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Posted: 1/10/2026 11:08:50 PM EDT
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So, I am on the hunt. I found out the unfortunate way that a BA 12.5" 308 Barrel with Carbine length Gas does not accept drop-in/Clamshell type handguards. Through some research I found that there are two ways of measuring gas system length for 308AR barrels. The more popular way being to size the journal a bit closer to the receiver (~3/16") in order to use a "standard" length AR15/Small frame gas tube. The other way is the Armalite spec for gas systems. This spec is the correct length to be compatible with AR15/small frame drop in furniture. My question is, which manufacturers are known to produce 308 AR barrels with the Armalite spec for the gas block journal? I've seen Faxon's 13.9 mentioned, but nothing about their other barrels. Fulton Armory seems to offer a specific gas tube for their 308 barrels, but not sure if it is a length thing or some other factor. There are a few mentions of Wilson Combat using the AR10 spec, but nothing in the way of solid confirmation that I've found yet. I am not picky on exactly which length to use, just trying to make a fun, handy rifle on an M5 receiver set. Prefer to avoid stainless, good chrome lining or nitride would be best. |
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Originally Posted By NTXGlocker: what barrel? Chose the barrel. Then you know journal size. I choose 16" barrels with rifle gas and .750 gb journal. Seems like you should select a barrel. That'll answer your question, that is if I understand the question. He's talking about the distance from the receiver front, to the gas block journal, not the journal diameter. The Armalite journals are moved about 3/8" further toward the muzzle. If you use one of those barrels with a conventional length gas tube, the tube doesn't go into the gas key very far. Tony |
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I have an M5 I built with a Faxon 16" barrel (mid length gas) a few years ago. I bought all the parts I needed before learning that there is a slight difference in gas tube lengths. I ended up opening the AGB screw all the way for reliable operation, so I took a closer look at how far the gas tub protoobes into the upper. I don't remember where I bought that mid length gas tube but it does look a bit shorter than what research said it "should" be - but it works fine at this adjustment so I just let it ride. |
"I guess you already know that there are angels masquerading as people walking around this planet and your mom was the bravest one of those." - Idgie Threadgoode, Fried Green Tomatoes
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Originally Posted By TonyRumore: He's talking about the distance from the receiver front, to the gas block journal, not the journal diameter. The Armalite journals are moved about 3/8" further toward the muzzle. If you use one of those barrels with a conventional length gas tube, the tube doesn't go into the gas key very far. Tony I was genuinely hoping you would be one of the ones to show up here. I had half baked plans of making a sort of "swollen up" version of the "Gun from Heat" with the BA 12.5" Carbine length that I acquired, but have been stymied by this discovery on gas block journal placement. Do you have any pertinent intel on manufacturers other than Faxon and Fulton that use the Armalite spec for journal placement? Originally Posted By piperpa24: I have an M5 I built with a Faxon 16" barrel (mid length gas) a few years ago. I bought all the parts I needed before learning that there is a slight difference in gas tube lengths. I ended up opening the AGB screw all the way for reliable operation, so I took a closer look at how far the gas tub protoobes into the upper. I don't remember where I bought that mid length gas tube but it does look a bit shorter than what research said it "should" be - but it works fine at this adjustment so I just let it ride. That is two specific Faxon models "known" to use the longer spec now, which is helpful. If, I can decide to stomach/deal with one of their profiles. I know they make nice stuff, just haven't quite decided if I feel like dealing with the visible aesthetics of their profiles. |
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Thank you for the extra information. Shows that there is even more variation than I realized between DPMS and Armalite specs. Unfortunately, none of the MFGs I've been able to find list or show the dimensions I need to make a purchase decision without having to send a bunch of emails and try to communicate what I'm actually asking to whichever poor bugger is answering the tech/customer service email address. |
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