View Full Version : Gas port sizing
I am having a barrel cut shorter and will need to open up the gas port i assume. So whats the best procedure to follow to get this set right? I want to get it setup so I can shoot it suppressed and unsuppressed with as little excessive gas as possible. With that said while im opening it up id like to also take reliability into account.
So how do those of you go through the process of getting the correct gas port size?
I am having a barrel cut shorter and will need to open up the gas port i assume. So whats the best procedure to follow to get this set right? I want to get it setup so I can shoot it suppressed and unsuppressed with as little excessive gas as possible. With that said while im opening it up id like to also take reliability into account.
So how do those of you go through the process of getting the correct gas port size?
Send it to a machine shop that knows what they are doing...
A discussion here with a range of numbers posted (http://m4carbine.net/showthread.php?t=1481)...use google and search this site...there are many threads debating different issues that affect port sizing...
I read through that link among others and have a gas port range but also know each rifle is different. I don't like sending out work when its something i can probably do so thats why i want to learn the method. For all i know they would just drill the largest gas port they know works and thats not what im after. So I was trying to figure out if there was a certain procedure people used to determine they have the correct size to be reliable yet not overgassed.
I havent found a thread on actually drilling the holes per a certain rifles performance. Everything has just said this rifle used this port size and so on. Within that the same "type" barrel could have a range of port sizes. Thats why i wanted to drill my own so it would have the best size for my setup.
So I was trying to figure out if there was a certain procedure people used to determine they have the correct size to be reliable yet not overgassed.
I test cyclic rates.
If you're doing this yourself, and you're set on some certain goal, drill the smallest you think will work and go up from there to get function the way you want it.
I read through that link among others and have a gas port range but also know each rifle is different. I don't like sending out work when its something i can probably do so thats why i want to learn the method. For all i know they would just drill the largest gas port they know works and thats not what im after. So I was trying to figure out if there was a certain procedure people used to determine they have the correct size to be reliable yet not overgassed.
I havent found a thread on actually drilling the holes per a certain rifles performance. Everything has just said this rifle used this port size and so on. Within that the same "type" barrel could have a range of port sizes. Thats why i wanted to drill my own so it would have the best size for my setup.
That may be due to the fact you shouldn't use a drill to modify your gas port. Drills don't make round or clean holes.
MK18Pilot: What other options are you suggesting? Something like an EDM or drilled and then reamed? I do have some machine tool experience by the way so I'm not going to just jump in with a dremel. If you have a good method I'm all ears. I'm just trying to learn what ways people who have done it before have found to work well.
Rob_s: That was the method I had in mind. In your opinion do you think it would be best to get it where it functioned fine and then go a couple thousandths more for reliability?
Todd.K: I didn't consider cyclic rates. In my case I cant use that method but that is interesting. I assume you have different rates in mind for different type builds.
EzGoingKev
03-06-12, 03:35
I test cyclic rates.
This would be with a full auto lower, correct?
That may be due to the fact you shouldn't use a drill to modify your gas port. Drills don't make round or clean holes.
Just how clean and round does a gas port need to be?
Just how clean and round does a gas port need to be?
How long do you want your port to last?
A reamer is the proper tool to finish the port. That being said, what size hole does a 1/16 drill make when its chucked up in Black & Decker? Not 0.0625. Its disingenuous to foment the idea that you can properly modify a gas port in your garage with a drill motor or drill press and drill. If you have access to a turret mill and the proper cutting tools, cut away.
This would be with a full auto lower, correct?
Yes, or high speed camera. It's not the best DIY project unless it's just a plinking or game gun.
I can make a gun that will run when clean, and choke when it's cold and dirty. Getting to just functioning may not be enough for serious use reliability and most people don't have the tools to know for sure.
Let's take this further...
I have tried once to do this. I over gassed the hell out of a 16in barrel. Afterward, while troubleshooting what I thought was a a lost cause, I found two things out:
1. Mark18 pilot, you're right about drilling. It just doesn't keep a proper, even concentric hole for the port. The implications of this are a multitude of **** all.
