Trouble with .308 not cycling properly

[Adamc55]

Hey guys, first post here but this looks like a very informative community so thought I'd ask here first. 

I recently finished an lr308 build and I'm having trouble getting it to cycle properly. First few shots I was stupid and had 3 rounds in the mag. I ended up with a bunch of pinched brass from the bolt ramming into them while returning to battery. I had no tools with me to adjust the gas block so I cut my losses and stopped shooting it for the day.

2nd time out I had tools with me, so I was able to make some adjustments to the gas block. I didnt go crazy with it but same deal, pinched a few rounds (made the same dumb mistake as the first time) and then would just eject brass without stripping the next round. At this point I thought maybe the bolt is moving too fast since I always hear 308's are over gassed.

3rd time out, yesterday. I finally wised up and was only loading one round in the mag to see if the bolt would lock on last round. Negative. I tried gas block full open, full closed and everywhere in between. It would go from barely lobbing out the spent brass onto the shooting bench about 1' away to just re inserting the brass into the barrel. Now I am convinced the bolt is not moving far enough during fire.

So far my thoughts are maybe the buffer and spring combo is wrong, which would be strange because it seems a lot of people run them. The charging handle is really heavy to pull though. Or maybe the gas ports are not lined up, which I intend to check today but when I installed it I was able to blow through the barrel with the other end plugged and felt a good amount of flow. I also just recently saw that sometimes a small space is needed between the gas block and shoulder on the barrel and I know mine is right up against it.

I guess I just wanted to make sure I didn't miss something or if anyone had any other recommendations on what to check. The bcg was well lubricated but the buffer /spring were not.

 

Here's a list of build parts that matter-

Aero m5 upper/lower

Aero lower build kit

Faxon .308 bcg

Faxon 18" heavy fluted match barrel

Fortis r.e.d. muzzle brake

Seekins ar15 .750 adjustable gas block 

Rifle length ar15 gas tube

Milspec carbine buffer tube

Sprinco orange carbine length buffer spring

Expo arms 5.3oz .308 carbine buffer

 

[jtallen83]

Sounds like all good quality parts. Where does the gas tube end inside the upper? Should be in the center of the cam pin cutout. Might pay to slide the gas block forward and look for the shadow of carbon to see if the port is completely open. Could be the tolerances are tight and it needs more rounds through it to mate the surfaces. Any marks on the lower right in front of the buffer tube?

and Welcome from Iowa, via Missouri, be sure to introduce yourself in the  introduction category. 

[Armed Eye Doc]

I addition to what @jtallen83 said,

Will it hand cycle ammo?

Will it hold open on an empty mag with hand cycling?

What is the internal depth of the buffer tube?

[Adamc55]

Ok, so I had a few free minutes and tore the rifle down a little. I discovered some troubling things. First off the shadow line from the gas block port covers the barrel port plenty, so positioning there is good. But I did notice marks on the lower in front of the buffer tube like jtallen83 mentioned. I removed the buffer and spring and slid the bolt into the tube. It makes contact in the worn off spots. It's weird because it locks back by hand but it's very obvious that it is mechanical contact that is keeping the bolt from going all the way back. The buffer is 2.5" long end to rubber end. The spring is only 11" long which seems short. But regardless the bolt could never go further back into the tube.

[Adamc55]

This all explains why the recoil felt really hard and sharp

[jtallen83]

Drop a quarter or two in the buffer tube and see if that will stop the carrier from hitting. Also a picture of the gas tube in position inside the upper would help, many times we see AR15 tubes falling short in 308’s. Real Armalite parts in these areas often help. You never hear of the Armalite buffer kit, tube, spring, and buffer, having issues, uses a standard H3 buffer in a bit longer tube, that said your parts should work fine once you stop the contact, I have a build with an Aero buffer kit that had a similar issue, same marks on the lower, and 50 cents solved it. The Armalite rifle length gas tube is a touch longer giving a bit more dwell time. 

 

[Adamc55]

I will try the quarter trick for sure. Here is a pic of the gas tube in the upper

[Armed Eye Doc]

1 hour ago, Adamc55 said:

 

The buffer tube looks like it could be screwed in one more turn.  It is barely hanging on to the retainer pin.  That, along with a buffer tube that is longer than 7 inches internal depth, can contribute to the issue that you appear to be having. 

[Armed Eye Doc]

46 minutes ago, Adamc55 said:

I will try the quarter trick for sure. Here is a pic of the gas tube in the upper

This appears to be OK, but can you get a picture looking up though the mag well?  The gas tube should extend to the middle of the cutout.

[jtallen83]

As @Armed Eye Doc mentioned it does look like the buffer tube could go another turn, had to file a notch in one one of mine to get that turn and not bind the detent. 

[Adamc55]

1 hour ago, jtallen83 said:

As @Armed Eye Doc mentioned it does look like the buffer tube could go another turn, had to file a notch in one one of mine to get that turn and not bind the detent. 

If I remember correctly I tried to do another turn but it was hitting the high point of the stop. I'll try and make a notch for it like you said. I also ordered an Armalite h3 buffer, same weight but a little longer.

Edited September 10, 2019 by Adamc55

[jtallen83]

3 minutes ago, Adamc55 said:

I also ordered an Armalite h3 buffer, same weight but a little longer.

Hold up there! You need the correct length tube to use the Armalite buffer, it works as a system. What is the inside depth of the tube you have? 

