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[ARCHIVED THREAD] - Gas-port Erosion of AR-15 Barrels (Page 1 of 3)
Posted: 1/9/2016 1:07:50 AM EDT
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Gas-port Erosion of AR-15 Barrels
The bore-scope view pictured below shows the gas port of a 20" AR-15 stainless-steel barrel with a rifle length gas system that has not yet been fired. 
The next bore-scope view shows a 20" AR-15 stainless-steel barrel with a rifle length gas system after only 1000 rounds have been fired through it. This view illustrates one of the reasons that certain high-end barrel manufacturers make the extra effort to ensure that the gas port is drilled in the center of a groove. 
The shorter the gas system, the more rapid the erosion. The bore-scope view below shows the gas port of a 14.5" chrome-lined CMV AR-15 barrel with a carbine length gas system after only 500 rounds have been fired from it. 
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I'm curious about what material the pictured barrels are made of? Or would all materials show the same result? Also, if these were un-lined/un-treated barrels, would chrome lining or nitriding slow the process of erosion?
Forgive the line of questioning, but while we got you here...... |
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I'm curious about what material the pictured barrels are made of? Or would all materials show the same result? Also, if these were un-lined/un-treated barrels, would chrome lining or nitriding slow the process of erosion? Forgive the line of questioning, but while we got you here...... The first two pics are from stainless-steel barrels. The third pic is from a chrome-lined barrel. |
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In! I love Molon threads. So much to learn. Thank you for putting these together. +1. Good stuff. I saw this brought up in a thread within the last few days. One member was remarking on how lighter weight barrel ports tend to deteriorate faster. Unsure of the validity of that but it made sense. Make sure to post more of your findings if you continue to photograph gas ports. |
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Any idea when this erosion starts to affect the accuracy, or rather precision, of the barrel? Gas port erosion should not affect accuracy, but the corresponding muzzle erosion will. I'm not here to argue but I am not sure I agree with you Medic and I think Molon made the comment below directed at just that. Quoted:
This view illustrates one of the reasons that certain high-end barrel manufacturers make the extra effort to ensure that the gas port is drilled in the center of a groove. Problem is most of us do not have a bore scope to check a bore. I wonder how an adjustable gas block would impact this; slow the process, speed it up or show zero difference? |
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I'm not here to argue but I am not sure I agree with you Medic and I think Molon made the comment below directed at just that. Problem is most of us do not have a bore scope to check a bore. I wonder how an adjustable gas block would impact this; slow the process, speed it up or show zero difference? Quoted:
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Any idea when this erosion starts to affect the accuracy, or rather precision, of the barrel? Gas port erosion should not affect accuracy, but the corresponding muzzle erosion will. I'm not here to argue but I am not sure I agree with you Medic and I think Molon made the comment below directed at just that. Quoted:
This view illustrates one of the reasons that certain high-end barrel manufacturers make the extra effort to ensure that the gas port is drilled in the center of a groove. Problem is most of us do not have a bore scope to check a bore. I wonder how an adjustable gas block would impact this; slow the process, speed it up or show zero difference? No need for argument. I used to be a Garand collector. We had barrels with way worse corrosion than that, and it made no impact on accuracy. |
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It would be interesting to follow a barrel through its use over time. Quoted:
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What about higher mileage barrels 4-6K? Any pics? It would be interesting to follow a barrel through its use over time. That is exactly what I am wondering what goes first the port or the barrel. |
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That is exactly what I am wondering what goes first the port or the barrel. Quoted:
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What about higher mileage barrels 4-6K? Any pics? It would be interesting to follow a barrel through its use over time. That is exactly what I am wondering what goes first the port or the barrel. I've seen photos of rust under the chrome lining of barrels, I wonder if this could be one of the start points for that. |
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I've seen photos of rust under the chrome lining of barrels, I wonder if this could be one of the start points for that. Quoted:
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What about higher mileage barrels 4-6K? Any pics? It would be interesting to follow a barrel through its use over time. That is exactly what I am wondering what goes first the port or the barrel. I've seen photos of rust under the chrome lining of barrels, I wonder if this could be one of the start points for that. Gas port erosion increases the amount of gas entering the system, and its velocity. This is due to the fact that the erosion pattern makes the simple hole more like a nozzle. The effect is the same as having a larger gas port. You will see an increase in cyclic rate and increased bolt velocity. If the erosion is sufficiently bad you will see increased wear on parts. Neither of those shown are that bad, yet, but given time... You can have an overgassed rifle long before your have a shot out barrel, depending on where the gas port is located and how fast it erodes. |
