New guy, first large frame AR, waterboarding info

[Argos1923]

Hello all,

I'm located in south Texas near San Antonio. I've been shooting for about 15 years, got into gas guns in the last 4 years, assembled a few AR-15s of various shapes and sizes (and shot them), and just assembled my first large frame AR. I've had some cycling issues with it and reading up on this forum has been very informative and interesting, so I thought I'd join up and answer the waterboarding thread here.

1) I do not have the Armalite recoil system

2) Buffer weights: 4.6, 5.4 oz

3) Buffer length: 3.25"

4) Buffer tube internal depth: 7 5/8"

5) Springs: Sprinco red, LMT 308

6) Gas system length: rifle

7) Gas tube length: 15 1/8"

8) Photo into magwell:

9) Photo of bolt face:

10) BCG clearance:

11) Gas journal diameter: 0.750"

Gas port diameter: 0.073" (nearest I can measure with calipers, a 5/64" drill bit will not fit)

12) Malfunction: Issues cycling with 146gr M80. Hotter 168gr .308 works fine. M80 works fine with a suppressor.

The upper is a Larure 18" Stealth 7.62 ultimate upper and the lower is an LMT MARS-H. I initially tried shooting the M80 with an Odin Works adjustable gas block and the LMT 308 spring with H3 buffer, and I either got no feeding at all, or jams. I diagnosed the gas system (block alignment, leaks, carrier/tube interface, gas rings) and am left with the conclusion that it's just undergassed.

The LMT spring is longer but with a lower spring rate than the Sprinco red, but the tension in battery is definitely higher. I switched back to the Larue fixed gas block and the Sprinco red/H2 that comes with the upper kit, and now it seems to feed M80 decently, with case ejection at 4:30 - 5:00, but won't lock back on an empty mag. With a suppressor on it cycles and locks back reliably, with case ejection at 3:00. Any weaker spring I could try than the Sprinco red, or would it not store sufficient energy to feed a round and go into battery? I also have an Armalite EA1095 which I haven't tried, but it seems stiffer and higher tension in battery than either the Sprinco or the LMT.

Thanks for reading and nice to be here.

Edited February 18, 2022 by Argos1923

[98Z5V]

2 hours ago, Argos1923 said:

The upper is a Larure 18" Stealth 7.62 ultimate upper

One important detail skipped in your post, and an important one.  What's the gas system length?  You nailed everything else, recoil system, all the other details.  Gas port diameter. 

What's the gas system on this gun?  It's either mid-length, or rifle-length.  That info right there will give me the information that I need to inform you on gas port diameter. 

EDIT - for adjustable gas blocks, you ALWAYS start with those things WIDE OPEN, maxed the fuk out on adjustment.  If you don't, you'll never know if the gas port diameter is large enough in the first place.  My $0.02 on adjustable gas blocks.  I don't own one, and never will, suppressor or not.

Edited February 18, 2022 by 98Z5V

[Argos1923]

11 hours ago, Argos1923 said:

 6) Gas system length: rifle

It was buried up there somewhere, but it’s rifle length gas. It seems to use normal AR-15 length tubes. The BA rifle length tube I bought for the adjustable gas block is the same length as the Larue gas tube that came with the kit. 

8 hours ago, 98Z5V said:

for adjustable gas blocks, you ALWAYS start with those things WIDE OPEN, maxed the fuk out on adjustment.  If you don't, you'll never know if the gas port diameter is large enough in the first place.

With the adjustable gas block, I did have it all the way open to start during the first range session. I later tried it 3/4 and 1/2 open, but no improvement. Worried that the adjustable block somehow had more restriction even when open than the fixed gas block, I’ve since switched back to the fixed gas block which I ran for the second and more successful range trip. 

[unforgiven]

Welcome from Indiana brother.

Is the oldest VFW still open? I helped my grandfather at the original Marketa. They were on the westside by Casiano park if you know the area.

[98Z5V]

14 hours ago, Argos1923 said:

6) Gas system length: rifle

Gas port diameter on an 18" rifle-gas .308 Win gun with a 0.750" gas block journal size needs to be 0.093"~0.096". 

[Armed Eye Doc]

Welcome to the forum.  I thought that gas port sounded a little small.  @98Z5V is the gas port whisperer.

[Argos1923]

11 hours ago, unforgiven said:

Welcome from Indiana brother.

Is the oldest VFW still open? I helped my grandfather at the original Marketa. They were on the westside by Casiano park if you know the area.

Thank you. I’m not familiar with that area of town, I’m outside of the city toward hill country. But I did use to live in Indiana, about 30 mi east of Bloomington. 

Edited February 19, 2022 by Argos1923

[Argos1923]

11 hours ago, 98Z5V said:

Gas port diameter on an 18" rifle-gas .308 Win gun with a 0.750" gas block journal size needs to be 0.093"~0.096". 

Oh darn. Thank you for looking at my specs and for the information. How do you deburr the port after drilling it out?

[98Z5V]

2 minutes ago, Argos1923 said:

 

How do you deburr the port after drilling it out?

Go slow with the bit, and there shouldn't be any burrs.  If you have a burr after that, the first projectile out of the barrel will take care of it.

[unforgiven]

4 minutes ago, Argos1923 said:

Thank you. I’m not familiar with that area of town, I’m outside of the city toward hill country. But I did use to live in Indiana, about 30 mi east of Bloomington. 

Bloomington is a nice town fished lake Monroe. 

[Albroswift]

@Argos1923Great job on the waterboarding, got to be the best yet. Agree gas port sounds small, and the symptoms reflect that. Welcome to the forum.