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Hello from NW PA


Gmtech
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Hi all

Just officially joined forum today and wanted to introduce myself. I have been reading threads here for awhile now.  Anytime I search something on Google your forum comes up with lots of ideas.  I purchased my first ar (5.56) about a year and a half ago and wondered why I hadn't done it sooner.  I also have a 6.5cm bolt gun and reload for both.  Before I got the 6.5 I was gonna get a 308 rifle but the more research I did and with the goals I wanted to achieve the 6.5 made more sense....but I still wanted that 308.  My first shooting experiences were with a 308 and I wanted that nostalgia.  I'm an auto technician and troubleshoot and repair mechanical systems so I thought ill build my own ar308...and that is when the troubles started.  From my searching I have a very common issue that can be caused by many things.  1 round loaded in mag bolt will not hold open against catch...carrier catches on feed lips

Build details to follow

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Aero m5 lower
Aero m5 LPK
Geissele SSAE trigger
Magpul miad grip
Aero 308 rifle buffer kit for A2 fixed stock
Luth ar mba1 stock
Aero m5 upper
   Shoulda got the m5e1 cuz In the end I got
Aero m5 enhanced 15" handgaurd
Criterion dpms 22" m118 lr barrel
Fulton armory bcg that criterion headspace
Raptor charging handle
Lantac awm 5 round mag

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I will attempt to answer preliminary checks.  Gas block is open 25 clicks (30 is max according to seekins) like stated earlier with one round in mag round is fired..ejects (weakly to 3 oclock) and bolt carrier is hung open by mag feed lips.  I worked my way to this agb setting from fully closed.  I've been to the range 3 times this week and just this afternoon got it to work perfectly with S&B 150gr .308 SPCE..ejects to 4 o'clock and over the next bench to my right.  150gr rem corelokt, 149gr win m80 7.62, and 165 gr handloads all barely eject and carrier hangs on feed lips.  Why does that one round work..I jumped back and forth between ammo with same results

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Attaching pic of gas tube halfway to cam pin cutout.  I feel like I am lubing the hell out of it.  Inside length of buffer tube is 9 and 5/8".  Spring has 39 coils and is 12 and 5/8".  Weighed buffer on triple beam at 5.2 oz.  Buffer is 5.21" long.  When pulling charge handle full back there is 0.25" gap between bolt face and bolt stop.  The barrel was predimpled by criterion.  Gas block screws r tight.  Dont see much carbon blowby on barrel just a little bit at base of gas tube where it is in block.  Stock screw is not protruding into buffer tube.  

20220102_195718.jpg

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I see lots of talk about opening up the gas port in the barrel but I would think criterion has that figured out and don't think I could bring myself to do it.  I did not measure it before installing the block.  Changing to an armalite buffer setup would be less invasive and quick

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Why not try opening up your gas block completely? Generally with the fixed stock setup from aero should be fine, unless I am wrong, it’s usually the carbon ones that are the issue. But you are right, criterion generally are pretty good with their barrels and gas ports. And these things work great with regular gas blacks, why restrict the gas flow at all unless running a suppressor. My suggestion is open the thing up all the way and see if it runs better at all, also a lot of times even a slight hair in misalignment can take away from your gas. What was the distance from the gun that the brass was throwing at 3 o’clock?

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When it was ejecting at 3oclock and the bolt was catching on the feed lips it was tossing the brass maybe a foot...switch to the s&b and its tossing it 6 feet or more at 4 o'clock.  I'll try full open again...I have gone full open...actually past full open..threaded this seekins "select" adjustment screw out so far when I fired it it damn near blew the adjustment screw out the front of the block..😆 

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If it doesn’t function reliably with the gas block fully open, I would take it off, check the gas port size in reference to the data collected here. If it is within the tolerances people have identified as acceptable, then it might be as simple as the gas block not being properly aligned with the gas port. One of the easiest things to make an oopsy on, even for experienced builders. 

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It has 30 total rounds...never built my own before...is it to soon to get worked up...how often does simply shooting them eventually work itself out...I will check my block alignment...not sure what I can do about it...criterion dimpled it...the set screws from seekins r tapered so it all just kinda centers itself...right?

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Thanks so far for the input...I went ahead and removed the gas block to check...I can clearly see a faint outline of the gas block opening on the barrel and the barrels gas port is all inside of the outline...so I am confident my alignment is good...then I attempted to check gas port dia.  It is larger than .078" but smaller than .085"

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15 minutes ago, Gmtech said:

Thanks so far for the input...I went ahead and removed the gas block to check...I can clearly see a faint outline of the gas block opening on the barrel and the barrels gas port is all inside of the outline...so I am confident my alignment is good...then I attempted to check gas port dia.  It is larger than .078" but smaller than .085"

Check out the post from December 30th. I am no expert on gas port sizes, especially with your length barrel, but seems you aren’t the first to be under gassed. I believe your gas port diameter is to small. @98Z5V has way more knowledge and should be able to confirm this. 

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Yes I'll be interested to see what he has to say...I was just looking at that thread and it appears it may be to small...22" criterion barrels with rifle length gas system and .875 block journal....do u think its a good idea or not to call criterion and see what they have to say

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You have an adjustable gas block. If you drill it out a bit it's not going to hurt anything. The one type of ammo that functions has the proper gas impulse to run your gun. If you open the gas up a bit the other ammo should also run your gun. 50 rounds isn't even broke in.

The barrel maker will tell you they are right regardless of what the truth is.

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