MarkLeupold

X-Caliber Barrel & Manufacturing 18" rifle length gas system, 6.5 Creedmoor, AR10 HBAR contour (.750" gas journal) Port is .081" or .082" when you choose their "default gas port" (the other option is slightly oversized for use with an adjustable gas block) Runs just fine on a JP FMOS BCG, A5 extension, H3 buffer, and Tubb's AR-10 Flatwire spring

Small update on this project X-Caliber barrel came in. 18" RLGS, 6.5 Creedmoor, .750" gas journal. Port is either .081" or .082" (Tubb's flatwire, H3 buffer, A5 extension). I ended up putting a Superlative gas block on it, mainly for the fact that it was a clamp on block that didn't look like a polished turd more than needing the adjustability. That being said, I did play with the adjustment a little bit. I set the block while shooting Hornady Match 140gr ELD-M. The rifle ran with the port full open and didn't feel like it was beating itself up at all. I turned it down without my suppressor first and I think I turned it down to within 3 clicks of closed before the rifle failed to lock back, all the while ejecting pretty strongly to the 3-4:00. I didn't really notice any change in the ejection strength, pattern, or recoil impulse while turning the gas down. I opened the gas up two clicks, added the suppressor, and the rifle started displaying some occasional ejections to 1:00 with the rest staying around 4:00. I opened the gas up three more clicks (now 8 clicks from closed, 10 clicks from full open) and left it there. Ejection has remained strong. I shot 95 rounds in a match today and it never hiccupped once.

Small update: While still curious on the 16" RLGS port size, I have decided that I'll most likely be getting an 18" RLGS 6.5 Creedmoor barrel. I believe that I can spare two inches of barrel for a couple more powder options, specifically RL16, which I have a good bit of on the shelf already.

A side note of interesting information that I found: Criterion puts a .086" port on their 16" RLGS 6.5CM Hybrid barrels. Of course, this is without any knowledge of what recoil system they'd be designing for, but it is another data point to have.

I'm not so set on the RLGS if a mid length gas would work better, that's just generally what the internet will tell you a 6.5 Creedmoor needs because port pressures are higher than 308... It's not wrong...I currently simulate the 6.5 Creedmoor about 30% higher port pressure for mid gas than the 308 does. Move the port out to a rifle length and the pressures are similar to my mid gas 308, if not a little lower for the Creedmoor. As for dwell time, the 308 has roughly .23 ms (0.00023 seconds) of dwell time whereas the rifle length system has about .1 ms (0.0001 seconds) of dwell time, just under half. Short dwell time has come more into vouge recently with barrels like 16" RLGS and even 14.6" RLGS from FOR Systems, albeit on 5.56 rifles. It supposedly has more overlap in the operating parameters when moving from suppressed to unsuppressed (especially with high back pressure cans), so it's gained quite a lot of attention for that. In my reading, the rifle needs enough gas volume to fill/accelerate the carrier, and if the port is the correct size, even short dwell barrels should function. That being said, I understand the large frame guns are a different beast and I am still learning. What makes the short dwell barrels tricky? I have a HuxWrx suppressor, so I don't usually account for it affecting rifle function an appreciable amount; lengthening the gas system in my case is more of an effort to create another soft shooting rifle that has balanced function and doesn't want to tear itself apart. I'm also not vehemently opposed to going to an 18" barrel, which is [barely] more common on 6.5 Creedmoor, but I grew fond of the 16" barrel with the suppressor in tree stands this deer season and it's a good thought experiment to keep my mind occupied for the winter months.

The LR308 build I did about two years ago continues to run flawlessly and I am very satisfied with it...but that being said, it's probably time to put another in the safe. As always, gas port is the question. I emailed X-Caliber barrels and their sales team wasn't able to get me a number, but did say that I could request a port size on a barrel. I would really like to know what they would put on the barrel standard, but if I can't be sure, I'd like to spec out my own. The build would include: Barrel: 16" 6.5 Creedmoor Gas: Rifle length Gas Journal: .750" or .875" I haven't decided yet Buffer: H3 AR-15 (3-1/4" length, 5.4 oz) Buffer Tube: A5 (7-5/8" depth) Buffer Spring: Tubb's SR25 flat wire Carrier: JP Full Mass So, my references based on research on this forum: An 18", 308 Win, RLG, .750" journal needs a ~.095" port [.001566 in³ volume, 5.5" dwell] A 20", 308 Win, RLG, .875" journal needs a .090" port [.001803 in³ volume, 7.5" dwell] A 16", 308 Win, RLG, .750" journal "probably needs bigger than .094" (later found in a different thread that this would probably need to be maxed out at .125") [.002712 in³ maxed out, 3.5" dwell] The 16", 308 Win, RLG, .936" Fulton barrels needed a .105" port to run right [.002719 in³, 3.5" dwell] A 20", 6.5 Creedmoor, RLG, .936" journal "could use a...port diameter of .070" [.001293 in³, 7.5" dwell] An 18", 6.5 Creedmoor, RLG, .875" journal, .087" port was "too much gas" (Seekins AGB turned in 2.5 turns for good function) [.001816 in³, 5.5" dwell] A 20", 6.5 Creedmoor, RLG+2", likely .875" journal, .088" port after being modified by PROOF for function [.001858 in³, 5.5" dwell] A 20", 260 Rem, RLG, unknown journal, .071" port (probably getting opened up to .080-.090") [assuming .936" journal - .001330 in³, 7.5" dwell] My supposition from this information is that I need roughly .002000 in³ of port volume. This equates to a .091" port on the .875" journal or a .102" port on a .750" journal. If anyone has any experience with a 16" rifle length gas 6.5 Creed/260 Rem, I'd love to hear about it!

