Gas tube length discrepancy

[Hoi Toider]

Folks, I’m new to the forum and am building an AR in 6.5 Creedmoor. The build consists of a Palmetto State Armory complete .308 lower with collapsable stock. The upper, which I assembled from scratch, is a PSA Gen2 .308 receiver, a Faxon 16” mid-length 6.5 Creedmoor barrel, a Toolcraft dual ejector BCG, and an AR Stoner set-screw secured, non-adjustable gas block and mid-length tube. The forearm is a Magpul MOE which requires the front retaining ring which requires a tiny bit of space between the back of the gas block and the shoulder on the barrel. Upon test firing the rifle, the action wouldn’t even try to cycle. It stayed in battery and had to be manually recocked and dropped to load another round. I assumed this was a gas system problem. I reexamined my gas block installation and decided to set the block further back on the barrel, which required filing a few 1/100s of an inch off the back of the block in order to center the barrel gas port in the opening in the gas block. With this adjustment, the rifle now functions perfectly about 80-90% of the time, but once in a while I get a stovepiped empty casing and a failure to reload. This seems to happen once out of every 10 shots I fire. Otherwise, it goes along fine.

This morning, I read through the lengthy post on this forum about the vagaries of finding AR308 parts that work together, especially as regards the gas and buffer systems. I weighed my carbine length buffer (3.84 ounces) and measured the length and no. of coils on my buffer spring (11.25” length, 29 coils). I also observed while cleaning the weapon that my gas tube lacks 7/32” reaching to the center of the bolt cutout milled into the inside top of the receiver. Obviously, the gas system and buffer setup work to a great degree, but just as clearly the recoil is either too light or too heavy to operate the weapon with 100% reliability. Is the discrepancy in the length of my gas tube (11.75” overall per Midway) enough to make the action unreliable? Would I profit by using a heavier or lighter buffer? Is it even possible to get a mid-length gas tube 12” long instead of 11.75”? BTW, I have been firing Defender Ammo hunting loads with 130gr Sierra Game King bullets at an MV of 2830fps (out of a 26” bbl) and am using a D&H 5-shot mag and an ASC 5-shot mag with no apparent difference in function. Other than “Don’t make a 308 platform upper your first attempt at building an upper” is there any other advice you folks can give me. I would appreciate any guidance you can offer here.

[Matt.Cross]

5 hours ago, Hoi Toider said:

Folks, I’m new to the forum and am building an AR in 6.5 Creedmoor. The build consists of a Palmetto State Armory complete .308 lower with collapsable stock. The upper, which I assembled from scratch, is a PSA Gen2 .308 receiver, a Faxon 16” mid-length 6.5 Creedmoor barrel, a Toolcraft dual ejector BCG, and an AR Stoner set-screw secured, non-adjustable gas block and mid-length tube. The forearm is a Magpul MOE which requires the front retaining ring which requires a tiny bit of space between the back of the gas block and the shoulder on the barrel. Upon test firing the rifle, the action wouldn’t even try to cycle. It stayed in battery and had to be manually recocked and dropped to load another round. I assumed this was a gas system problem. I reexamined my gas block installation and decided to set the block further back on the barrel, which required filing a few 1/100s of an inch off the back of the block in order to center the barrel gas port in the opening in the gas block. With this adjustment, the rifle now functions perfectly about 80-90% of the time, but once in a while I get a stovepiped empty casing and a failure to reload. This seems to happen once out of every 10 shots I fire. Otherwise, it goes along fine.

This morning, I read through the lengthy post on this forum about the vagaries of finding AR308 parts that work together, especially as regards the gas and buffer systems. I weighed my carbine length buffer (3.84 ounces) and measured the length and no. of coils on my buffer spring (11.25” length, 29 coils). I also observed while cleaning the weapon that my gas tube lacks 7/32” reaching to the center of the bolt cutout milled into the inside top of the receiver. Obviously, the gas system and buffer setup work to a great degree, but just as clearly the recoil is either too light or too heavy to operate the weapon with 100% reliability. Is the discrepancy in the length of my gas tube (11.75” overall per Midway) enough to make the action unreliable? Would I profit by using a heavier or lighter buffer? Is it even possible to get a mid-length gas tube 12” long instead of 11.75”? BTW, I have been firing Defender Ammo hunting loads with 130gr Sierra Game King bullets at an MV of 2830fps (out of a 26” bbl) and am using a D&H 5-shot mag and an ASC 5-shot mag with no apparent difference in function. Other than “Don’t make a 308 platform upper your first attempt at building an upper” is there any other advice you folks can give me. I would appreciate any guidance you can offer here.

Unless you are interested in drilling your gas port up in size, I'd recommend correcting the gas tube length as your first course of action.

[98Z5V]

To completely fix what it sounds like you're describing, you'll need to address all three things:  gas tube length (Armalite AR-10 Carbine gas tube is what you need, measuring 12 1/16" long), punch up the gas port diameter if it's not already between 0.085"~0.090" for that 16" midlength gas barrel, and address the entire recoil system. That's the only guarantee that the gun is going to function reliably 100% of the time.

That spring you listed doesn't even match any AR15 carbine spring specs.

[Hoi Toider]

Thanks for these replies.

[mrmackc]

missed that it was a 6.5

Edited January 30, 2020 by mrmackc