Possible Undersized Gas Port in Criterion Barrel

diggler1833

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Looking for input from people with more knowledge than mine.

I thought an 18" rifle gas barrel in .308 should have somewhere in the neighborhood of a .096" gas port? I have a short stroking issue and while I lack the proper gauges, I can't get a piece of .078" hot wire through the gas port on the barrel. It will let a 1/16" allen key in with some room to spare (.065" max width).

If it's off as I believe that means I got two defective parts in one order...
 

Burning sensation

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Great. Now I feel like an ass.
My issues with my 2 AR10 builds and every AR15 build have been the opposite. Over gassed, then way over gassed once I add a suppressor.
My initial thought is that a suppressor would help if it is actually a gas problem. Expensive solution, I know. I’ve been spoiled hunting suppressed enough that all my hunting rigs have them, or they don’t get used to hunt, but I understand that’s not for everybody(although it should be).
The gas port on my 20” WC measures about 0.08” using the internal jaws on my dial caliper. I know that’s not the most precise method for measuring ID. The attached picture shows it is smaller than 0.096”. My calipers are set to 0.095” because they run about 0.001” small.
I’ve had at least one problem with my each of my AR10 builds. 2 holes for hand guard were not threaded. My mag would drop after every shot (any mag). The last was the bolt would lock open after every 4th or 5th shot.

4CD62E75-2D30-4C21-A010-26B6D9FC68BF.jpeg
 

Ravenworks

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Aren't their barrels drilled @ 750 ?
I know mine are right at 750 and I've never had an issue.

I would call Josh 1st thing tomorrow morning before everyone on the internet confuses you.
What spring combo?
With a 308 AR there is a LOT OF STUFF that has to work.

Using a dial caliper to measure a gas port is NOT the proper way to do it.
 

diggler1833

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Rifle manually cycles smooth as anything I've got, and locks back on both types of empty magazines that I have (manually).

It will cycle heavy loads like Federal 180gr soft point and Federal 175gr GMM, but will not cycle any kind of military ball. It will not lock back on an empty mag on anything.

No leaks from carrier key as I've shot that with a compressor after liberally oiling. Bolt will rotate in BCG if it is placed on end...so good seal there.

No fitment issue with gas tube, and is appropriate length. Gas tube is not upside down (how is that possible anyway?). BCG slides freely over tube.

Gun was run wetter than all get out.

No binding of buffer or spring, no buffer tube strikes. I even cut four coils off of the spring as I have another...no difference in ejection. Spring was part of a entire kit. Not running a heavy buffer or extra power spring.

Non-adjustale gas block...burn ring over barrel gas port is perfectly centered.
 

loveha

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Carbine or Rifle Length buffer tube?
If it is a carbine length buffer, is it a Carbine (3 oz) buffer, or an H (3.8 oz) buffer?
If it is rifle length could be that the 5 oz is to much.
 
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Rifle manually cycles smooth as anything I've got, and locks back on both types of empty magazines that I have (manually).

It will cycle heavy loads like Federal 180gr soft point and Federal 175gr GMM, but will not cycle any kind of military ball. It will not lock back on an empty mag on anything.

No leaks from carrier key as I've shot that with a compressor after liberally oiling. Bolt will rotate in BCG if it is placed on end...so good seal there.

No fitment issue with gas tube, and is appropriate length. Gas tube is not upside down (how is that possible anyway?). BCG slides freely over tube.

Gun was run wetter than all get out.

No binding of buffer or spring, no buffer tube strikes. I even cut four coils off of the spring as I have another...no difference in ejection. Spring was part of a entire kit. Not running a heavy buffer or extra power spring.

Non-adjustale gas block...burn ring over barrel gas port is perfectly centered.
Ported correctly for heavies, little small for M80 / 147-150 FMJ.
I wouldn’t say “wrong” as personally I’d prefer to run heavier bullet.
Burn rate of powder in the 147 - 150 FMJ ammo is giving you wrong time / pressure curve, too little gas volume / gas pressure at port.
If you want to run mil ball type ammo, open up port some. Open port for 147-150 mil type ammo, probably will be on over gassed side for 175-180 once open port though.
 
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bfoosh006

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Find someone with a proper pin gauge set... Gunsmith, or someone.

Knowing what your gas port size truly is important in this case for sure.

How many rounds through the firearm ?

