AR gas question (update)

[LanceS4803]

Still no resolution to problem of failing to hold open after last round on empty mag. (Previously it also wouldn't load next round, didn't encounter that today.)
This is 1 of 3 rifles, all identical builds except the problem rifle has a 16" Hanson profile build barrel, while the other two are standard profiles.
Mid length gas, pinned gas blocks and carbine buffer setups, same bolt carriers.
Pulled pin and screws from gas block (non-adjustable) and checked barrel gas port which is free and clear, (gauging pin drops right through into bore) measuring a little over .070, which is spec for that barrel. Gas block lines up perfectly with gas port in barrel. Forced air through gas tube, it exited from around loose gas block. I didn't unpin the gas block from the tube and didn't pull the brake to completely remove block from barrel. Reassembled and forced air through gas tube, and it was exiting barrel.
Gas key tight and staked, no interference with gas tube when installed.
Checked carbine buffer weights, all 3oz. All springs are 11".

Range Testing
Put problem upper on known good lower from other rifle, still failure to hold open. (Problem lower worked fine with known upper.)
Put bolt carrier and charging handle from known upper into problem upper, still failure to hold open. (Problem rifle bolt carrier and handle worked fine in known rifle.)
Put spring and buffer from known good lower into problem rifle, still failure to hold open. (Problem spring and buffer worked fine in known rifle.)
Shooting M855 green tip. (Also tried a couple of different Federal SP loadings.)
Ejection pattern in 3 to 4 o'clock, about 8-10' away.

At this point it is obviously the gas system. I am wondering if I have an internal defect in either the block or the tube causing a slight obstruction. Am considering just ordering new ones.
Can't think of anything I am missing.

 

[Omega]

Did the known good upper/lower have the same gas system? With a mid length gas system, a carbine gas system lower may not have enough gas to push the extra 2". Many normal ARs are over gassed, but when you start playing with barrel length, different length gas tubes, heavier buffers and lighter/heavier springs it can make it difficult to get a rifle to cycle properly. The buffer, spring, and adjustable gas block (if equipped) is used to "tune" your weapon so you only get enough gas to properly cycle your weapon with chosen ammo. The BCG can also affect it if you have an auto vs semi, the auto is a bit heavier, but many choose to change the other parts as they are cheaper.

 

[backyardtndeer]

Could be something as simple the gas block leaking too much. Put a little gun grease around the gas port on the barrel before tightening the block.

 

[LanceS4803]

On all three rifles, same:
barrel length (same brand)
gas system (mid-length, same gas tubes and gas blocks)
Bolt carrier group
buffer/spring

I found my stash of muzzle break washers, so time to pull the whole thing apart, maybe re-pin the gas bloc/tube.

 

[rifle02]

I can't say right now I am familiar with Hanson profile. I will say this the diameter of the barrel at the Gasport is also a factor. Gas Port size depends on the length of gas system as well as the barrel diameter at that point. There is a chart somewhere on the internet that I have seen detailing gas Port size for all different factors. You might search for that chart and get an idea, but somehow .070 seems small in my mind. Also from your post I think you said your gas block was leaking? That is definitely a problem.

 

[DaveB]

Sounds to me as if the rifle is just a complete bust.

Send it to me, I will give it a good home.

Or

if you have a buffer spring that is of lesser strength try that. Or, look it up to see if carbines have springs that are of lesser strength.

 

[19_A_CPT]

I had to drill my barrel's gas port out from the .068 to .093 for both my 450 Bushmaster builds. Although i dont think you need anywhere near that amount of gas, upping the volume may be your answer if the alternative choices don't yield you success.

 

[LanceS4803]

So, as an experiment I coated the gas block/gas tube with oil. I plugged the receiver end of the gas tube and the chamber and pressurized at the muzzle.
Lo and behold, air came blasting out from around the gas tube where it is inserted into the gas block.
I found the gas block hole spec and gas tube diameter spec, and will tear it down and measure. Hopefully I have an undersized gas tube and it will be an easy replacement. Otherwise, I might just upgrade to an adjustable gas block.

 

[backyardtndeer]

So, as an experiment I coated the gas block/gas tube with oil. I plugged the receiver end of the gas tube and the chamber and pressurized at the muzzle.
Lo and behold, air came blasting out from around the gas tube where it is inserted into the gas block.
I found the gas block hole spec and gas tube diameter spec, and will tear it down and measure. Hopefully I have an undersized gas tube and it will be an easy replacement. Otherwise, I might just upgrade to an adjustable gas block.

Thanks for reporting back with your updates, may be helpful to someone else. I might be inclined to swap out the block/tube assemblies from one of the others before buying another.

If found to be a defect in the block or tube, would be curious how aero would respond.

 

[LanceS4803]

Well, no new good info.
From a poster on arfcom who looked at the mil spec drawings, the regular AR sight base hole is .1805" tolerance +/- .001
Gas tube diameter is .180", +/- .0006

Using gauge rods, the gas block hole is .1785
Caliper on the gas tube is a very consistent .178
The fit is incredibly tight, as might be expected. Viewing through a bore camera didn't reveal any internal defects in the block.
I'm out of ideas. Don't really want to start taking apart other rifles, pulling flash hiders, etc. to put them on this barrel and then reverse everything.
BTW, a Hanson profile barrel is slightly thicker full length, without being overly heavy.

 

[infoman jr.]

If you've exhausted all options, and it sounds like you have gone through the steps that make sense, you could always drill out the gas port.

 

[Omega]

Ok, it may sound stupid, but are you sure the hole is facing the right way, down into the block? I have also read where some have sealed the tube into the block with some Loctite, but it sounds like it shouldn't need that. How about posting some pics, to get an idea of what you are dealing with.