For me, it was seeing posts every day about how "AR-10's are so hard to tune," "You need an adjustable gas block for a large frame rifle," "building an LR308 sucks and mine wouldn't run for anything so you should just buy a complete one." I just wasn't willing to accept that answer, so I set out on my first to prove them wrong. About a year ago, I started researching gas dynamics and how it pertained to the direct impingement gas system. I had hoped that I could simplify the whole system as a dynamic equation since it really is just a multiple spring dampened mass problem. All that needed to be solved was the initial force and duration of the gas on the carrier. As a simplified system, I didn't figure it would be perfect without a lot of refining, but it would probably get me close. That researching did lead me here, where I found the legwork done for me on so many common calibers and configurations. I still have dreams of completing that work, more for my own amusement than anything, but also to help other people online see that the large frame rifles aren't THAT hard when done correctly. Maybe they have to be a bit more meticulous in their research and parts selection than with an AR-15, but they don't require an adjustable gas block and they sure as hell can be tuned reliably. And look at that: my first one went off without a hitch. Even less problems putting it together and running than my AR-15 did. So I applaud you, sir, for your effort and contribution to the ones willing to listen.

I would love to learn more about how you run these numbers and what you're looking for when you do. Is the cylinder volume the total volume of the barrel and gas system to the gas port?

Of course.

Yep, you're right. Bonehead move. It's a midlength gas system. 11-3/4" gas tube

Thanks for all the welcomes, guys!

New info: Lilja 16.25" .308 Win barrel. This barrel has a 1-8" twist, a .750" gas block journal, and a .089" gas port. JP FMOS BCG + LAW folder dingus H3 buffer (3-1/4" length) Tubb's AR-10 flat wire spring A5 buffer tube (7-5/8" depth) Runs like a champ. Maybe slightly overgassed if 2:30 ejection means anything on the large frame rifles, but it shot extremely smooth today.

Got the new rifle put together and out to the range today for the first time. I did make some modifications to the original plan, specifically the barrel, and picked up a Lilja 16.25", 3 groove, 8 twist instead. I also swapped the H2 for an H3 buffer as is recommended around here. This barrel has a .750" gas block journal and a .089" gas port. The gas and recoil system is as follows: 16.25" barrel (measured from the end of the receiver extension to the muzzle), .750 gas block journal, .089" port, midlength (11-3/4" long gas tube) gas system JP FMOS carrier LAW folder mechanism (I figure the little doohickey at the rear adds a couple ounces to the reciprocating mass) A5 receiver extension (7-5/8" depth) H3 buffer (3-1/4" long) Tubb's AR-10 Flat Wire Boresighted, zeroed, then put three shots under an inch at 100. The next five went in two inches (but if you take that pesky flier out, the other four were inside .75"). Made easy hits out to 600 yards today with GGG .308 175gr Sierra MatchKing ammo, but the biggest thing about this build is how soft-shooting it is! I started shooting long range with a Remington 700 SPS in .308 back when I was in junior high. The thing probably only weighed 9 pounds or so and I remember that thing feeling like it was kicking the snot out of me. Granted, I have become a more seasoned shooter since then and learned to handle recoil a lot better, but this thing just felt so smooth and light. It was awesome! I think I've got the large-frame bug...

Yep, mine's also a .750" gas block. Carbine length tube is the AR-15 length, so it's pretty short. Gas tube looks to end in about the right spot on the cam pin cutout. I'm way more comfortable with drilling around on a gas tube if it does need to be opened up some! 😅 I'll get her running right and then start all over when I get my suppressor for it.

Awesome, nice and easy solutions! Yep, thanks for the concern, but I'm informed on the subject. That's why it's in its full form only in a drawing right now.

And I guess I do have a noob question to start off with: I've got a Cross Machine and Tool UHPR 11.9 Gen 2 rail on this rifle and it doesn't have any handy alignment features on it. Does anyone have experience with a good way to consistently align the handguard with the upper receiver? With any luck, this will get a clip on thermal unit in the future and I want to make sure everything is as aligned as it can be when the time comes.

My early conceptualization of the build:

Hello from central Iowa. Came across 308AR.com this year while planning out my newest rifle build and, boy howdy, there's a lot of good information here I don't think I could've found anywhere else! My build is pretty much complete, just need to get it out and test fire it once I get the time to. The important stuff: Aero M5 upper and lower 11.75" Bartlein .308 Win barrel, carbine gas, the port was in the ~.082 size JP High Pressure Full Mass BCG VLTOR A5 buffer tube Carbine H2 buffer Tubbs AR-10 flat wire spring (I came across the recommendation of at least an H3 buffer and no flat wire springs too late) BRT EZ-Tune gas tube, they gave me a .062 gas port for my setup. We'll see if it works. It also has a LAW folding adapter on it. I'll certainly be around trying to soak up information and probably asking a few noobish questions since this is my first venture into large frame rifles. I'll definitely be back later when I try to suppress this rifle with a CGS Hyperion QD, preferably without an adjustable gas block...it'll be an adventure, and I'll need some guys that know more than me!