I have a BA 18" SPR barrel that seems to be under gassed for whatever reason. BA apparently drills the gas port about .01" smaller than crane spec and what every other company does. I've found that the barrel shoots full power 5.56 okay, but it doesn't care much for 223. With cheapo steel case it will short stroke nearly every round, with nicer Norma 223 I was only getting an occasional short stroke. Before anyone asks, yes my gas block is properly aligned. I dimpled the barrel using a jig on a drill press. I've also fiddled with buffers a bit and found it's more reliable on a lighter buffer, but I don't really want a gun that is that picky. I'd like to improve the reliability by opening up the port to proper spec. I have access to a drill press so it should be pretty easy.

My question is mainly about coating/treatment of the bare metal after drilling. The Barrel is nitrided and I've noticed that nitride barrels always seem to have all the machine work done before nitriding so that every surface is treated. Not sure on chrome lined barrels, is the gas port chrome lined?

I don't want to have corrosion issues or problems with accelerated gas port erosion due to the bare metal. Is this a legitimate concern or am I over thinking it? The barrel is 4150 steel FWIW. Let me know what you guys think.

I think you are over thinking the gas port corrosion issue after drilling.  Keep the bore oiled and you are not going to have any problems.

As to enlarging the gas port, I do a lot of wildcats, therefore, I always have my barrel maker start with a small gas port and I enlarge to function using the powders that I feel will work.  I haven't mucked up yet by going too large and I've done well over two dozen SS and CrMo barrels both CL and non CL, but I have not done any nitrided barrels.

 I use numbered drill bits and enlarge one or two numbers at a time depending on the function I am seeing and the decimal gap between the numbers, (sometimes there is a .001 difference between numbers, other times there is a .003 difference between numbers).  Be sure to use a drill press or stick a SOLID brass rod down the barrel past the gas port when you drill.  Under no circumstances do you want to hit the off side of the barrel with the bit.  I use to use a drill press and really fuss over it, that was pure BS, now I stick a brass rod down the barrel and use a hand drill.  I've done three times more that way than I have the drill press way and it's much better IMO.  Just be sure to use the SOLID brass rod.  When you are removing only .001-.003 inch of metal, the bit will grab through in nothing flat so that SOLID brass rod is a must.  If doing a nitrided barrel, I'd suggest using a cobalt or some other super bit.  For stainless steel or Cro-Mo barrels, even CL, standard HS bits are fine and dandy.  Do not let them get dull because that will throw up burrs on the inside.  A sharp bit will plow on through with no burrs and HS bits are so cheap to replace there is no reason to over use them.  When they get dull, I resharpen them for other uses but always use new or as new sharp bits on gas ports.

Thank you for the input, that is very helpful. I've had good success using cobalt bits with titanium nitride coating from mcmaster for dimpling nitrided barrels for gas blocks. Mcmaster has pretty much every size imaginable, my plan was to just enlarge the port to a size consistent with the port size on some of the equivalent barrels from other manufacturers that don't have this issue.

BA uses a .093 or thereabouts, whereas everyone else uses at least .099 or larger. I'd probably just target .1".

The brass rod idea is fantastic. I can probably find something on mcmaster, and I'll probably couple it with a drill stop for extra safety.

I sent three stainless barrels away to get nitrided. One of the three didn't run reliably. It was so slow that the bolt didn't have enough running start to strip the next round from the magazine 100% of the time. I removed the gas block and discovered the port was gunked up. I didn't inspect it prior to assembly which was a mistake.

I carefully ran quality drill bits with plenty of cutting oil with a hand drill until it was cleared of debris. Runs like a champ now. I suspect all you need is to clean it out. If you have to go larger go slow in small increments reassembling the rifle and testing as you go.

My barrel is mid gassed and I'm running a .078" gas port.

Is it rifle gas?  I have a BA 18in SPR barrel with rifle gas.  It does not like a rifle buffer system but will cycle anything with a regular carbine buffer system and H2 buffer.

You mentioned that your GB is aligned properly.  Have you made sure your gas tube is not slightly off center and causing the carrier to hang up?

Makes sense to me, but maybe consider using a reamer instead of a drill bit.  This is the job they’re made to do — enlarging a hole vs making a hole.  No tendency to grab.

Not meant to derail thread, but this is great information.

I’ve never had to resize a gas port, but I always wondered how you would remove the debris on the inside of the barrel AND not hit the off side of the barrel.

I always read your post on this kind of stuff.

Reamer > twist bit.


Always use a brass rod inserted into the barrel to prevent contact with bore on opposite side.

Good suggestion, I will be sure to inspect it before doing anything.

The BA rifle length 18" SPR gas port is sized at .093 which is on par with a rifle length 20" barrel. Crane spec is right around .099 I believe. The MK12 originally used a rifle stock/buffer. If it can't cycle one I'd say that's a pretty clear indication it's under gassed.

