Cycling Issue

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Speaker

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Hello
I had dropped off all the parts for a AR build with a local 'smith in August. Most of his reviews were positive but I'm thinking I might have given him something that's not in his expertise. After several attempts to get some info out of him this is what I got ......

"I thought the gas tube was too long so I ordered a new one. That is not the case.
The gas block has been fitted to the shoulder of the barrel, at its correct point. When the bolt is dropped down, it will not seat fully into battery, making me think there is something to do with the gas tube/bolt. When the bolt does seat completely into battery, it forces the gas tube ever so slightly forward that it turns the rifle into a single shot. Now, I am trying to figure out what the actual issue is. Is it the gas key, gas tube, or bolt itself. "

The barrel did not come with a gas port. Could he have drilled the port too small?

Any suggestions much appreciated.
 
You did not give specifics on the AR build so here is some general info. Depending on barrel configuration, the gas port are different size at different locations. With all the prints I have the gas port location is figured with the use of a 0.030" shim between the gas block and barrel shoulder. So if it's jammed tight it may be too close. Like said, alignment is critical to some degree. Closer to the chamber smaller the port due to higher pressure. You also need 5" of barrel after the port for reliability. Shorter it is the less time for things to happen.
 
Blue68f100 said:
You did not give specifics on the AR build so here is some general info. Depending on barrel configuration, the gas port are different size at different locations. With all the prints I have the gas port location is figured with the use of a 0.030" shim between the gas block and barrel shoulder. So if it's jammed tight it may be too close. Like said, alignment is critical to some degree. Closer to the chamber smaller the port due to higher pressure. You also need 5" of barrel after the port for reliability. Shorter it is the less time for things to happen.
Click to expand...

It's a 6.8 build.
The barrel is this:
https://columbiariverarms.com/produc...-configurable/

The FSB is this:
https://www.brownells.com/rifle-part...prod80559.aspx

It's the same FSB on the Colt 901's and 6940's

I had dry fitted everything together before I took it to the smith and everything fit well, no binding and not flappin' in the breeze either. The only thing that appeared to be an issue was the length of the gas tube (too long). Apparently he got a shorter tube and tried it but had the same issue.We had also discussed the size of typical 6.8 gas ports because the barrel came without a port. I don't know. It seemed like it should work but doesn't.
 
What length gas system did you go with?
Use pin set to measure or Number bits will get you close.

Rifle requires 20 barrel length with a 0.0935"-0.980" port . Located 12.125" from the Barrel extension nut face.
Mid Length 16" barrel with a 0.078"-0.810" port. Located 8.75" from the barrel extension nut face.

Make sure the rings splits on the bolt are distributed around the bolt and not all in alignment.

Are you using a heavy buffer, Heavy bolt?

Don't know where your located at so I'm probably not very close to you.
 
Do you know what length gas tube you ended up with? If by chance he trimmed the end that goes into the bolt to make it fit, he removed the expanded area used for a seal. So you will have too much gas leakage to operate the bolt. The std mid length gas tube is 11.75" long. These vary some but are with in 1/16" of the length.

I would confirm that the gas port is aligned on the gas block, and find out what length gas tube is being used. You can measure to see if the gas port is in the right location. 0.3" from the step on the barrel. And check the distance to the barrel extension nut.

Most gas blocks have a ~0.125" port to allow for some movement. Do some measurements and see if it's aligned properly. Also makes sure there is not a metal shaving in the port.

The BCG is the std weight which is good, the 4oz is whats considered H1, all should work fine.
 
Blue68f100 said:
Do you know what length gas tube you ended up with? If by chance he trimmed the end that goes into the bolt to make it fit, he removed the expanded area used for a seal. So you will have too much gas leakage to operate the bolt. The std mid length gas tube is 11.75" long. These vary some but are with in 1/16" of the length.

I would confirm that the gas port is aligned on the gas block, and find out what length gas tube is being used. You can measure to see if the gas port is in the right location. 0.3" from the step on the barrel. And check the distance to the barrel extension nut.

Most gas blocks have a ~0.125" port to allow for some movement. Do some measurements and see if it's aligned properly. Also makes sure there is not a metal shaving in the port.

The BCG is the std weight which is good, the 4oz is whats considered H1, all should work fine.
Click to expand...

Don't know the length of the tube but I will soon because I have asked for my rifle back.

Thanks for your responses.
 
Got my rifle back today. I can tell you exactly what's wrong with it. The gas tube is too long.
 
He might have said he ordered a new tube but it was obvious he didn't replace it.

The other day I read a post by someone else using the same setup who had the same issue. They discovered the gas tube that comes with that particular FSB is in fact 12", a Colt 901 proprietary length. So he replaced it with a mid length and all was well.

There were some other aspects of this individuals work that made me lose confidence in his abilities. I believe he may be good at correcting cosmetic issues with aged firearms and personalizing new ones but lacks practical knowledge of the mechanics behind them.
 
Last edited:
Speaker said:
There were some other aspects of this individuals work that made me lose confidence in his abilities. I believe he may be good at correcting cosmetic issues with aged firearms and personalizing new ones but lacks practical knowledge of the mechanics behind them.
Click to expand...

Understood. Two sides of the coin - those who can do great cosmetic work, but do not understand critical fit/function issues and those who can build a gun that runs like a thoroughbred, but looks like …. something else. Then the much smaller group that can do both. Those are the ones that produce functional art, usually at a premium price point.
 
So last week a friend of mine with the right tools put in a standard mid length gas tube and mentioned that the gas block seemed loose. Funny thing was when I got the rifle back from the gunsmith I looked at the gas block and thought he had secured it with a pin and mangled the pin in the process but it was actually a mangled set screw. My friend put in a new set screw and tightened it. Over the weekend I did a test fire and everything cycles normally.
 
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