that doesn't improve things I'll open it up a tad further.
     

    S197

    Gunny Sergeant
    Full Member
    Minuteman
    Nov 22, 2019
    915
    789
    It's a 0.093" port

    Cases are ejecting around 5 o'clock, not locking back on empty mag, and was running a carbine H2 buffer. I started with an adjustable gas block, but even with it all the way open it didn't want to cycle properly. I swapped the gas tube and eventually replaced the block with a non-adjustable version, but same results. Next I replaced the buffer spring and also tested with an adjustable buffer. As the weight was reduced, the ejection pattern improved but the bolt had difficulty returning to the locked position.

    I'm leaning towards trying to open it up to 0.096" and if that doesn't improve things I'll open it up a tad further.
    An H2 is to heavy, I would go with an H which should be 3oz’s. the whole point of a rifle length gas is for a soft recoil, opening the gas port should be a last resort on a high end barrel.

     
    • Like
    Reactions: rpoL98

    Bigtrucknut

    Private
    Minuteman
    Dec 9, 2020
    58
    36
    NoVA
    And I wise man once advised me to stick an appropriately size wooden dowel down the bore far enough to cover the gas port. This will prevent you from running the drill bit into the opposite side of the rifle bore when the drill breaks through the port while you are exerting pressure on the drill.
     

    msgriff

    Sergeant of the Hide
    Full Member
    Minuteman
    Feb 10, 2021
    420
    486
    Rogue Valley OR
    I believe that the Colt 605 cloners (15.5"/16" RLGS) open up the gas port to 0.110" (#35), but there's nothing wrong with working your way up. There are shops that are familiar with your current barrel and gas system specs and actually know the correct port diameter, and the reputable ones will also deburr/chamfer the outside and inside of the port.
     
    • Like
    Reactions: Dogtown

    TheOE800

    Supporter
    Supporter
    Full Member
    Minuteman
  • Feb 14, 2017
    2,816
    1,739
    USA
    Chucking or hand reamer #41 wire gauge.

    Proof Research specs a 0.093” port for 16” with Noveske intermediate length or 18” rifle.
     

    TonyTheTiger

    Like a Boss
    Full Member
    Minuteman
    Feb 14, 2017
    4,063
    5,586
    It's a 0.093" port

    Cases are ejecting around 5 o'clock, not locking back on empty mag, and was running a carbine H2 buffer. I started with an adjustable gas block, but even with it all the way open it didn't want to cycle properly. I swapped the gas tube and eventually replaced the block with a non-adjustable version, but same results. Next I replaced the buffer spring and also tested with an adjustable buffer. As the weight was reduced, the ejection pattern improved but the bolt had difficulty returning to the locked position.

    I'm leaning towards trying to open it up to 0.096" and if that doesn't improve things I'll open it up a tad further.
    All my RLGS 16" barrels are between .098-.102.
    My 17" RLGS +1" is .106 and cycles everything perfectly. I would definitely try a lighter buffer before drilling though.
     
    Last edited:
    • Like
    Reactions: LRBuck and rpoL98

    Dogtown

    Ke = (mv^2)/2
    Full Member
    Minuteman
    Jun 21, 2007
    5,327
    2,211
    CONUS
    Yup, I have gone down the lighter buffer route. As I went lighter it ejected just fine but wouldn't load and lock-up reliably.
     
    • Like
    Reactions: rpoL98

    bobke

    Sergeant
    Supporter
    Full Member
    Minuteman
    Supporter+
  • Sep 17, 2010
    1,330
    526
    Hill Country, Texas
    Another thing to check is condition of your gas rings. Have a 20” +2 gas Proof 6.5 CM with a JP BCG/HP bolt that was leaving a round in port and hindering feeding and after turning up the gas and fooling with SCS springs, turned out a fresh set of gas rings brought everything back to full function. One piece JP gas rings are small in diameter and have needed to be replaced sooner than I’ve expected across several large/small frame AR’s. Just something else to check before drilling.
     

    Aaron Crist

    Private
    Minuteman
    Aug 22, 2020
    4
    8
    Most all the 16" barrels we make with rifle gas, end up around 7/64" (.109"). When dwell is this short, you can jump more on port size and not see an over gas issue. Good time to do it is now, when its winter. These have all been with FSB installed. Ive gone as low as .098" and as high as full port .125", This is dependent on weight of reciprocating parts, surface finish, and ammo.
     
    • Like
    Reactions: S197 and Dogtown

    TonyTheTiger

    Like a Boss
    Full Member
    Minuteman
    Feb 14, 2017
    4,063
    5,586
    Started at 3.8oz and went down to 2.1oz. On the heavy end it wouldn’t eject, and on the light side it wouldn’t go into battery without slapping the forward assist. Sadly there wasn’t a middle ground where it all worked.
    That doesn't surprise me with an .093 port, I was just curious if you'd tried a gutted buffer or anything.
     
    • Like
    Reactions: rpoL98

    Constructor

    Private
    Full Member
    Minuteman
    Started at 3.8oz and went down to 2.1oz. On the heavy end it wouldn’t eject, and on the light side it wouldn’t go into battery without slapping the forward assist. Sadly there wasn’t a middle ground where it all worked.
    If using a standard buffer and carbine spring try a .096, if you want to use an H or H2 or a stronger spring like a Springco or SSS you will need to try a .098 or .0995(#39). It really depends on the pressure your ammo creates. If using M193 or 855 it should be fine, if using 223 ammo you may need to go a little larger. In the last 10-12 years I've ported over 70,000 barrels and the only barrels ever ported with anything over .0995" were 300 blackout barrels.
     

    Rocketvapor

    Major Hide Member
    Full Member
    Minuteman
  • Dec 10, 2018
    1,657
    1,054
    South East Louisiana
    "but the bolt had difficulty returning to the locked position"

    Can you drop a single round from the mag without having to use the forward assist?
     

    S197

    Gunny Sergeant
    Full Member
    Minuteman
    Nov 22, 2019
    915
    789
    Started at 3.8oz and went down to 2.1oz. On the heavy end it wouldn’t eject, and on the light side it wouldn’t go into battery without slapping the forward assist. Sadly there wasn’t a middle ground where it all worked.
    If you dropped down to 2oz, then you will need to open up the gas port.
    do you have a vise and press?