Mine runs 5.56 just fine with the carbine and H buffers I've tried. It really doesn't like weak wolf ammo, and nice quality Norma 223 seems to eject but not pick up the next round approximately once in every 10-15 shots.

I will look into purchasing a reamer rather than a drill bit.

This is a good suggestion as well. I just removed the bolt from my carrier and slid it in and out of the upper to verify that the gas key isn't snagging. It appears to be perfect.

Does yours cycle weak 223 ammo with the H2?

Here may or may not be a problem using a reamer.

 In order for a reamer to enlarge a hole, the reamer must first enter the hole.  How do you do that without a tip cutting bit?  By having a taper on the end of the reamer of course.  And that taper MIGHT, and I say MIGHT because I have no idea what a reamer that small looks like, hit the off side of the barrel by the time you ream out the entire gas port to full size.  The reamers I have used all have a tapered end that are anywhere from 1/4 inch to 3/8 inch long.  I've used them for reaming holes that are not depth critical, but if the gas port sized reamer have a long taper, they might ream the top of the hole to full size but the bottom of the hole MIGHT be in the taper of the reamer, not cutting it to full size, or by the time you do cut it to full size, you might be into the off side of the barrel.  ON a 5.56 barrel the bore is less than a quarter of an inch.  That's not going to allow for much taper on the reamer.  And if you have a brass rod in the barrel you darned sure won't be reaming the bottom of the hole to full size if there is a taper on the reamer's end.

I fully admit I might be way off base here and maybe a reamer with no taper on the end can be used to enlarge a hole by 2 or 3 thousands, but IMO I see the potential for more problems caused by a reamer in the gas port than it solves if the end is tapered.  And if it is not tapered, how to you get a reamer of X size into a hole that is X-.002 or .003 smaller?  Or do those small reamer have a cutting end to them?  My larger 1/4 inch reamers do not have a cutting end so the reamer is tapered.
 
Also, gas port size is not rocket surgery.  It is not the type of precision work that I think some imagine it is.  The gas port is going to enlarge anyway by the time you shoot a few thousand rounds through it.  I know you've seen the sectioned barrels showing gas port gas cutting so what good did that super fine (and much more expensive) reamer do?  I never heard of any barrel maker who drills the gas port then reams it to size. That's exactly why you see here all the time, on $89 barrels or $400 barrels, people comment that when they receive their new barrel there is a slight burr on the inside, where the gas port is located. And we always tell them it's normal, shoot it out and it will be gone in two or three shots.  

Ream it if you want, it won't hurt anything, unless of course it is tapered and you get into the off side of the barrel or find that after reaming there is no change in function since the bottom of the hole is as small as it was when you started.  Then tell us how much you love that reamer.

Reamers do cut material, that's how they work. You wouldn't drill a .125 hole and then ream it with a .125 reamer.  You generally want to drill .005-.010" under, then ream to size.

https://www.mscdirect.com/product/details/72010002

Smaller sized reamers usually have a pointed tip, but you can grind that off (not completely though) if you need clearance.

^^^^^
Drill slightly undersized/ream to dimension.

Twist bits do not produce round holes by nature and tend to deliver oversized holes.


I wouldn't want the GP diameter any larger than needed for proper function personally.

The type of reamer that I meant is called a “chucking” reamer — perhaps I should have been more specific. They’re not tapered, and they do cut at the front end, but only near the diameter.  You’d hardly notice the cutting edge;  it just looks like a 45-degree chamfer.  

I suppose the most common use for one is when you want a precise hole (correct diameter, round, parallel).  That may be overkill for a gas port, but I like them just because they cut so nicely when enlarging an existing hole. (Twist drills... not so much.)

I’ve made wildcats myself and have done the same thing you describe:  start with an undersized port and open it up one small step at a time until the gas system works.  More than once I’ve taken some reamers and a little egg-beater drill to the range, and it worked great.

One caveat:  I’ve never tried opening up the port on a nitrided barrel.  Not sure what difference that might make.

Good discussion here on drill vs reamer. I see merit to both arguments. Here's my take, someone correct me if I'm wrong.

Upside to a reamer is that it produces a proper round hole. This provides a more professional finished look, and a true machinist would certainly prefer this option. Downside to a reamer is it's more expensive.

Upside to a drill is that it is cheaper, about 1/5 the cost of an equivalent reamer. Downside is that the hole isn't perfectly round. To play devil's advocate here, I'd argue that we don't actually care if the hole is perfectly round. If the concern is that a drill produces a larger hole than intended, then the bit can just be undersized.

I understand that the reamer is truly the 'correct' tool for the job and a competent machinist would always use one. The question I have here is whether it matters in a practical sense in terms of the roughness of the hole inside the bore. Does a reamer actually produce cleaner results? I.e. is this just a matter of principle for machinists to produce results that look satisfactory, or will the reamer actually lead to better accuracy?