Will the rifle lock back on a single mag fed round ? Consistently ?... or not at all ?
 
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Sieg

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Use a drill bit to check the port hole?

How far does the bolt travel past the bolt lock when pulled back to it's farthest position? My understanding is it should be an 3/32 to 1/8". I used washers in the bottom of the tube to set the travel length.
 

bfoosh006

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FWIW, I just checked my Criterion rifle length gassed, 18" 6.5 CM gas port... .080" ( uninstalled )

I realize not truly helpful in context to your 308 RLGS gas port size.. but
 
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rpoL98

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use drill bits, the shank end. fractional drill bits, wire gauge drill bits, and alphabet size drill bits. it's the next-best thing, short of acquiring a pin gauge set. Pin gauge set actually pretty reasonable on Amazon, chinese I'm sure, though.
 

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It will cycle heavy loads like Federal 180gr soft point and Federal 175gr GMM, but will not cycle any kind of military ball. It will not lock back on an empty mag on anything.
So, it's possible for an AR to be so badly overgassed that the malfunctions seem like short stroking if you don't know what you're looking for. However, the sentence above indicates that's unlikely to be the problem, so it sounds like it's undergassed as you suspect.

That's unusual, most 308 ARs are pretty overgassed because people expect them to run military ball which is significantly underpowered compared to good commercial 308.

The solution is easy for a guy like me, just indicate the barrel in the mill and drill the port larger. If you don't have the equipment to do that though, be really careful, or find someone who does. I wouldn't step up more than .005" on the port size without re-testing, proceed slowly.

Up to you if you want it to run with military ball. You could port it for that, and use an adjustable gas block for full power 308 or suppressed loads. Or just port it for FGMM and similar ammo and only shoot that.
 
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diggler1833

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Carbine or Rifle Length buffer tube?
If it is a carbine length buffer, is it a Carbine (3 oz) buffer, or an H (3.8 oz) buffer?
If it is rifle length could be that the 5 oz is to much.
Carbine length buffer tube with short, standard carbine buffer. No H buffer.
 

diggler1833

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Ported correctly for heavies, little small for M80 / 147-150 FMJ.
I wouldn’t say “wrong” as personally I’d prefer to run heavier bullet.
Burn rate of powder in the 147 - 150 FMJ ammo is giving you wrong time / pressure curve, too little gas volume / gas pressure at port.
If you want to run mil ball type ammo, open up port some. Open port for 147-150 mil type ammo, probably will be on over gassed side for 175-180 once open port though.
Definitely. It has to run ball ammo. I'm giving it to my dad to hold onto for a few years until he realizes that he's too old to have a defensive .308 :D. He stocked up on ball ammo about a year ago anyway.

Definitely agree that if it runs 147-150gr boolits well, it is going to be over gassed for the 175gr +. However, once I finally get this thing running, it will only see ball and 165gr handloads for hog hunting in a few years when I get it back. I fear handing my dad something with an adjustable gas block that is only going to cause him problems.

I don't think it's ported correctly for heavies. Best I can tell mashing the end of a q-tip with the swab cut off into the port and measuring that is that it is .073" +/- a couple thousandths for user error with a janky setup. Even with the heaviest, slowest round that I can find I can't get the bolt to lock back on an empty mag until I do it manually.
 

diggler1833

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It should still hold the bolt open on FGMM 175. That’s why I think he should be able to get it done with a tuned buffer/spring or a JP SCS.
Agree, but you can't find a setup lighter than mine that I know of...and I cut four coils off of the buffer spring as an experiment with no benefit. Maybe I'm way off, but after trouble shooting everything I can find it looks like a gas issue. I definitely appreciate the input though.
 

diggler1833

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Find someone with a proper pin gauge set... Gunsmith, or someone.

Knowing what your gas port size truly is important in this case for sure.

How many rounds through the firearm ?

Will the rifle lock back on a single mag fed round ? Consistently ?... or not at all ?

I did cut the end of a q-tip off and jam the stick into the port, and then measured the stick with my calipers. It is going to be the fastest thing that I have available without spending additional money. It came to .073", again +/- a few thousandths because that is a less than ideal way to measure.

I still am only finding info out there that says an 18" rifle length on a .308 should be in the .096 range. However I am not the authority on what it should measure, so I'll be hitting Criterion up in a few hours.
 

diggler1833

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Use a drill bit to check the port hole?

