Gas Tube/Key Fitment

[MiddleLuke]

I put my 308AR together the other day. Aero receiver set, BA barrel with mid length gas. Today I was looking through some forums and found that to make sure everything is lined up, put the bolt carrier in with no bolt and slide it forward and if there’s any resistance then something probably isn’t aligned. So I tried it out and felt resistance when the key came to the tube. I couldn’t get a good view to see where it was making contact. Took everything apart and was trying to find what I did wrong. I notice the barrel nut is slightly touching the gas tube. I looked down the end of the receiver and the gas tube seems to be coming in Perfectly straight. So on a whim i took off the gas tube and seen how it fit into the gas key when not on the rifle. It’s here I noticed that the tube didn’t slide right into the key. It was some resistance. There’s like a mushroom head on the tube that creates the resistance. Below is a picture of the little bit it goes in before resistance starts.  So now I’m pretty sure that’s where I’m getting resistance. No the question is.. is that normal? Or should I get a new gas tube to install? 

[Albroswift]

I believe the "Mushroom Head" is designed to allow break in and good gas seal without wear or resistance on the tube. The "Mushroom" is the same diameter as the tube, except for where it is cut down couple thousands for the inch or two right before the "Mushroom". I would think a small amount of resistance should be normal, but I'm sure someone more expert then myself will come along and confirm.  

 

Edited March 30, 2020 by Albroswift

[MiddleLuke]

11 minutes ago, Albroswift said:

I believe the "Mushroom Head" is designed to allow break in and good gas seal without wear or resistance on the tube. The "Mushroom" is the same diameter as the tube, except for where it is cut down couple thousands for the inch or two right before the "Mushroom". I would think a small amount of resistance should be normal, but I'm sure someone more expert then myself will come along and confirm.  

 

Yeah, kinda what I thought but I’m in the same boat. 

[Albroswift]

@MiddleLuke

Got me stirred up so I went out to the shop, tried several tubes in a Toolcraft BCG with only 30 or 40 rounds through it. None had any resistance but they were all really close. no significant rubbing when aligned. 

[MiddleLuke]

@Albroswift  Wellll crap. HAHA Sorry to get you wondering. I appreciate you checking though! I’ll order another one. 

[jtallen83]

Paint some dykem on the tube and see where it rubs, sometimes you can tweak them a bit. I have the best luck when using a tube specifically for the large frame AR, Armalite and White Oak Armament are two that I prefer. The large frame tends to need a slightly earlier bend to clear some barrel nuts.  

[98Z5V]

2 hours ago, MiddleLuke said:

@Albroswift  Wellll crap. HAHA Sorry to get you wondering. I appreciate you checking though! I’ll order another one. 

Don't waste the money on another one.  Shoot the gun.

[98Z5V]

Anybody here ever buy a new motorcycle, and immediately take apart the engine, measure piston ring end-gap?  Check combustion chamber volume?  You buy a new bike and test the fuel pressure right out of the pump, if it's EFI?  You know, to make sure it's in spec? 

No, you buy a new bike, fill up the tank, and you go rip that thing.

OP, if you don't obviously have something wrong with your gun - then go shoot it.

[Sharpshooter]

^^^

What he said.

[MiddleLuke]

@98Z5V @Sharpshooter Enough said! I’ll give it a whirl, didn’t need much convincing. I appreciate the input 98. 

[Albroswift]

10 hours ago, 98Z5V said:

Anybody here ever buy a new motorcycle, and immediately take apart the engine, measure piston ring end-gap? 

After following this forum over the last couple of years seems like large frame AR builders ain't up to the same rigorous QA/QC that Harley or Ducati uses . 

[MikedaddyH]

12 hours ago, 98Z5V said:

Anybody here ever buy a new motorcycle, and immediately take apart the engine, measure piston ring end-gap?  Check combustion chamber volume?  You buy a new bike and test the fuel pressure right out of the pump, if it's EFI?  You know, to make sure it's in spec? 

No, you buy a new bike, fill up the tank, and you go rip that thing.

OP, if you don't obviously have something wrong with your gun - then go shoot it.

Let's go one step further, rack the BCG about 100 times. This will prep the metal surfaces then lube up your BCG really good and shoot it ! 

[MiddleLuke]

Just now, MikedaddyH said:

Let's go one step further, rack the BCG about 100 times. This will prep the metal surfaces then lube up your BCG really good and shoot it ! 

Started doing this last night to see if it made any difference. Only 75 times left to go HAHA

[98Z5V]

14 hours ago, MiddleLuke said:

@98Z5V @Sharpshooter Enough said! I’ll give it a whirl, didn’t need much convincing. I appreciate the input 98. 

Yep, go rip it up, man, see what it does.  If something happens during break-in, try to gather as many details as you can, report them back.  There's nothing that we can't figure out, that's for sure. 

Edited March 31, 2020 by 98Z5V

[MiddleLuke]

Happy with how it came out. Thanks for the input everyone. Gonna full send tomorrow.