Armalite Adjustable Gas Block Help

[Silver]

So, I installed some floating handguards and non-adjustable low profile gas blocks on my ARs and had mixed results.  The AR-15 survived and is firing well.  The AR-10 however has gone bolt action on me failing to feed any round (tried 5 different types -- surplus plinkers to match ammo) I put in it.

I researched the problem and one potential issue that may be the cause is that the S&W M&P10 supposedly has a smaller port on the gas tube than any similar AR-10.  FTF is not an uncommon problem with the rifle for some folks even if unmodified.

I had previous FTF problems before the mods but generous oiling and a solid cleaning seemed to fix it.  That didn't work this time around.

Buying into the theory that the port on the gas tube may be just too small, I've replaced the gas tube and paired it with a new adjustable gas block.

The Armalite adjustable gas block did not come with instructions and given the holes in the block, I'm not sure how it functions. 

Watching videos of other adjustable gas blocks it seems there is an additional hole and you simply adjust flow by tightening or loosening the screw. This one has two screws however and they intersect each other though it doesn't look like either one would fully seal off the chamber/opening of the other.

The question is, how do the two adjustment screws work in conjunction with each other? I can see the need for one and how tightening or loosening it could create the right pressures but not how the two would operate together.

Any thoughts?  Diagram attached.

Thanks in advance.

[Dan W]

I am not familiar with that gas block, but I think (read wild guess) one screw is the gas adjuster and the other is probably a lock screw

[JAK]

Based on other adjustable gas blocks I have used, the screw on the side should be the locking screw and the one in front the adjustment screw.

John K

[shooter]

 you said FTF. fail to feed. do you mean the bolt isn't going back far enough to grab a new cartridge? sounds like not enough gas. a bigger tube should have helped, but if it didn't , I think that maybe the gas port in the barrel itself is to small. adjusting the gas down will not make a difference if you need more gas,
   You might have to inlarge the gas port.

[Dan W]

   You might have to inlarge the gas port.

I agree...open up the gas port and then use the adjustable gas block to tune it in

[Silver]

Yes, FTF.  Instead of opening up the original tube, I purchased a new Armalite gas tube which I understand has a significantly larger port.  I went with the adjustable gas block in case the port is "too large."

On the FTF.  Every round ejected fine.  Out of 21-24 rounds I had two rounds halfway feed (not sure of the proper term - front half of the round was in the chamber with the lower half still in the mag and the bolt resting up against the side of the round.)  All the rest of the rounds remained fully in the mag.

Sounds like shortstroking and as you all said, insufficient gas pressure.

Thanks for the specifics JAK. I suspected something like that but wasn't sure which is which.  In the videos I watch of people tuning their gas blocks, they seemed to be loosening (they started with the screw fully or near fully closed) the one on the side through gaps in the handguards.

That said, the allen wrench that came with the gas block is rather long which seems to allow for adjusting the front screw of the gas block as if it were under a good length of handguard.

[SS_N_NE]

Because of patents on gas blocks and that Armalite seems to keep the adjusment a secret (guess you would need to contact them for specifics) it seems simpler to try alternately tightening / turning the 1 or 2 screw and realize which one is locking the other. Or, find the one that locks the other in mid turn...that screw is the lock...other is the adjuster.   Saw somewhere that Armalite uses a .099 port in their barrel for AR-10. Are you certain the port on the block and barrel are lined up? Could be a very simple but confusing issue.  Also ensure the entire gas system is free from any obstructions (silly but simple issue). Carbon from shooting or something from a cleaning (pipe cleaner/ patch/q-tip" in the barrel port, block, gas tube, port even in the gas key). With the adjustment screw backed out the gun should fully cycle. As the adjustment screw is turned in, the felt recoil should be reduced until the gun no longer cycles. I realize that typical instructions are the reverse, but the gun should be set up to cycle with an unrestricted block port. The adjustable block just slows things down by limiting gas to the BCG.

[Silver]

Learning can be so embarrassing sometimes.  Had the sudden epiphany that the gas port is in the barrel and not the hole in the gas tube lol.  Thus, if I need to open up the port a bit, that requires drilling the barrel and not simply drilling the gas tube (or replacing the gas tube with one that had a larger opening). 

Duh.

That said, went to the range to try out the new gas block and, another moment of shame, apparently I over tightened one of the bottom set screws on the gas block though I'm not sure how, I really didn't put a lot of twist on them.  Now the gas block is like the handguard, free floating. 

Anywho, back to the drawing board.

[shooter]

 did you strip out the set screws?  yes you need to very slightly open the port in the barrel,   if you need I can rethread the gas block for you

[SS_N_NE]

Drill bits come in a lot of different sizes...more than increments of 1/64".

OK....why do I bring that up? I am a machinist. I use 115 pc. drill sets, since there are fractional, letter and number bits. Don't just grab a fractional bit and drill out your barrel. First, you want to have an exact size and work in small increments. That is going to require a set of number bits or at least the right size individual bit(s). A second thing to consider is dealing with the bur created when a twist drill goes through steel. In other words there is going to be metal sticking out inside your barrel. Measures have to be taken to ensure the bit can not get across the bore and damage the other side of the barrel. Nicks and burs in a rifle bore can lead to a lot of strange shooting problems. Bur removal and bore inspection are not cheap DIY items inside a gun barrel.