How far does the bolt travel past the bolt lock when pulled back to it's farthest position? My understanding is it should be an 3/32 to 1/8". I used washers in the bottom of the tube to set the travel length.
I like that idea, but the bolt won't even come back far enough to pick up another round of ball ammo.
 

diggler1833

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I realize that the .308 is significantly more finicky than the small frame builds, of which I have done three and they all run flawlessly.

I literally did 12+ hours of Google Fu on this subject before I posted here. I don't really like posting a question unless I've exhausted all means. I feel like I'm taking away from people's days when I start asking questions...so my shit had better be together before I waste the time of others.

Gas block, gas tube, gas key, are all free of debris or at least they all allow for airflow without any discernible leaks...I literally got an obstructed gas tube a few weeks ago, which I posted about here. Everything has been checked twice now.

Alignment of all parts is good to go. Carbon deposits are all centered over holes, carrier moves freely over gas tube, buffer and spring move freely with no strikes. With the amount of lube I added it runs like butter when cycled manually.

There is no unusual wear pattern on the trigger or bottom of the BCG, suggesting that there is excessive friction on the BCG during cycling. I've found two instances of that on the interwebs in my search.

The rifle has about 60 rounds on it.
- 10 Rounds, 180gr Federal Power Shok...Cycles but will not lock back on and empty mag.
-6 Rounds, 175gr 7.62x51 FGMM...Cycles but will not lock back on an empty mag.
-6 Rounds, 150gr Federal American Eagle ball...will not cycle at all. Will at least reset the trigger though.
-40 Rounds, 147gr ZQ1 ball...will not cycle at all. Occasionally resets trigger.

Everything that I've read, points to an undersized gas port. I can open it up if I dare. I do have at least a 3/32 bit and a 2.5mm bit... but that kind of brackets me around the .096" that I'm reading about and neither of them are carbide. I also have a drill press, but no way to truly indicate on it either so my likelihood of breaking a bit off goes up a little. I work on ranch equipment all the time, but I'm definitely not a professional gunsmith. I've seen guys do it with a hand drill on YouTube.. but looks half-assed at best, and I'd like to keep the accuracy of the barrel...If I wanted a barrel with a bunch of burs and a solid 2 MOA guarantee, I would have saved a few hundred bucks on this project. This will eventually be a hunting rifle, not a blaster.

I will probably add an adjustable gas block later, but not now as I'll be handing this over to my dad for a few years. I do not want to hand him something that he has to dick with...so slightly over gassed is better than the alternative. When I get it back, I've got 1,500 (165gr) soft points of various makes that I intend on using for hogs, and I'll probably add the adjustable gas block then. So while I'm using heavy ammo now, it is just for function testing if that makes sense.

I expect that if the port is undersized that Criterion or Primary Arms will make it right. My biggest question of all was that I was correct in assuming that the gas port should be about .096" in diameter?

To all who have provided input so far: Thank you.
 

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I like that idea, but the bolt won't even come back far enough to pick up another round of ball ammo.
If the bolt can't travel back past the bolt lock it won't lock. That indicates a lack of bolt travel due to buffer tube length, buffer length, spring length or?

My Ruger SR-762 Carbine came with a 7.75" deep buffer tube, 3.25" long 3oz buffer, 13.75" spring w/ 37 coils of .071 diameter wire.
FWIW - The SR-762 uses a Gas Piston vs Direct Impingement.
 

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I realize that the .308 is significantly more finicky than the small frame builds, of which I have done three and they all run flawlessly.

I literally did 12+ hours of Google Fu on this subject before I posted here. I don't really like posting a question unless I've exhausted all means. I feel like I'm taking away from people's days when I start asking questions...so my shit had better be together before I waste the time of others.

Gas block, gas tube, gas key, are all free of debris or at least they all allow for airflow without any discernible leaks...I literally got an obstructed gas tube a few weeks ago, which I posted about here. Everything has been checked twice now.

Alignment of all parts is good to go. Carbon deposits are all centered over holes, carrier moves freely over gas tube, buffer and spring move freely with no strikes. With the amount of lube I added it runs like butter when cycled manually.

There is no unusual wear pattern on the trigger or bottom of the BCG, suggesting that there is excessive friction on the BCG during cycling. I've found two instances of that on the interwebs in my search.