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Gas port erosion increases the amount of gas entering the system, and its velocity. This is due to the fact that the erosion pattern makes the simple hole more like a nozzle. The effect is the same as having a larger gas port. You will see an increase in cyclic rate and increased bolt velocity. If the erosion is sufficiently bad you will see increased wear on parts. Neither of those shown are that bad, yet, but given time... You can have an overgassed rifle long before your have a shot out barrel, depending on where the gas port is located and how fast it erodes. Quoted:
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What about higher mileage barrels 4-6K? Any pics? It would be interesting to follow a barrel through its use over time. That is exactly what I am wondering what goes first the port or the barrel. I've seen photos of rust under the chrome lining of barrels, I wonder if this could be one of the start points for that. Gas port erosion increases the amount of gas entering the system, and its velocity. This is due to the fact that the erosion pattern makes the simple hole more like a nozzle. The effect is the same as having a larger gas port. You will see an increase in cyclic rate and increased bolt velocity. If the erosion is sufficiently bad you will see increased wear on parts. Neither of those shown are that bad, yet, but given time... You can have an overgassed rifle long before your have a shot out barrel, depending on where the gas port is located and how fast it erodes. lysander, I'm not claiming my thought process is correct here, and I have pretty much no experience in dealing with or calculating the impact of gas port erosion. Here's what came to my mind after reading your description. If the erosion starts toward the muzzle end rather than chamber end, would the gas really tend to accelerate to cause that nozzle effect you mention? If the erosion was going from the port to the chamber, I can see a tiny bit earlier cycling episode therefore contributing to a slight overgassing...but very, very slight I would think. I'm getting the concept of a kind of "ram air" effect that you may be touching on, but is the amount of the erosion channel long enough and deep enough to really create this effect in this case to a degree that makes any real difference? I'm not trying to argue with you, but it is a an interesting concept. Is there a definitive test or evidence established from a barrel maker or other reliable source that confirms this? I'm geeky enough to enjoy digging into the concept. |
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That is exactly what I am wondering what goes first the port or the barrel. Quoted:
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What about higher mileage barrels 4-6K? Any pics? It would be interesting to follow a barrel through its use over time. That is exactly what I am wondering what goes first the port or the barrel. Man, no kidding. I had no idea it was occurring this quickly |
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Any idea if there's a difference in wear-rate between poly-rifling and non? All else being equal? I haven't collected any long term data on that subject matter. One casual observation that I've made from bore-scoping my AR-15 barrels; barrels with traditional rifling and a right hand twist sometimes start to show erosion of the rifling on the right edge of the rifling first. I haven't noticed this phenomenon in barrels with polygonal rifling. Again, that's just a casual observation. |
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lysander, I'm not claiming my thought process is correct here, and I have pretty much no experience in dealing with or calculating the impact of gas port erosion. Here's what came to my mind after reading your description. If the erosion starts toward the muzzle end rather than chamber end, would the gas really tend to accelerate to cause that nozzle effect you mention? If the erosion was going from the port to the chamber, I can see a tiny bit earlier cycling episode therefore contributing to a slight overgassing...but very, very slight I would think. I'm getting the concept of a kind of "ram air" effect that you may be touching on, but is the amount of the erosion channel long enough and deep enough to really create this effect in this case to a degree that makes any real difference? I'm not trying to argue with you, but it is a an interesting concept. Is there a definitive test or evidence established from a barrel maker or other reliable source that confirms this? I'm geeky enough to enjoy digging into the concept. The flow through a straight wall tube is thus (left picture): 
Note that the actual area (effective orifice size) is smaller that the diameter of the hole. The grey areas are stagnant areas of turbulence, the area that gas can flow through is the minimum space between the two stagnant places . As the upstream side erodes the shape of the port become more shaped as shown on the right, the effective area of the port increases because the flow can stay attached to the walls. As the port erodes, the amount of gas flow will increase, and you will see an increase in cyclic rate, then at a point the gas flow will plateau, as increased wear (increased radiusing of the port entry) will expand beyond the optimum flow path. That is until the erosion gets so bad the actual diameter of the port increases many thousands of rounds later. As to why the muzzle size erodes first, look at the impact angle of the gas flow: 
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Wonder what effect the size of the gas port, if any, has on rate of erosion?
I'm assuming a piston AR is susceptible to the same wear? Gotta be a hole for the gas to get to the piston eh? So much for "mid-lengths are just a fad" heh.... 
