Question for the AR pros...gas key resistance at gas tube

[Dentite]

Hey gang,

Just built my first AR. Pretty fun to put it together. I took my time and I think I have everything right. Assembled a friends at the same time and his is fine.

The only problem is that the BGC won't easily seat all the way onto the gas tube.

At least I think that's the problem. Is there anything else that would hold up the BCG from seating all the way onto the gas tube and into the receiver?

I first wondered if the bolt was hanging up so I removed the bolt and there is still resistance. I can get it to close all the way onto the gas tube if I let it get a running start using the buffer spring (pulling back and letting go about 1/3 of the way back).

The gas tube looks perfectly straight and not binding on the barrel nut.

So then I took the bolt out of my factory Colt 6920 and put it in the new build. What do you know, it fits with only very slight resistance.

So then I took the BGC from the new build and put it in the Colt. It fits fine, closes with no resistance, etc.

Is that weird or what?

I first thought either the gas tube is too large or the key is too small or something's not lined up. But the Colt BCG works fine in the new build and the new BGC works fine in the Colt upper.

Colt has probably 500 rounds though it.

Any thoughts? Thanks.

 

[USAF_Vet]

Checked the binding bolt with the Colts, checking specifically for the position and alignment of the gas key. I'd also check the length of the gas tube versus one you know operates well, like the one on the Colt.

One or the other sounds slightly out of spec or is slightly misaligned. Check your gas block for proper alignment as well. If it's off slightly, it'll cock the end of the tube to one side.

What I'd do is loosen the gas block, and close the bolt. If it closes with no resistance, check your gas block position and tighten it down. If its the type of gas block that is held in place with set screws, does the barrel have dimples set in place from the factory, or not? If there are factory set dimples for the gas block, they might be off by a hair, causing the misalignment of your gas block. You can remove the dimples with a file, just leaving that area flat, or fill them in with a metal epoxy and sand it smooth. Check the barrel alignment in the upper before doing any permanent changes.

In a nutshell
Check for
Barrel alignment
Gas block alignment
Gas Key alignment
Gas tube alignment/ length

Also, make sure everything is well lubed.

 

[Dentite]

Thanks for the info, I'll see if loosening up the gas block makes a difference. This barrel has a milled flat for set screws, but I think it's a moot point because I'm using a gas block with a clamping style attachment (two screws squeezing the gas block onto the barrel) as opposed to set screws.

Thanks.

 

[Jackal]

My money is on canted gas block as well.

 

[MistWolf]

Before doing anything else, remove the BCG from the rifle and check to make sure the gas tube is free floating where is passes through the barrel nut and centered inside the upper. The barrel nut alignment could be off which will push the gas tube to one side or the other

 

[Ibmikey]

USAF has the answer, follow his instructions and reclamp gas block with the bolt and carrier in battery, then you will be filling the air with hot brass.

 

[mac_hunter870]

Had an issue like this and thought it was a gas key/gas tube alignment issue but come to find out the slot in the receiver which aligns the barrel with it's little pin was off just a little bit and wouldn't let one brand of BCG/bolt locking lugs close and lock but another brand BCG/bolt would. Pulled the barrel and carefully filed the slot in the front of the receiver in the direction that I needed the barrel to slightly rotate to get the bolt locking lugs to enter the back of the chamber on the barrel...reassembled and it runs fine now with either BCG/bolt. The gas block and gas tube alignment was never the issue for this rifle. Just an experience I had that may help solve your issue.

 

[lysanderxiii]

Before doing anything else, remove the BCG from the rifle and check to make sure the gas tube is free floating where is passes through the barrel nut and centered inside the upper. The barrel nut alignment could be off which will push the gas tube to one side or the other

This.

The next thing would be to remove the bolt and carefully push the carrier home, watch the gas tube through the bottom of the upper (removed from the lower) and see if the gas tube moves to one side as the carrier goes forward.

 

[Dentite]

Thanks for the advice guys, I will do the recommended things listed above and report back.

Thanks.

 

[NWcityguy2]

Is it possible your gas tube is bent up or down and is contacting the gas key that way? There should be little to no resistance with two in-spec parts.