2. The bitch of it is, I still haven't had it fully explained to me how the port must change diameter for barrel length. I understand somewhat the workings of the ar so the gassing I understand, it's the port size that's a bit confusing. Add a suppresor in the mix and let the cursing begin.
Now, this was years ago, long before i found this site.I don't have the time I used to. But after I get through typing this, I'm using the orange search button.
I guess my main question is where to find a reamer that small, and where to find the data on gas ports and their relation to barrel length and suppressor use. I don't have a lot of the toys needed to play this game.
Companies like MSC and maybe Travers have reamer this small. I've never used one that small but they are available. When i was looking at MSC they were about $25 for the cheaper ones.
I was going to drill the barrel and using one of my pin gages to select the correct reamer to finish it. I defiantly agree that this isn't hand drill territory. If it were a barrel with no port I wouldn't bother but since it is drilled almost to the correct size that simplifies the process.
The way I understand it the further out the port the larger it is. Also the closer it is to the end of the barrel it is the larger it gets. I assume its a result of the pressure drop since the bullet is out of the barrel faster. In my head I see it as high pressure volume in a small hole close to the chamber vs a lower pressure volume far from it. To make up for the lower pressure the port is enlarged to allow the same volume to flow through to cycle the action.
A suppressor simply adds back pressure since the gasses are contained and slowly let to exit. This bumps up that pressure so its best to have a way to knock it back to the bare barrel rate in order to keep the function consistent. I guess you could argue that since a suppressor usually caused extra fouling through the chamber a little more gas may be helpful from a reliability standpoint.
How long do you want your port to last?
A reamer is the proper tool to finish the port. That being said, what size hole does a 1/16 drill make when its chucked up in Black & Decker? Not 0.0625. Its disingenuous to foment the idea that you can properly modify a gas port in your garage with a drill motor or drill press and drill. If you have access to a turret mill and the proper cutting tools, cut away.
How much faster does a slightly oblong hole wear than one perfectly clean & round? How clean and round does a gas port need to be? What is the tolerance for the hole diameter? Roundness? Straightness? Angularity? Are you certain that the reamer you are using will give you a better surface finish than a drill bit? What surface finish is required? Will the reamer give a consistent hole size?
Part of what I do for a living is drill holes. There is more to hole quality than most folks realize. For example, did you know that for many drill operations, a drill bit will give better hole quality than a reamer? While employed with McDonnell-Douglas, I participated in a study to find out why we were having problems with hole quality, costing us millions of dollars in re-work. Standard procedure for drilling close tolerance holes through stack ups of aluminum and aluminum and titanium was to start with a #30 drill bit, open it with a #20 (equivalent) core drill and finish with a .1850 reamer. Yet we were having problems with egg-shaped holes, over sized holes and Hi-loc & Huck Bolt fasteners seizing during installation. (Hi-locs & Hucks are slightly larger than the hole size and coin the surrounding aluminum when driven in to increase the strength of the material around the hole).
The investigation revealed that the problem was the reamers. Reamer flutes are formed by multiple cuts and in the case of the manufacture of the reamers we were using, each flute is cut one at a time. This caused warpage from stresses induce by the machining process. Then, the reamers were rolled by hand on a flat granite surface and the reamer pounded straight with a hammer. During the heat and stress of use, the reamers would wander, some flutes would collapse, some would open up. They would not drill the same sized hole each time. The process was out of control.
Another company that had been originally contracted to build the same assemblies had been using a double margin drill bit. Our engineers didn't want to use them because they produced holes slightly larger than the bit. However, it was discovered that they produced the same sized hole with good surface finish and were round, every time they drilled a hole. When we switched to simply using double margin bits, our hole quality was in tolerance, in control and it was a faster process using less expensive tooling. We saved millions of dollars from reducing rework alone.