Fair enough about the price difference.  And I have little doubt that you can get the job done with a twist drill.

To be clear though, my preference for reamers (in this context) isn’t so much about the precision of the hole, but rather how smoothly they cut — they’re not prone to grabbing in the existing hole and potentially causing something bad to happen.

Another reason I’ve used reamers is that I suspect they'd tend to leave a smaller burr on the back side of the hole than would a twist drill.

ETA: BTW, you can save a few bucks if instead of looking for a drill or reamer that’s exactly 0.100” you go with some standardized size. A number 39 is equivalent to 0.0995 — close enough.

There is no reason to buy or use a $18.00 reamer when you can accomplish the same goal with a drill bit. We aren't trying to mate moving parts or create a perfect diameter cylinder, all we need is a hole for gas to escape through. Gas erosion is going to ruin your perfect hole in no time anyways.

Buy a quality drill bit and use plenty of cutting oil. Go slow and straight so you don't break the bit or damage the opposing side of the bore.

Stick a cleaning rod down the bore to prevent drilling into the opposite side of the bore

PSA put out a bad batch of 20" nitride chf barrels that were drilled with a midlengtth spec gas port. I opened it up with a hand drill. Enlarging an existing hole is much easier than drilling a new hole.

Also, most barrel makers drill the port after nitriding is done, anyway. If you drilled it before, the nitriding can cause the extension to come loose. When you re-torqued it, the port would likely not be aligned with the index pin.

I have a similar barrel, the 18" rifle length gas from AIM surplus.  In all likelihood, it's a Ballistic Advantage probably near identical to the one you have.  I am using a bootleg adjustable carrier and had it running as much gas as possible, but I suppose that could be different than a normal milspec carrier.  Buffer tube setup is a standard carbine buffer and buffer tube.

I was getting some FTF issues and it felt under gassed, which I was not surprised to see out of an 18" rifle gas barrel.

Rather than mess with the gas port, I picked up a reduced power buffer spring and it cleared up all the issues immediately.  I'd suggest checking that out before you spend time, effort, money, and headache.

https://www.primaryarms.com/sprinco-m4-ar15-yellow-reduced-power-carbine-buffer-spring

Gents, I figured I'd report back here with my results. I Enlarged the gas port using a .0995" drill bit on a press. I used a 3/16" 303 stainless rod in the barrel as a backstop. Brass would probably be ideal for this application but it's a lot more expensive and the barrel is nitride so the hardness of SS is really not an issue here. For an SS barrel I'd probably use a softer material. I also used a drill bit stop on the drill. Prior to drilling I confirmed that the original port on the BA barrel was .093" which is the standard size for a 20", but is significantly undersized compared to nearly every other 18" rifle length barrel on the market, including the crane spec.

Barrel was clamped using some triangular blocks to keep it horizontal. I used a smaller drill bit first to ensure the gas port was vertical and aligned with the drill press chuck. The bit I used was a cobalt bit with titanium nitride coating from mcmaster. Cost less than $5 and went through like butter. I used an air compressor to blow any chips out of the barrel and re assembled the upper.

It was a nice cold day around 10deg F, and with the enlarged gas port it now cycled weak 223 with no issues.

Accuracy was my main concern after doing this, and I finally had a chance to test it yesterday with excellent results. This was a 6 shot group with pine valley munitions 77gr SMK at 100yd. This was shot from a bipod with a rear bag while laying in about a foot of snow. Rifle has a Larue MBT 2 stage, and was set up with a 9x magnification scope. I'm not the best shooter and there was one shot that I felt like I totally pulled but I don't really know which it was. Daylight was fading and it was cold so I called it quits. All things considered I'm pretty satisfied with this one, extreme spread measures just under 1.25".



Also note, the rifle was not zeroed with the 77gr at 100yd. I fired a quick group at 50yd with 55gr xm193 to get a rough zero and the backed up to 100 to see how the Sierra grouped. The dot above and to the right of my group was my point of aim.

A reamer still leaves a bur on the inside of the barrel.

You should finish the hole with a de-bur tool like this.  You may need a couple of different sizes depending on how large your gas port hole is.  They run $50ea but are good for a hundred barrels or so.

Tony

I had a 17” .093, and it didn’t like Wolf. Ran great on 5.56 and with a can it felt like a 20” in recoil impulse. It was perfect. I get why the OP wanted to open his up though. Not everyone has a silencer with two baffles or wants to.

Wish I had a 20” barrel with a .078 gas port, it would be perfect to cut to 16.5 and out my other AE can on.

I refuse to own a rifle that is picky about ammo, buffers, springs etc. That doesn't mean I don't try to optimize the system within those constraints, but not being able to shoot some of the most commonly available ammo is a total show stopper for me.