 

[Dentite]

Well I took the rifle apart tonight and took the gas block and gas tube off the rifle. I tried the gas tube into the gas key and that confirmed an interference. The gas tube was very tight to get into the key.

Scrubbed out the key with a bronze brush, no change. Decided to remove the melonite coating from the end of the gas tube. A few minutes with some 400 grit sandpaper and repeatedly checking fit. Once there was just a little resistance I lubed it up, put the rifle together and now everything is fine.

Hopefully that works. Not sure if the key is a little small or the tube was a little large but the tube is easier to modify that's for sure.

Thanks guys.

 

[briansmithwins]

A few minutes with some 400 grit sandpaper and repeatedly checking fit. Once there was just a little resistance I lubed it up, put the rifle together and now everything is fine.

Hopefully there won't be too much gas leakage. If the rifle starts short stroking or not holding the bolt open after the last round is fired you've got excess leakage. But at least gas tubes are cheap.

Good luck.

BSW

 

[bearcreek]

The gas tube was very tight to get into the key.
Hopefully that works. Not sure if the key is a little small or the tube was a little large but the tube is easier to modify that's for sure.

What brand of gas tube is it?

 

[lysanderxiii]

They can stand a lot of leakage before short stroking becomes a problem.

 

[Dentite]

Thanks guys. I do haven an adjustable gas block.

The tube is a melonited Spikes.

Gas key came installed on an Underground Tactical carrier.

I got that carrier because my barrel is a 6.5 Grendel Lilja and they recommend the UT bolt as a headspace match to their chamber.

 

[moxie]

I think this might be a case of over-improvement. That Spike's melonited gas tube is already stainless steel. I read the purported virtues of the coating but think perhaps all it did is increase the diameter of the tube which then you removed on the end with sandpaper.

 

[MistWolf]

You did check to make sure the gas tube was free floating as it passed through the barrel nut, did you not? Because if the barrel nut is over or under clocked, you have made your gas tube under sized which will not fix the problem and will introduce another

 

[carbine85]

The Nitride treatment wasn't the problem and you would have been better in the long run to leave it on there. These rifles will loosen up a lot after a few rounds down range.

 

[Dentite]

I think the coating was a factor because my friend assembled one the same night. Same barrel, barrel nut, gas block, carrier. One difference was he had a plain stainless tube and his had only a slight bit of resistance.

No, the barrel nut was not out of alignment...if you read above, the gas tube could not go in or out of the gas key without considerable effort OUTSIDE the rifle. Gas tube in one hand, carrier in the other. I did not adjust the barrel nut and it's fine after removing the coating.

I could see leaving it and letting it shoot in if there slight resistance, but if I closed the bolt all the way, I could barely get it open with serious effort pulling on the charging handle. I'm talking about barely getting it open type pulling.

I guess time tell if it works out. Worst case I buy another $15 gas tube.

Thanks guys.

 

[wally]

I think this might be a case of over-improvement. That Spike's melonited gas tube is already stainless steel. I read the purported virtues of the coating but

About the only virtue I can think of, would be that the black coating would help prevent the OCD folks from having to clean the carbon off it

 

[moxie]

Yup! And when do you even spend more than 2 secs. doing that!

 

[Baron_Null]

Melonite is not a coating, it is a surface treatment that is referred to as "salt bath ferritic nitrocarburizing". It's more akin to a heat treatment process than it is a plating or a coating. From my understanding, it should do little to nothing to change the diameter of the part.

It might be possible that the gas tube was sligtly oversized regardless of the coating, and that the gas key was undersized enough to cause interference, given that the BCG worked in the Colt, and the Colt BCG worked with th Spike's gas tube.

 

[carbine85]

Melonite is not a coating, it is a surface treatment that is referred to as "salt bath ferritic nitrocarburizing". It's more akin to a heat treatment process than it is a plating or a coating. From my understanding, it should do little to nothing to change the diameter of the part.

Correct. Typically you are better off with it then without it

 

[moxie]

Typically perhaps, but not this time.

Q.E.D.