The point of this is, it's not that difficult to drill a hole, but it is more than simply assuming that a reamer must be used or that it's a problem if a 1/16" drill bit produces a hole larger than .0625". Drill blocks can be used to ensure the angle is correct. I know I can set up to drill a gas port in a barrel using a hand drill with good hole quality. A good drill press would be better and a turret mill is simply over-kill. Considering the job the gas port performs, I know tolerances aren't as critical as they are when drilling for a Hi-loc or even a simple rivit
How much faster does a slightly oblong hole wear than one perfectly clean & round? How clean and round does a gas port need to be? What is the tolerance for the hole diameter? Roundness? Straightness? Angularity? Are you certain that the reamer you are using will give you a better surface finish than a drill bit? What surface finish is required? Will the reamer give a consistent hole size?
Part of what I do for a living is drill holes. There is more to hole quality than most folks realize. For example, did you know that for many drill operations, a drill bit will give better hole quality than a reamer? While employed with McDonnell-Douglas, I participated in a study to find out why we were having problems with hole quality, costing us millions of dollars in re-work. Standard procedure for drilling close tolerance holes through stack ups of aluminum and aluminum and titanium was to start with a #30 drill bit, open it with a #20 (equivalent) core drill and finish with a .1850 reamer. Yet we were having problems with egg-shaped holes, over sized holes and Hi-loc & Huck Bolt fasteners seizing during installation. (Hi-locs & Hucks are slightly larger than the hole size and coin the surrounding aluminum when driven in to increase the strength of the material around the hole).
The investigation revealed that the problem was the reamers. Reamer flutes are formed by multiple cuts and in the case of the manufacture of the reamers we were using, each flute is cut one at a time. This caused warpage from stresses induce by the machining process. Then, the reamers were rolled by hand on a flat granite surface and the reamer pounded straight with a hammer. During the heat and stress of use, the reamers would wander, some flutes would collapse, some would open up. They would not drill the same sized hole each time. The process was out of control.
Another company that had been originally contracted to build the same assemblies had been using a double margin drill bit. Our engineers didn't want to use them because they produced holes slightly larger than the bit. However, it was discovered that they produced the same sized hole with good surface finish and were round, every time they drilled a hole. When we switched to simply using double margin bits, our hole quality was in tolerance, in control and it was a faster process using less expensive tooling. We saved millions of dollars from reducing rework alone.
The point of this is, it's not that difficult to drill a hole, but it is more than simply assuming that a reamer must be used or that it's a problem if a 1/16" drill bit produces a hole larger than .0625". Drill blocks can be used to ensure the angle is correct. I know I can set up to drill a gas port in a barrel using a hand drill with good hole quality. A good drill press would be better and a turret mill is simply over-kill. Considering the job the gas port performs, I know tolerances aren't as critical as they are when drilling for a Hi-loc or even a simple rivit
You are THE man! The forum wouldn't be the same without you. As for my contributions, all bets are off...
Mistwolf
Thank you for the info, as usual you know your stuff.
One question I would ask is about bit walking. Even in a press( which I did not use my first and only time) on a round piece of metal I some times have a small variation in hole size. But my situation is different, a slight difference in a 1/4in hole for a 1/4in bolt doesn't mean much. In the aerospace industry that slight difference could be life and death. And I think it might make a difference in the gas port too.
Or do I need to find a better drill press.
Companies like MSC and maybe Travers have reamer this small. I've never used one that small but they are available. When i was looking at MSC they were about $25 for the cheaper ones.
I was going to drill the barrel and using one of my pin gages to select the correct reamer to finish it. I defiantly agree that this isn't hand drill territory. If it were a barrel with no port I wouldn't bother but since it is drilled almost to the correct size that simplifies the process.
The way I understand it the further out the port the larger it is. Also the closer it is to the end of the barrel it is the larger it gets. I assume its a result of the pressure drop since the bullet is out of the barrel faster. In my head I see it as high pressure volume in a small hole close to the chamber vs a lower pressure volume far from it. To make up for the lower pressure the port is enlarged to allow the same volume to flow through to cycle the action.
