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M5 build issues


2AUSA

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hey everybody. my 308 is not locking back on an empty magazine, and the ejection is kind of weird, around the 5:00 area. shooting federal 7.62x51 BPXM80CBL 150 grain fmj. At first, I thought it was short stroking but I think the bolt may be outrunning the magazine follower. I screwed up and ordered the wrong bolt (low mass), while I was sick, quarantined with covid. shocked when I opened the package and realized the mistake, I decided to try and make it work instead of waiting months, or who knows how long, to exchange it and get the right bolt.

Here is my setup, following the waterboarding post.

 

I have a criterion 18" barrel with a JP low mass operating system and headspaced bolt.

 

running a milspec carbine length buffer tube

Buffer tube internal depth is 7  1/16"

KAK heavy buffer 5.3 oz & 2.5" length

blitzkrieg components sprinco ar 10 carbine buffer spring orange

rifle length gas system
gas journal Dia 19.02 mm
gas port Dia  1.95mm

 

I'm not sure if it's my buffer system, short stroking, hammer wedging, gas port undersized or what exactly is going on. it's not cycling properly. I've tried several fixes, including a different bolt catch, gas blocks, gas tubes, different springs and different buffer (Aero m5) but nothing has fixed the issue. something is out of spec. at this point, I'm throwing money at it and getting the same results. 

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31 minutes ago, Armed Eye Doc said:

A standard BCG and a 5.4oz buffer is the right combo for most 308ARs. You need to replace the mass that is missing from your lightweight BCG.  One option is to replace you receiver extension with one that is 7 5/16" and get the xh buffer and spring listed under 308 here.

https://heavybuffers.com/xhcar.html

7 5/8" internal depth is the number to look for.  That's what you'll need to run a 3.250" long AR15 carbine buffer on a .308AR platform.

You've maxed out your BCG weight, with that KAK buffer.  Stainless body, 2.500" long, and it only hold two weights- and it's already got 2 tungsten weights in it, just to get it to that 5.3oz.  That Aero M5 carbine buffer you have is an aluminum body, 2 tungsten weights, and it weighs 3.8oz.  The stainless body on the KAK is what gets it weight up to "normal."

Your recoil system is solid.  Perfect, for what we suggest when you run the 7.000" internal depth AR15 Carbine Receiver Extension.  Spring - perfect.  Your gas tube length is perfect.

Your gas port diameter is a problem...   1.95mm = 0.076771653543.   That gas port diameter is 0.076"  That's WAY too small. 18" Rifle Gas needs a gas port diameter that's 0.095".  I think I've given the range here in the past at 0.093"~0.096".   That gas port diameter HAS to go up.  It needs drilled.  No other way around that. 

Next - if you keep that receiver extension, the real spec is 6 15/16" internal depth.  A long time ago, commercial AR15 manufacturers just started making it 7.000" internal depth - and that works, and it kinda "stuck" as the "spec."  However, if you're really at 7 1/16" internal depth - that's too much.  There's a quick way to fix this, and that's stick a quarter in the receiver extension, and THEN install the spring and buffer right in top of it.  Quarters are 0.069" think, and will take up that extra 1/16" that you've got going on there.  You do NOT want to risk smashing that BCG into the ears of that lower receiver, when you do get this gun running...  

My $0.02 on what I saw.  See what a difference "the details" make?  :thumbup:

 

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Let's sort out that gas port diameter problem, and stick a quarter into the receiver extension - and see what happens with the gun - before getting into addressing that lightweight BCG.

I have an idea of how to handle that BCG-weight issue - 2 ideas, actually.  Let's just make the gun run first, though.  It's gonna cost you a drill bit that's about 0.095" (an Irwin drill bit from any Ace Hardware), and it'll cost you a quarter...

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8 hours ago, 98Z5V said:

Let's sort out that gas port diameter problem, and stick a quarter into the receiver extension - and see what happens with the gun - before getting into addressing that lightweight BCG.

I have an idea of how to handle that BCG-weight issue - 2 ideas, actually.  Let's just make the gun run first, though.  It's gonna cost you a drill bit that's about 0.095" (an Irwin drill bit from any Ace Hardware), and it'll cost you a quarter...

thank you, 98Z! I will get the drill bit this afternoon and install george in the buffer tube. I'll report back with the results.

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  • 2 weeks later...

hey guys, I haven't forgot about the forums here just been really busy at work. I'm going to try and drill the barrel out this weekend. in the meantime I ordered a full mass operating system from JP and it came in the mail today.

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1 hour ago, 2AUSA said:

hey guys, I haven't forgot about the forums here just been really busy at work. I'm going to try and drill the barrel out this weekend. in the meantime I ordered a full mass operating system from JP and it came in the mail today.

Let us know what that buffer weighs...  

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  • 2 months later...

well, im back in the land of the living. work got hectic,  but today I finally had a chance to do some work. so I broke down the lr 308, opened the gas port up on the drill press and went out to shoot..

it looks like 98Z5V's recommendation of opening up the gas port diameter was right on the money. the bolt finally locks back after the last round is fired.

I tuned the gas block and it is now kicking the brass out, instead of dropping them out at my feet. from what I could tell, it was ejecting at the 3:00 position but I will have a friend shoot it so I can watch and make sure. I thought one round may have kicked out at the 2:00 position (I wasn't paying attention) and that caught my eye, but the rest seemed to go straight out at 3:00.

I have a sprinco orange spring and a kak 5.3oz buffer. 

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On 3/25/2021 at 11:05 PM, 98Z5V said:

I have an idea of how to handle that BCG-weight issue - 2 ideas, actually.  Let's just make the gun run first, though.  It's gonna cost you a drill bit that's about 0.095" (an Irwin drill bit from any Ace Hardware), and it'll cost you a quarter...

I also went ahead and ordered a full mass bolt from jp. I didn't install it in the gun, I did the work with the low mass. but having an extra m5 lower lying around, I am glad I have an extra carrier. I may still install the heavy one, though. the rifle is heavy anyway, this criterion hybrid barrel is a freaking crow bar. 

 

ps, I forgot to install the quarter in the buffer tube. should I try that? i don't want the bcg slamming into the ears of the rifle.

Edited by 2AUSA
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update:

 

I installed the quarter in the buffer tube, took it back out with a friend to shoot and it looks like it's ejecting right at the 3:30 mark. should I consider this one finished? I may order a proper sized buffer tube. who would you guys recommend going with for a new tube?

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8 hours ago, 2AUSA said:

I installed the quarter in the buffer tube, took it back out with a friend to shoot and it looks like it's ejecting right at the 3:30 mark. should I consider this one finished?

 

7 hours ago, edgecrusher said:

 For now, it’s working and I’d leave it alone as it’s mostly correct with the mixed parts. Many here have been running a combination like or similar to that for a long while.

^^^   I'm with him.  If it runs like that, then leave it, and run it.   :thumbup:

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The Armalite kit is what I put on my LR-308 after the gun shop put a AR-15 spring and buffer in when they changed the but stock for me.  Took it out 2 days ago and it worked very well.  Didn't even notice the recoil from the 147 gr. Armsco ammo.  I need to add a scope instead of the EOTEC.  The EOTEC works very good at close range.  Old eyes need all the help they can get.   😊😊😊

   Steve

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On 6/15/2021 at 10:19 AM, edgecrusher said:

Not bad for a bunch a knuckleheads 

That would definitely define this crowd.

On 6/16/2021 at 12:10 AM, 98Z5V said:

Even blind monkeys find bananas once in awhile.   :thumbup: Sometimes we strike gold, but it's pure luck, and we're not starving. 

:thumbup:

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