Anyone now which gas tube for a 18” Faxton 308 barrel with “rifle” gas system with an Aero upper. Would a 15,125  be the right one, or the older style 15.375

The longer AR10 one. I got mine from Wilson Combat.

Faxton?

https://www.ar15.com/forums/AR-15/-308-AR-Gas-Tube-Length-Question-/121-774074/?page=1&anc=bottom#bottom

https://www.ar15.com/forums/AR-15/Time-to-diagnose-a-sick-rifle-pt-2/121-772760/?page=1&anc=bottom#bottom

https://www.ar15.com/forums/AR-15/6-5-Creedmoor-build-not-running-until-I-cut-buffer-spring-/121-773940/

https://www.ar15.com/forums/ar-15/New-LR-308-Build/121-746059/?r=7997129&page=1&anc=7997129#i7997129

https://www.ar15.com/forums/industry/308-Big-Gunner-308-barrel-standard-or-Armalite-gas-tube-/700-292403/&page=1&anc=bottom#bottom

https://www.ar15.com/forums/ar-15/AR308-Incorrect-Gas-Tube-Length-/121-763130/?r=8261731&page=1&anc=8261731#i8261731

15.5"

@bfoosh06
Faxon uses the Armalite gas port location for their large-frame AR-10 barrels.  if you use the common AR-15 gas tube with a Faxon AR-10 barrel (308 or 6.5CM), it will end up looking short, like that high-mileage pic.  Using the Armalite AR-10 gas tube with a DPMS-pattern AR-308 barrel that uses the Armalite AR-10 gas port location just makes the gas tube the right Goldilocks length inside the upper receiver for that application, sort of a "dwell time" thing of the the gas tube inside the gas key with the bolt carrier moving rearwards before it uncouples from the GT.

I personally have not had to enlarge Faxon DPMS-pattern large-frame AR-10 gas ports, but then I've always been using the Armalite AR-10 gas tubes: rifle, or Armalite's version of carbine which corresponds approximately to the AR-15 mid-length gas.

It does appear that depending on temps, ammo, if you are on the ragged edge of "will it work, yes or no", and the builder insists on using the AR-15 GT instead of the Armalite AR-10, then when the temps drop, or you switch to a weaker batch of ammo, like how Tula steel-case is for AR-15's, it may be necessary to enlarge the gas port since you're having a shorter dwell of the gas key sucking on that GT.  Similar to if you're having too short of a dwell time on the barrel, you have to compensate by drilling the gas port.

Faxon's premise for the AR-15 GT in AR-308 usage is that even with the short suck, there is enough energy, momentum imparted to the bolt carrier to successfully do its job.

There are some other barrel mfrs that also use the Armalite AR-10 gas port standard, Anderson is one of them, although almost nobody uses Anderson barrels for their large-frame AR's, so that never comes up.   I think Wilson is another, although I don't have first-hand knowledge on that.  Using the Armalite AR-10 gas port location allows the use of AR-15 drop-in handguards with AR-10's, since it compensates for the thicker DPMS LR-308 barrel nut, Delta ring, which pushes the handguards out forward that extra 3/8".  The Old-School Armalite AR-10's used the AR-15 drop-in handguards, to facilitate that, with the bigger-all-around barrel nut design, required that extra 3/8" of A2 FSB gas port location.  or did the AR-10 pre-date the AR-15?  dunno.

BA, Criterion, and Rainier use the AR-15 gas port location for their DPMS-pattern AR-308 barrels.

my apologies for the Wall of Text.

Thanks guys, I knew you guys would know

Ok found one in stock.  14.95.   12.00 shipping ouch

you're lucky, most of the time, when you need one, it's out-of-stock for 6 months, everywhere.  no shit.  even pre-pandemic.

Mind sharing the source? I have one on-order from Armaltie, but it's out-of-stock.

here's one.  Fulton Armory.

Awesome!

FYI for those looking, Wilson Combat's rifle-length tube is 15.5". I'm going to go with Fulton's since I need some M14 spare parts.