A suppressor simply adds back pressure since the gasses are contained and slowly let to exit. This bumps up that pressure so its best to have a way to knock it back to the bare barrel rate in order to keep the function consistent. I guess you could argue that since a suppressor usually caused extra fouling through the chamber a little more gas may be helpful from a reliability standpoint.
At the risk of sounding like a smartass, I kinda know that. Don't take this the wrong way.
What I am searching for is port size in proportion to barrel length and supressor use. I ran out of time yesterday but I feel like I have seen a chart or somthing somewhere on here. Gotta make more time...
Dirtyboy333
03-07-12, 17:41
Listen to RobS's & of course Mistwolf's suggestions... Drilling gas ports is not as complicated as some make it seem. I've done this multiple times eyeballing it with a simple hand drill (Dewalt) and HSS and carbide bits. It has worked perfectly and I've gotten the exact results i've wanted.
ETA: Drilling a stainless SIG 550 valve was extremely tough and I couldnt even get started but after finding the right bit it became easy. There might be a chart but I would just use trial and error (staying on the small side of course). I didn't realize your port was already drilled until i re-read your post. In this case the trial and error method should be cake other then snapping a few bits.
Inkslinger
03-07-12, 18:10
For you guys drilling out your gas ports, are you able to drill without leaving a burr? Correct me if I'm wrong, but wouldn't the drill leave a hell of a burr compared to milling? How does the burr effect rifling.
Dirtyboy333
03-07-12, 18:21
For you guys drilling out your gas ports, are you able to drill without leaving a burr? Correct me if I'm wrong, but wouldn't the drill leave a hell of a burr compared to milling? How does the burr effect rifling.
Sometimes it will leave a burr but within a couple shots it will be gone. The gas ports that i've drilled in Sig Valves de-burred with a lil sand paper.
Inkslinger
03-07-12, 18:44
Sometimes it will leave a burr but within a couple shots it will be gone. The gas ports that i've drilled in Sig Valves de-burred with a lil sand paper.
That's kind of what I thought. I guess the amount of material being removed wouldn't leave that heavy of a burr. Definitely not something I would want to try by hand. You guys got balls.
You are THE man! The forum wouldn't be the same without you. As for my contributions, all bets are off...
You humble me. I re-read my post and realized it's tone is high handed and the questions posed in a STFU manner. MistWolf the technician gets so focused on the job at hand, he is completely lacking in manners. I know this. Unfortunately, the technician does not. I apologize for any insult or disrespect.
The questions about hole quality and tolerances are legitimate even though poorly asked
Sometimes it will leave a burr but within a couple shots it will be gone
Had one like that pass through a friends hands last summer.
Straightness gauge wouldn't pass through and it was determined to be said burr and when reported back to the shop 'they that sold it' told him the same thing: shoot a few rounds and it'll go away.
They were 100% right.
On the 3rd round the burr came off and immediately proceeded down the gas tube, through the key and into the bolt ring area where it wadded up two rings to the point that it took two people mortaring it hard to get the bolt to move enough get things taken apart.
Freind was really excited to see his BFN rifle getting slammmed on the ground like that to try and open it up to fix.
Dirtyboy333
03-07-12, 23:19
Had one like that pass through a friends hands last summer.
Straightness gauge wouldn't pass through and it was determined to be said burr and when reported back to the shop 'they that sold it' told him the same thing: shoot a few rounds and it'll go away.
They were 100% right.
On the 3rd round the burr came off and immediately proceeded down the gas tube, through the key and into the bolt ring area where it wadded up two rings to the point that it took two people mortaring it hard to get the bolt to move enough get things taken apart.
Freind was really excited to see his BFN rifle getting slammmed on the ground like that to try and open it up to fix.
Man that sucks!!! :( Interesting though as it's the 1st time i've heard that.
fixit69: Ha not a problem im not worried about it. From what you wrote i thought you might have a grasp but just didnt know. As for the suppressor part i dont think anyone can really put a number on how much it will change. All the various suppressors have different amounts of back pressure so i think its hard to say.
