Hi all,
I'm slowly putting my AR10 together, and have finally received the missing piece. My JP adjustable gas block, (JPGS-5B) arrived.
I went to test fit the parts,  and it appears the gas tube is about 1/4 too long,  (generic ML GT from from CDNN)
If I seat the GT into the block with the pin,  and attempt to mount the GB,  then the bolt won't close,
if I let the bolt close,  obviously it pushes the GB forward of the port in the barrel,

I'm sure someone has encountered this,  before, so what did you find?  an obstruction in the gas key<  (didn't see any)
an out of spec GT? an out of spec Gas block?  call me befuddled.

AP receiver and BCG, 18" mid length barrel, ML GT and JP adjustable gas block.

(there's mention of .025 play, to accommodate conventional handguard retainers,  but this is about 1/4 of an inch?)

Thanks for the education.

are you sure you have the gas tube roll pin through the GT roll-pin hole, and not in front of the GT?

i think it's highly unlikely that you got an Armalite AR-10 gas tube by accident, since those are hard to find and usually out-of-stock everywhere, and sort of rare.

Sounds like you got an armalight ar10 gas tube.

The handguard retainer is about 1/32" in thickness so you'll need to account for that when placing your gas bock.

Try verifying that the gas block's port aligns with the gas tube's port, using a straightened paperclip to check that the passages align.  Then verify that the barrel's gas port is aligned with the gas block's passage.

Also, hold the carrier and gas tube in your hands and verify that they will smoothly telescope together.  I've seen too tight gas key bores and oversized gas tubes.

Also, on 308 builds it's easy for the gas tube to be flexed by the barrel nut or handguards.  If the gas tube shouldn't touch the barrel nut or inside of the handguards at any point.

UPDATE

I measured the GT and it's about  12"  to the flared part that goes into the gas key,  so the total length is 12" 1/8  long.  (some research on truenorth arms says it should be 12.5")
I measured the distance from the chamber end of the barrel to the gas block shoulder,  and it's 9 7/8 "  (like a mid length AR 15 barrel,  Hmmm...)
according to some research on the web,  a true ML AR10 is supposed to be 10 1/8",  so there's my missing 1/4"

it appears I have a 308 barrel, with a 556 mid length gas system.  (yes it really is a 308 barrel)  
Now to see if it will go together with a 556 ML gas tube.

so apparently,  my frankenbuild is even more of a monster,  with that extra .25" of gas pressure,
it's sure to cycle better than your plain jane, common, un remarkable AR10 mid length systems.

Who made the barrel?

The end of the gas tube should end at the center of the cam key cut-out like this:




Here is a list of gas tube lengths. This list is not 100% complete. Using gas tubes that are too long can prevent the BCG from going into battery. Tubes that aren't long enough can cause short stroking.

DPMS LR-308 Gas Tube (also AR15):

Pistol 6-5/8 - 6-3/4
Carbine 9-3/4 - 9-7/8
Mid-Length 11-3/4 - 11-7/8
Intermediate 13-3/4
Rifle 15-1/8 - 15-1/4
Rifle+2 17-1/8


Armalite Gas Tube:

Super SASS Carbine (308) 11"
Mid length Carbine (308) 12-1/16"
Rifle (308) 15-1/2 "
Carbine (223) 9-3/4 "
Mid (223) 11-3/4"
Rifle (223) 15-3/16"


Oddballs:

Wilson Combat Match Grade Intermediate 13-1/4"
Knight's Armament SR-15 Mod 1. Mid-Length 13"


There are a couple of Mfg's who make DPMS patterned barrels/extensions and drill the gas ports at Armalite lengths (Fulton Armory Criterion).

For gas block positioning, the vast majority of barrels (but not all) require a set-back of the gas block from the shoulder. This is to take into account the thickness of a handguard cap for drop-in or two piece handguards. The standard set-back is 0.030", but I've seen it vary between 0.020" to 0.035". The best way to ensure alignment is to use a dimpling jig (you can make your own out on an old gas block), but in lieu of that, this is what I do for set screw blocks. This is done before the barrel and/or gas tube are installed, so you may have to rely on measurements and feeler gauges or other methods if those are already installed.
Remove the set screws, rotate the gas block 180 deg. and center the breach end set screw hole over the gas port. Scribe or mark the barrel at the front of the gas block to help with alignment during installation (the breach end set screw is how the gas passage between the gas port and the gas tube is typically drilled in the gas block and should be in perfect alignment). For rotational clocking, I use the cross slide table on my mill to mark TDC on the barrel, but you can use a business card wrapped around the barrel against the shoulder as a flexible square. Line up with the center of the gas port, and mark the TDC of the barrel just in front of where the the gas block will be.

Thanks for the photo,  it reinforces what I've discovered,  when I place the block appropriately (1/32) i front of the GB stop,
the gas tube extends beyond the center of the cutout,  it actually goes just a hair past the butt end of the cutout.
If I was to have it end precisely in the center of the cutout,  the tube is actually over 3/8ths too long.

it is a Ballistic Advantage barrel,  it was in stock at a dealer in a small city,  when all others online were unobtainium,
I don't know if it's a good one from way back,  or potentially not.  I'll try a 11.75 ML GT and update this sometime in the future when it's in my hands.

Best

no,  can't post photos,  I'm a new account and I haven't bothered to set up a photo sharing site.
if you look at the picture in the thread and imagine that my GT ends at the 4" 20/32nds mark,  rather than where it is in the photo,

I opened a ticket with BA techs to see if this is a normal condition of their 308 Govt profile 18" barrels  (the base to Gas block ridge distance)

Best

BA barrels use AR15/DPMS308 length gas tubes. It does seem that you have a Armalite mid-length carbine (12-1/16") gas tube, when you need a AR15/DPMS308 mid-length (11-3/4). If you can't find one locally, Damage Industries sells them for a decent price and it should be free shipping.

If you do have an armalite tube I will buy you a membership and send you a mid length tube for it. They are out everywhere.

I heard back from BA,  This barrel is supposed to use an AR15 midlength gas tube not an AR10 ML gas tube.  
the next question regards what AR10 tube I possess,  the one I have is over 12" long, so perhaps it is one of those rare ones?

Im not sure but I hope its the unicorn I am looking for. I bought an LBE unlimited one while waiting for a armalite ar10 to come into stock and it is just past the center of the cam track cutout so this was worth a shot for me and my ocd. Thanks for the info.

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