Stripped threads can be drilled up a size and threaded for an oversize setscrew...or thread repaired with a heli-coil or similar thread repair system.  The thread repair kits are not especially cheap and should be done in a drill press or milling machine to ensure centered and square drill/threading for the repair threads.

[shooter]

Drill bits come in a lot of different sizes...more than increments of 1/64".

OK....why do I bring that up? I am a machinist. I use 115 pc. drill sets, since there are fractional, letter and number bits. Don't just grab a fractional bit and drill out your barrel. First, you want to have an exact size and work in small increments. That is going to require a set of number bits or at least the right size individual bit(s). A second thing to consider is dealing with the bur created when a twist drill goes through steel. In other words there is going to be metal sticking out inside your barrel. Measures have to be taken to ensure the bit can not get across the bore and damage the other side of the barrel. Nicks and burs in a rifle bore can lead to a lot of strange shooting problems. Bur removal and bore inspection are not cheap DIY items inside a gun barrel.

Stripped threads can be drilled up a size and threaded for an oversize setscrew...or thread repaired with a heli-coil or similar thread repair system.  The thread repair kits are not especially cheap and should be done in a drill press or milling machine to ensure centered and square drill/threading for the repair threads.

   add in metric bits to, plus all the ones that are just sold by the .001, 

[Silver]

My eyes suck, they used to be fantastic anywho, the set screw *seems* to thread in fine but when it is fully in, you can feel it tighten then suddenly get loose and begin to retighten.  I am unable to see anything wrong with the screw but I haven't taken a magnifier to it yet either.

I'm gathering my thoughts as what to do next.

Parts wise, I'm sitting on the original gas tube inside a non-adjustable gas block.  I also have the new, stripped adjustable gas block.

What I should probably do before I take any non-reversible action (widening the gas port) is to make 100% certain I'm aligning the gas port and gas tube perfectly.  If I can determine that and the rifle still isn't operating correctly, then I can feel better about drilling into the barrel.

I really appreciate the cautionary notes about "just drilling the hole wider" because I definitely wouldn't have considered everything you mentioned.  "more than increments of 1/64" That was comforting to hear also.  I was doing some math and trying to figure out how 64ths would get the necessary increments needed and I was concluding they wouldn't.

[SS_N_NE]

My eyes suck, they used to be fantastic anywho, the set screw *seems* to thread in fine but when it is fully in, you can feel it tighten then suddenly get loose and begin to retighten.  I am unable to see anything wrong with the screw but I haven't taken a magnifier to it yet either.

Yeah...my eyes have never been good. Glasses, bifocals and old age don't help much. Optivisors are my friend. I would definitely pull the block and take a look at things to be sure what the issue may be especially since you could be making a problem that you do not have yet.  Almost sounds like the set screws are short and passing completely thru the threads...which seems odd. If the screws are passing thru the threads, maybe a trip to the hardware store for some longer setscrews could fix the problem.  Is the block the correct size for the barrel...tight slip on fit? Not loose (rattle around loose)? a wrong block could be indicated by short setscrews. 
I would measure the distance of the barrel port to the block stop on the barrel and then measure the block port to ensure the ports line up. It is good practice to dimple the barrel by drilling shallow marks into the barrel to better maintain the setscrews. As you know from removing the original block...the mounting systems are very different.
And, a good point by Shooter (guess I didn't want to be too confusing) drill bits come in any size needed. MSC, Enco...etc are machine supplies that have all size of hole making bits.
Don't be afraid to seek out a gunsmith...sometimes their experience is cheap compared to the results of inexperience.

[Silver]

Yeah, the block is the right size.  It's not a clamp on.  It is not tight but it is snug (I can slide it on without marring the barrel but it does take a tiny bit of pressure).

Thanks for the rest of the advice.  I'm going into "slow down, be patient" mode because everything up til now has been reversible.  I'm going to be very deliberate before I take any non-reversible actions.

Thanks for your guys' advice.

[Silver]

Fixed.  Even though I am lacking appropriate measurement tools, I was able to determine the gas block needed to come forward about a 32nd or 64th of an inch (guesstimate).  My AR is back to semiautomatic and kicking like a mule.  Feeds and ejects perfectly.  Glad no drilling was not needed.  Lots of lessons learned.  Thanks for the advice and suggestions.

[shooter]

oops, forgot to tell you to space it the thickness of the front handguard cap. they sell small washers that are about .034 thick. if you don't run a handguard, cap the gas ports wont line up. cant believe I forgot that

[Silver]

I do think I read that elsewhere but forgot about it. One of my lessons learned is that many things on a gun are measures in thousandths of an inch...lol.  probably common sense to a lot of shooters.

[SS_N_NE]

cant believe I forgot that

uuuhh...that makes two of us!  Seems the savy low profile block manufacturers realize that hand guards are likely to swap out and drill the block port oversize. You would think the adjustable items would be on to that trick.