For the burrs i was concerned about this too. In my experience if you step drill the last couple holes the bur will be minimal. This will probably be the case in opening gas port since chances are your going in a couple thousandths increments. I wish i had a borescope so i could see out of curiosity. It would be similar to reaming the hole as someone else said. Another method ive read was to use the cerro safe and melt that right at the gas port location. Then drill and just push it out. That way you have a blind hole so the burr isnt really there. I'm sure to some degree depending on how technical you wanted to get there would be one but this would make it very small. That method however isnt very time effective if you trying to figure out your port size right then.
The issue ive read about with shooting out a burr is that you may could scratch the barrel if it rides the bullet down the bore. Most people say thats how its done many times though. I would be pissed though if it came back through the gas port like GTifosi said and caused another issue. That sounds like a fairly large burr they left.
Sneaking up on the final size would be a good way to go.
The issue experienced at this end was likely the result of either a dull drill, too fast a feed speed on the press, or both.
It wasn't a huge piece of flash by any stretch of the imagination, just big enough to prevent a straightness gauge from passing through.
I don't recall specifically which, but it was a 'shit brand X', not anything with a real name behind it and the barrel has since been permanantly taken out of service (I think now he uses it for a pry bar or cat whomper actually) and the upper sold for weight as scrap alunimum.
I've never even thought about a burr when upsizing a port, I guess it can happen though.
You need one of those back chamfer tools I've seen on a mill if you were overly concerned about it. I had this wierd part that needed a hole and a chamfer, the chamfer was on the inside. There are chamfer/edge break tools that when reversed can dive into the holes, then when you reverse the spindle the blade pops out, you then do your path, reverse and pull out.
Look at the port paths on the ones that get use, the fluid flow destroys all sharp angles and smoothes it out and makes it at an angle.
I used to race dirt bikes and I had to pay some one to take the head intakes and dremel them smooth for fluid flow. Don't think a tool is going to produce ideal fluid flow with normal axis constraints. I consider myself a pro with the stupid dremel, using the grinding bits, I have opened up ports very carefully. I tend to take the tool down to the internal rifling and try adjusting that area without adjusting the top, then doing the top, if you really wasted the bottom you would still control the gas by the top, and a really used one has that area toasted anyhow. The dremel tool with the small grinding tools don't burr edges either.
You can really go wrong with the dremel though, if you do go too far it sucks, but there are a few other areas to rig to bring it back into shape, the rear of the bolt tail, and the gas tube-key interaction tolerances can be increased to bleed off excess gas. I would do the gas tube head. It goes fast so be careful.
Gas flow is very sensitive to its cross sectional area, small changes have big differences.
If you're doing this yourself, and you're set on some certain goal, drill the smallest you think will work and go up from there to get function the way you want it.
The only way to be sure to get your barrel (rifle) to do what you want it to do. Depending upon how much you are cutting the barrel back, you may wish to try it after the cut and crowning. I would go up one numbered drill at a time using the same ammunition I expect to use. Pressure is but one aspect of the cycle and dwell time will also affect the rate.
You can always take more metal away, but putting it back... well that's a tad more difficult.
Inkslinger
03-08-12, 18:05
Had one like that pass through a friends hands last summer.
Straightness gauge wouldn't pass through and it was determined to be said burr and when reported back to the shop 'they that sold it' told him the same thing: shoot a few rounds and it'll go away.
They were 100% right.
On the 3rd round the burr came off and immediately proceeded down the gas tube, through the key and into the bolt ring area where it wadded up two rings to the point that it took two people mortaring it hard to get the bolt to move enough get things taken apart.
Freind was really excited to see his BFN rifle getting slammmed on the ground like that to try and open it up to fix.
Thats crazy. To you guys that just shoot out the burr in question, do you see any effects in accuracy? Wether the first few shoots or long term.
A bore scope of this would be great. What if any burr? After shooting? Leftover material? How it appears on the lands and groves?
SicTransit
03-08-12, 18:37
I had a nasty burr in one of my rifles that aggressive brushing wouldn't remove. So i said **** it and shot it out. Although I was worried it didn't seem to hurt anything.
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