The rifle has about 60 rounds on it.
- 10 Rounds, 180gr Federal Power Shok...Cycles but will not lock back on and empty mag.
-6 Rounds, 175gr 7.62x51 FGMM...Cycles but will not lock back on an empty mag.
-6 Rounds, 150gr Federal American Eagle ball...will not cycle at all. Will at least reset the trigger though.
-40 Rounds, 147gr ZQ1 ball...will not cycle at all. Occasionally resets trigger.

Everything that I've read, points to an undersized gas port. I can open it up if I dare. I do have at least a 3/32 bit and a 2.5mm bit... but that kind of brackets me around the .096" that I'm reading about and neither of them are carbide. I also have a drill press, but no way to truly indicate on it either so my likelihood of breaking a bit off goes up a little. I work on ranch equipment all the time, but I'm definitely not a professional gunsmith. I've seen guys do it with a hand drill on YouTube.. but looks half-assed at best, and I'd like to keep the accuracy of the barrel...If I wanted a barrel with a bunch of burs and a solid 2 MOA guarantee, I would have saved a few hundred bucks on this project. This will eventually be a hunting rifle, not a blaster.

I will probably add an adjustable gas block later, but not now as I'll be handing this over to my dad for a few years. I do not want to hand him something that he has to dick with...so slightly over gassed is better than the alternative. When I get it back, I've got 1,500 (165gr) soft points of various makes that I intend on using for hogs, and I'll probably add the adjustable gas block then. So while I'm using heavy ammo now, it is just for function testing if that makes sense.

I expect that if the port is undersized that Criterion or Primary Arms will make it right. My biggest question of all was that I was correct in assuming that the gas port should be about .096" in diameter?

To all who have provided input so far: Thank you.
Just because other manufacturers have larger gas ports in their barrels doesn't mean that you should.

Did you call Criterion up?
The three rifles that I have will eat any ammo you throw at it.

The bolt not locking back needs to be looked at.
Pull the charging handle back and measure the free space between the bolt face and the stop.

I'm willing to bet that it's the buffer and spring.
 

diggler1833

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Criterion said that the smaller port is normal for them, and my measurement was in spec.

Suggested that I need at least a 5.4oz buffer, so I'll try that. Said that the rifle is essentially coming out of battery while the case is expanding and not utilizing all of the gas.

I will happily report if I'm a moron if that fixes it.

If not...I guess it's drill time.

I hope they're right.
 

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Criterion said that the smaller port is normal for them, and my measurement was in spec.

Suggested that I need at least a 5.4oz buffer, so I'll try that. Said that the rifle is essentially coming out of battery while the case is expanding and not utilizing all of the gas.

I will happily report if I'm a moron if that fixes it.

If not...I guess it's drill time.

I hope they're right.
This is the key right here--" It will cycle heavy loads like Federal 180gr soft point and Federal 175gr GMM, but will not cycle any kind of military ball. It will not lock back on an empty mag on anything."

I've ported a few thousand 18" 308 barrels(.093) over the last 14 years and no one ever calls back to say the rifle doesn't cycle.
 

diggler1833

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This is the key right here--" It will cycle heavy loads like Federal 180gr soft point and Federal 175gr GMM, but will not cycle any kind of military ball. It will not lock back on an empty mag on anything."

I've ported a few thousand 18" 308 barrels(.093) over the last 14 years and no one ever calls back to say the rifle doesn't cycle.
Thank you Sir. Criterion told me that their port size was acceptable when I called them this morning. Maybe I confused the guy?

I just swapped uppers onto a proven lower (rifle length) that I sold my buddy...got even less ejection with 180gr Federal 10 minutes ago. The cases dribbled out like the ball ammo I had been using.

Don't know why I didn't try that earlier.

Going the drill route.
 

diggler1833

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If your certain your gas block is centered.

You can also cut some links out of the buffer spring.

I'd just widen the gas port... Don't go to big.
I have a second spring, so I did cut four coils out of one for shits and grins. I didn't find that it really made a difference.

The carbon deposit from the gas block on the barrel around the gas port is beautifully centered. The gas block sits a few thousandths off the shoulder, and it lines up perfectly with the dimple. I've had this gas block off five times now as of today.

I inspected my gas block when my second gas tube came in, and there were no obstructions leading to the port in the gas tube.

