I have a 22” Bartlein 6mm ARC AR. It is a rifle +2 gas system. I purchased the barrel and bolt from Precision Machine in WV. I have about 400 rnds through the barrel and get mediocre accuracy. The port fell on a land and it’s giving serious erosion issues. I believe that it is so bad that I am getting bullet deformation. I post here to have some experts give their opinions, maybe I don’t have an issue, please help me decide. I chose 6ARC for a precision auto loader build due to its resemblance to my 6BRN and PPC cartridges. I will post a picture of the build and some development targets shot at 100m. And some borescope photos. I appreciate you gentlemen and your knowledge. Thanks for your time.
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6 ARC Gas port erosion
- Thread starter Shield9
- Start date
Short list of things that can take a .75 MOA AR-15 for 20 shots to 1-3MOA:
- Gas tube binding on upper or carrier
- Muzzle device (seriously, swap muzzle devices and you'll see distinct differences between them)
- Adjustable gas; set it as low as it will reliably function
- Barrel thermo fit or bedded to upper
- handguard/bipod pressure changes POI (usually not a huge issue prone or off a bench but you can move it vertically and horizontally easier than you think).
Also the other load development things that always affect things are still at play. Try swapping bullets and/or powders.
Gas port in a land trends to shooting worse but isn't an outright guarantee of anything one way or the other.
- Gas tube binding on upper or carrier
- Muzzle device (seriously, swap muzzle devices and you'll see distinct differences between them)
- Adjustable gas; set it as low as it will reliably function
- Barrel thermo fit or bedded to upper
- handguard/bipod pressure changes POI (usually not a huge issue prone or off a bench but you can move it vertically and horizontally easier than you think).
Also the other load development things that always affect things are still at play. Try swapping bullets and/or powders.
Gas port in a land trends to shooting worse but isn't an outright guarantee of anything one way or the other.
That port doesn't look horrendous. It does look like it has carbon and copper in it. I'd probably scrub it out a little better and then patch the whole barrel with a light abrasive paste, focusing on the port area. The groups look good. What do they look like after 20 or 30 rounds? Do the groups wander when hot or fouled?
There's nothing wrong with your Gas Port, it just looks bad because it's on the dead center of a land. Keep shooting the barrel and it will eventually break in making a smooth transition.
But this is also the reason I only use 4 Groove Bartlein Blanks, it's so much easier to put the port in the center of a Groove.
But this is also the reason I only use 4 Groove Bartlein Blanks, it's so much easier to put the port in the center of a Groove.
It looks like it started out like a large burr from from drilling the gas port.
Like the one shown.
It Broke off leaving a ugly spot on the edge of the gas port.
It has been lapped in that area and appears someone tried to deal with it causing it break off.
Doesn't appear to be a big deal at this time.
One can drill it out larger dia with a new sharp drill and cutting oil, as I did with this barrel's gas port, cutting off the burrs first, without trying to champher them off as one edge appears.
Then lapp, so as not to break off the small burrs.
Then add an adjustable gas block, to deal with the larger diameter gas port.
Like the one shown.
It Broke off leaving a ugly spot on the edge of the gas port.
It has been lapped in that area and appears someone tried to deal with it causing it break off.
Doesn't appear to be a big deal at this time.
One can drill it out larger dia with a new sharp drill and cutting oil, as I did with this barrel's gas port, cutting off the burrs first, without trying to champher them off as one edge appears.
Then lapp, so as not to break off the small burrs.
Then add an adjustable gas block, to deal with the larger diameter gas port.
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Thanks for your advice. I have addressed most of those issues and continue to test new powder/bullet combinations. I seriously addressed the barrel nut/handguard pressure by welding a bag rider to the lower receiver. The upper is lapped and thermo fit. I appreciate your input.
Sir, I shoot some pretty long strings and don’t see poi wandering. I am worried about one area in particular and will certainly some abrasive on the port. It appears the land is rolling up towards the muzzle, creating a lip. Thanks for your advice.
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Yes sir, I will continue shooting it. It’s a 4 groove Bartien, also. I guess the shop that finished this blank wasn’t concerned with that detail. I have the same barrel in 6.5Gren from them that is in the groove, I was expecting that for this barrel too.
Sir, thank you for listing the options. My concern is the land rolling up into the bore. I was hoping it would bevel itself with firing. Now I want to try to lap it out with abrasives. Possible?
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What I do is use number drills to find out the size of the gas port, or gauge pin.
Say its .089 #43 drill. Then redrill with .0935 #42 this cuts the stubborn area and leaves a fine burr on can easily lapp.
It looks like this set up in mill for drilling, use dark heavy cutting oil at moderate speed slow feed and a new sharp quality HS drill. Then lapp. This works. Adjustable gas block can be added if necessary.
Also can be filled and redrilled but that should not be necessary.
The gas port location on the rifling form is a non issue, the bullet has already taken the rifling form sealing the bore. If it's half bore half lands its just as good as if it were centered in the bore groove or land. The physics doesn't care...as long as its smooth.
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I looked at all my gas gun ports, .308,458,.300s, 5.56s, 6.5 gren. I don’t scope them regularly like I do with this 6ARC. This 6ARC is by far the ugliest. Any idea of the root cause?
take the mirror off the bore scope and take a couple pics from each side. That will show if the edge is rolled up. I don't think it is. I had a WOA barrel that had a way worse erosion in 40rds than that. It was double the size. Mine would grab a patch as you pushed it through the bore t the port. That was why I scoped it. Then it went from shooting 1/2 inch groups to 1.5
groups. They gave me my money back after I shipped the my upper for them to test and look over. All they said was it did not shoot like they like.
groups. They gave me my money back after I shipped the my upper for them to test and look over. All they said was it did not shoot like they like.
You need to know how bad the burrs on the gas port are.
Bore scope it out.
If its bad, I set up and drill the gas port to the next largest size, with a new quality, sharp drill with dark heavy cutting oil.
This removes the heavy burrs.
Next you lapp.
I use a fixture that fits over the barrel extension centers the rod in the bore and can not flex, and has a stop nstalled & adjusted, to limit bore travel so ya don't exit.
The barrel is held in aluminum jaw, with V block, in a Bridgeport milling machine vise.
The type of lapp depends on what you have left to remove...polish off tiny burrs, or something more aggressive.
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, I tried… I put 220 grit lapping compound on a patch, went straight to the gas port area and made 2-3” strokes. I repeated this with approx 1500 strokes, changing to newly impregnated patch every 100ish strokes. I have a photo of my “finished” lap but it honestly looks like I wasted my time. I took a new batch of loads to the range today. Shot about 50 rounds. I am not pleased with the groups, not bad but I didn’t put a 22” Bartlein on to get mediocre accuracy.
I put a bore scope in my barrel when I got home and again after I cleaned it.
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Honestly, I’m about to rebarrel this rifle. I will order a barrel from a shop that guarantees the port in a flat. I appreciate those who told me to be happy and shoot but that detail is always in my head. It’s like having a car that is running 11 second times but was built to break in the 10s. This gun is shooting .5-.8s and I want .2-.4s.
I tried to scope it without the mirror as you suggested. My cheap scope doesn’t work well like that. Horrible photo quality. I think that the presence of copper in the port indicates bullet jacket deformation. I took a couple more photos. Lmk what you think. Thanks