Science and observable fact are awesome. Thanks Molon. |
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This was from the 10,000 round lucky gunner test of the BCM barrel with the brass cased ammunition. Gun went 10,000 rounds with breakage or stoppages. Barrel was toast though. https://scontent-lax3-1.xx.fbcdn.net/hphotos-ash2/t31.0-8/fr/cp0/e15/q65/703571_10151253376704464_1261259578_o.jpg?efg=eyJpIjoidCJ9 Those were bushmaster not bcm barrels. Not saying that a bcm barrels would have performed differently, just wanted to point it out for accuracy. Additionally the port errosion is not what caused the barrels to be toast,if I recall correctly,. The rifeling was shot out due to the firing cycle. |
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Those were bushmaster not bcm barrels. Not saying that a bcm barrels would have performed differently, just wanted to point it out for accuracy. Additionally the port errosion is not what caused the barrels to be toast,if I recall correctly,. The rifeling was shot out due to the firing cycle. Quoted:
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This was from the 10,000 round lucky gunner test of the BCM barrel with the brass cased ammunition. Gun went 10,000 rounds with breakage or stoppages. Barrel was toast though. https://scontent-lax3-1.xx.fbcdn.net/hphotos-ash2/t31.0-8/fr/cp0/e15/q65/703571_10151253376704464_1261259578_o.jpg?efg=eyJpIjoidCJ9 Those were bushmaster not bcm barrels. Not saying that a bcm barrels would have performed differently, just wanted to point it out for accuracy. Additionally the port errosion is not what caused the barrels to be toast,if I recall correctly,. The rifeling was shot out due to the firing cycle. Just noticed I put BCM. Had the BCM accuracy page open in a seperate tab. |
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Stainless steel barrels may not rust but are much softer than 4150 Chrome lined mil spec steel barrels and as such will erode and wear faster. The first two pics in the original post are from stainless-steel barrels. The third pic is from a chrome-lined CMV barrel. |
| Looks like some air/oxygen gets pulled in from gas port/gas tube and mixes with the hot gases causing even higher temps. This process starts right about when the bullet end reaches the center of the gas port, and seems to continue until a few millimeters after the gas port. |
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Looks like some air/oxygen gets pulled in from gas port/gas tube and mixes with the hot gases causing even higher temps. This process starts right about when the bullet end reaches the center of the gas port, and seems to continue until a few millimeters after the gas port. It's much simpler than that, just solid particles in the gas flow impinging on the exposed corner of the hole. Smokeless propellant has less solid combustion products, but it still has some, that small amount of smoke, that's solid combustion products. |
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It looks more like heat damage to me. It's like a plasma jet that slowly cuts into the metal over time. I have seen this this before near oxygen injectors in rocket boosters. Well, the flame temp is around 3000 degrees at the chamber, not a lot cooler 12" away, so there is a heat element to it. But there isn't much free oxygen in the bore. Mostly carbon dioxide, nitrogen and water. |
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Any idea when this erosion starts to affect the accuracy, or rather precision, of the barrel? I don't have any correlating data on that, but it is interesting to note that when bore-scoping a barrel after a few hundred rounds fired without any cleaning, there is noticeable copper fouling in the area of the gas-port erosion. ... |
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So it would seem that the longevity of a barrel may best be measured in respect to the speed at which the gas port erodes? In respect to a gun like an MK18 it would seem as though this very well could be the case....though I'd prefer Molon's thoughts on this. I believe you will find the throat erosion limits barrel life, not throat erosion. This test shows a nice picture of some worn out gas ports, the worst was the one shooting Federal, which maintained the best accuracy through the entire 10,000 test. The mean radius when from .61 to .81.... |
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I believe you will find the throat erosion limits barrel life, not throat erosion. This test shows a nice picture of some worn out gas ports, the worst was the one shooting Federal, which maintained the best accuracy through the entire 10,000 test. The mean radius when from .61 to .81.... Quoted:
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So it would seem that the longevity of a barrel may best be measured in respect to the speed at which the gas port erodes? In respect to a gun like an MK18 it would seem as though this very well could be the case....though I'd prefer Molon's thoughts on this. I believe you will find the throat erosion limits barrel life, not throat erosion. This test shows a nice picture of some worn out gas ports, the worst was the one shooting Federal, which maintained the best accuracy through the entire 10,000 test. The mean radius when from .61 to .81.... Gas port erosion? |
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Gas port erosion? Quoted:
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So it would seem that the longevity of a barrel may best be measured in respect to the speed at which the gas port erodes? In respect to a gun like an MK18 it would seem as though this very well could be the case....though I'd prefer Molon's thoughts on this. I believe you will find the throat erosion limits barrel life, not throat erosion. This test shows a nice picture of some worn out gas ports, the worst was the one shooting Federal, which maintained the best accuracy through the entire 10,000 test. The mean radius when from .61 to .81.... Gas port erosion? I think that is probably what he meant. ... |
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I have a stainless barrel that has only about 200 rounds through it and I already have erosion enough to feel it with a jag and patch when I clean it. Is there any fix or problem with the gas block that I should check? You could find a gunsmith to bore-scope your barrel to see what is actually going on in there, but I doubt that there is actually any problem that developed from only 200 rounds worth of erosion. |
[ARCHIVED THREAD] - Gas-port Erosion of AR-15 Barrels (Page 1 of 3)
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