After ~20 rounds the bolt is dirty, but not nearly as dirty as my 6.8, 6.5G, or my .223s get after 20 rounds.

.078 and .093 carbide bits were ordered and I received an email that they have already shipped.
 

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Make sure you tape the bit so you don't blow through and hit the rifling on the opposite side. Also, not sure, but I think you want to go slow to prevent burring as the bit goes through. Cutting oil wouldn't hurt either. Would help cut down on the heat either way doing both going slow with cutting oil.
 
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diggler1833

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Definitely putting a cleaning rod in the barrel to prevent strikes on the opposite side.

I recently may or may not have received 55 gallons of environmentally friendly gear oil off of a wind turbine jobsite. That stuff may or may not be some of the most awesome stuff I have or haven't used. LOL. Seriously, they let a buddy who worked on site have (4) 55 gallon drums of the stuff when they finished the project. It's like $3,750 a drum.
 

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Make sure you tape the bit so you don't blow through and hit the rifling on the opposite side. Also, not sure, but I think you want to go slow to prevent burring as the bit goes through. Cutting oil wouldn't hurt either. Would help cut down on the heat either way doing both going slow with cutting oil.
Wrong thing to do as far as slow, going slow is more likely to bend metal into the bore. When drilling gas ports we aren't concerned with saving a $4 bit. Spin it fast (cordless drills aren't that fast anyway)and apply very little to no pressure. There is already a hole there, it isn't like we are trying to drill a hole through a piece of metal plate, the barrel will cut like butter. If the hole is already .80 he is only cutting .0065" on each side.
ETA -stainless barrels are more likely to develop a burr since the material is so soft, it just bends out of the way of the bit. Criterion uses 410 ss so that will be better than 416.
 
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Yondering

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Everything that I've read, points to an undersized gas port. I can open it up if I dare. I do have at least a 3/32 bit and a 2.5mm bit... but that kind of brackets me around the .096" that I'm reading about and neither of them are carbide. I also have a drill press, but no way to truly indicate on it either so my likelihood of breaking a bit off goes up a little. I work on ranch equipment all the time, but I'm definitely not a professional gunsmith. I've seen guys do it with a hand drill on YouTube.. but looks half-assed at best, and I'd like to keep the accuracy of the barrel...If I wanted a barrel with a bunch of burs and a solid 2 MOA guarantee, I would have saved a few hundred bucks on this project. This will eventually be a hunting rifle, not a blaster.
If you care about the rifle at all, do NOT just use whatever drill bits you already have that are close. You don't need carbide if the barrel isn't nitrided, but you do need sharp drill bits to avoid leaving a burr. This isn't ranch equipment, and requires a much more delicate touch and attention to detail if you're going to avoid butchering it.
Also, you need to step up in sizes to get what you want, the difference between .080" and .096" is huge, even though it seems like small numbers. If you're right on the edge of function with ~.075" right now, just going up to ~.080" may be enough to make it cycle military ball ammo.

Go to a local machine supply store like Fastenal, Tacoma Screw, etc, (not Ace hardware or Home Depot) and buy some good drill bits intended for steel. You want a 135° split point in a good grade of HSS like M42 or maybe cobalt. Not carbide, it's not sharper and you'll probably break it in the hole without using a mill. Jobber length drills are fine and the most common. I'd buy a few sizes, probably #47, 45, 43, and 41. Those will give you reasonable size steps between the .073-.075" where you are now and the .096" number you found (which is a #41 and probably way overgassed IMO). Drill bits like these are only a few bucks each, so we're not talking about big money here. Also, stepping up in small sizes and using a sharp new drill bit each time will keep the burr down to little or nothing if you do a good job. You do NOT want to power through the hole, let the drill bit cut but on the slow side for feed rate is better. I'm speaking as a machinist here.

Personally I wouldn't do this job without at least a solid vise in the drill press to hold the barrel. You can chuck a #48 or 49 drill bit (whatever fits your gas port) backwards in the drill so just a bit of the shank is poking out, and use that to locate the barrel so the hole is aligned under the spindle. Then insert the correct drill in the chuck without moving the setup.

Also - put a cleaning rod or 1/4" steel dowel down the barrel so the drill bit hits that and not the other side of the bore.
 

rpoL98

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with the drill bit diameter, use your high-school math, the pi-R-squared thing, to calculate the cross-sectional area of the gas port hole. I usually go up in increments of 10% to 20% in CSA, depending on how-badly-undergassed symptoms. For instance, for a current gas port dia of 0.075, a new diameter of .0804 is 15% more gas.

agree with everything Yondering said. very slow feed rate to keep the cut very clean.
 

Yondering

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with the drill bit diameter, use your high-school math, the pi-R-squared thing, to calculate the cross-sectional area of the gas port hole. I usually go up in increments of 10% to 20% in CSA, depending on how-badly-undergassed symptoms. For instance, for a current gas port dia of 0.075, a new diameter of .0804 is 15% more gas.

agree with everything Yondering said. very slow feed rate to keep the cut very clean.
Yup. You can do the math, but the easy way is to just go up one or two number sizes on the drill bit at a time (the number sizes are not a consistent change from one to the next, but I like to jump .003-.005" at a time for tuning a gas system once I see some sort of function). I avoid jumping more than that if the rifle functions at all; it's easy to go too far then you've got an overgassed rifle just like every other 308 out there, instead of a precisely sized custom port that matches your rifle.

I actually made that mistake on my last barrel just a few days ago; a #51 (.067") was too small and only cycling the heaviest ammo with no lock back, much like the OP's rifle, so I stepped up to a #49 (.073") and it ended up being too much; it runs well but slightly overgassed, with ejection at about 2:00. (This is on a 12.5" 243 LBC, different cartridge, barrel length, and gas system, so the numbers don't apply to the OP but the principle is the same.) I'll end up using an adjustable gas block on that one, and feel kind of bad about missing the goal there. Fortunately it's just my own barrel and not a job for somebody else.

OP - as you can see, drill charts are a necessity for this stuff, but you can download them free online, just google search for "number drill chart". I like the Starrett chart: http://img-cache.oppcdn.com/img/v1....782x1220/6135660bf0eabdfa5a121ba102305911.jpg

 
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diggler1833

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If you care about the rifle at all, do NOT just use whatever drill bits you already have that are close. You don't need carbide if the barrel isn't nitrided, but you do need sharp drill bits to avoid leaving a burr. This isn't ranch equipment, and requires a much more delicate touch and attention to detail if you're going to avoid butchering it.
Also, you need to step up in sizes to get what you want, the difference between .080" and .096" is huge, even though it seems like small numbers. If you're right on the edge of function with ~.075" right now, just going up to ~.080" may be enough to make it cycle military ball ammo.

Go to a local machine supply store like Fastenal, Tacoma Screw, etc, (not Ace hardware or Home Depot) and buy some good drill bits intended for steel. You want a 135° split point in a good grade of HSS like M42 or maybe cobalt. Not carbide, it's not sharper and you'll probably break it in the hole without using a mill. Jobber length drills are fine and the most common. I'd buy a few sizes, probably #47, 45, 43, and 41. Those will give you reasonable size steps between the .073-.075" where you are now and the .096" number you found (which is a #41 and probably way overgassed IMO). Drill bits like these are only a few bucks each, so we're not talking about big money here. Also, stepping up in small sizes and using a sharp new drill bit each time will keep the burr down to little or nothing if you do a good job. You do NOT want to power through the hole, let the drill bit cut but on the slow side for feed rate is better. I'm speaking as a machinist here.

Personally I wouldn't do this job without at least a solid vise in the drill press to hold the barrel. You can chuck a #48 or 49 drill bit (whatever fits your gas port) backwards in the drill so just a bit of the shank is poking out, and use that to locate the barrel so the hole is aligned under the spindle. Then insert the correct drill in the chuck without moving the setup.

Also - put a cleaning rod or 1/4" steel dowel down the barrel so the drill bit hits that and not the other side of the bore.
I appreciate the input. Yeah, I went ahead and ordered new bits for the job already...hope that they're here within the next day or two.

Definitely using a cleaning rod down the bore. I've got an excessive amount of old aluminum shotgun rods that aren't doing anything but collecting dust.

My neighbor across the highway has a vice for his drill press, so I'll be doing it at his shop.
 

JS8588

Ballistic Hipster
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Apr 7, 2020
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This is why I always buy generously ported barrels over uh...those gassed conservatively. I know the preference on forums is for soft shooting, but I'd rather mess with buffer weights & springs than have to re-drill a gas port. YMMV
 
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