Here is a link to the kit I mentioned, the search function here will lead you to all kinds of info on the buffer system.

https://www.armalite.com/product/ar10rekit01-6-position-receiver-extension-kit/

Edited September 10, 2019 by jtallen83

[Adamc55]

2 minutes ago, jtallen83 said:

Hold up there! You need the correct length tube to use the Armalite buffer, it works as a system. What is the inside depth of the tube you have? 

Crap, totally missed that. I'll have to cancel the order. That's what I get for jumping the gun

[Adamc55]

Order cancelled

[jtallen83]

1 minute ago, Adamc55 said:

Order cancelled

Unless your buffer tube is some oddball inside depth the parts you have should be able to do the job, just a little tweaking needed. If you don't want to mess with tweaking it then the Armalite kit linked above will work, every time. See if you can get another turn on the tube. Put the rifle together and pull the charging handle all the way back and see what measurement you have between the bolt and the catch, I shoot for 3/16-1/8 inch, If the gap is bigger I would add a quarter. Put some sharpie on the marks from the carrier hitting the lower and check to see if it is still hitting, may need to fire a live round to know for sure as the bumper does compress some under recoil. 

[Adamc55]

52 minutes ago, jtallen83 said:

Unless your buffer tube is some oddball inside depth the parts you have should be able to do the job, just a little tweaking needed. If you don't want to mess with tweaking it then the Armalite kit linked above will work, every time. See if you can get another turn on the tube. Put the rifle together and pull the charging handle all the way back and see what measurement you have between the bolt and the catch, I shoot for 3/16-1/8 inch, If the gap is bigger I would add a quarter. Put some sharpie on the marks from the carrier hitting the lower and check to see if it is still hitting, may need to fire a live round to know for sure as the bumper does compress some under recoil. 

I think getting some more turns on the tube might do the trick. I'll try everything recommended and report back. Thanks for the help everyone 👍🏻

[Adamc55]

Trimmed some of the tube down and added a small notch to clear the detent. Looks to be just over 1/4" space between the bolt face and the stop. I think the next step will be to fire it and add quarters as needed

[jtallen83]

1 hour ago, Adamc55 said:

Gas tube is a bit short, the Armalite tube would go past the ideal halfway point, another manufacturer splitting the difference. I would wait and see how it shoots but the gas tube may be your next item to swap. 

[washguy]

Adam      the Sprinco orange spring is a spring I use in some of my pieces.. BUT its for overgassed  rifles and it is to be used only after you have the rifle running on a regular 308ar car spring.....for one that's why it wont cycle on one round and two that's why its so hard to charge....you don't start with an orange Sprinco......you are undergassed with that spring......use a car ar308 spring first to see where you are at....your buffer tube should be 7 inches long measuring the inside of the buffer..... and lube up the bcg  more than you have it...your buffer is the correct length..You Need to turn the buffer tube some more...one more turn.........Always one round check to see if it holds back.....you wont get it to cycle with that Sprinco Orange spring.....unless your port is opened up.....so if it doesn't cycle with a regular ol spring then its time to measure the gas port    ....Wash

Edited September 11, 2019 by washguy

[98Z5V]

6 hours ago, Adamc55 said:

Trimmed some of the tube down and added a small notch to clear the detent. Looks to be just over 1/4" space between the bolt face and the stop. I think the next step will be to fire it and add quarters as needed

 

You shouldn't have to add any quarters, if that receiver extension has a true depth of 7.000", and is truly "milspec" as stated.  If it has an internal depth longer than 7.000", then you need to add quarters to take up the additional length, over 7.000".  Each quarter is 0.069" thick. 

Your recoil system is good - That 2.500" KAK 5.3oz buffer is fine, and really good matched to the Sprinco Orange spring.  Receiver extention depth is what you need to look at - and it did need that additional turn it - it should be flush with the vertical surface of the lower receiver, and it needed that additional turn in.

You need an Armalite AR-10 Rifle-length gas tube.  For CERTAIN.

You may need to drill your gas port up in size.  I wouldn't suspect that from Faxon, but try to measurethe diameter of your gas port - numbered drill bits are the easiest way to do this.  Find the one that doesn't fit, go down one size and it should fit - that's your gas port diameter.  Fractional bits don't have enough "range" but can get you close. 

You shot this with the gas wide open on that adjustable block, and it didn't help you.  Part of that is the short gas tube, but not all of it.  An 18" .308 Win barrel with midlength gas will have a gas port diameter range of 0.080"~0.085", for a 0.750" diameter at the gas block.  Expect it to be a little different for rifle-gas.  You need to measure that gas port diameter...

Edited September 11, 2019 by 98Z5V

[98Z5V]

10 minutes ago, 98Z5V said:

  An 18" .308 Win barrel with midlength gas will have a gas port diameter range of 0.080"~0.085", for a 0.750" diameter at the gas block.  Expect it to be a little different for rifle-gas.  You need to measure that gas port diameter...

I just looked at the numbers and you have a dwell time of around (pretty damn close, if not exact) 5" on an 18" Rifle Gas system.  For a 0.750" diameter at the gas port, you'll need a gas port diameter (min/max) of approximately 0.090"~0.095".

[98Z5V]

I just went through the exact dwell time (length) numbers, and on an 18" rifle-gas system, you should be at exactly 4.875", from gas port location centerline to the muzzle end. 

[Adamc55]

7 hours ago, 98Z5V said:

I just went through the exact dwell time (length) numbers, and on an 18" rifle-gas system, you should be at exactly 4.875", from gas port location centerline to the muzzle end. 

Let me check some of those numbers on the rifle

[Armed Eye Doc]

1 hour ago, Adamc55 said:

Let me check some of those numbers on the rifle

Be sure to include the inside depth of the buffer tube.  That is a key to the recoil system functioning properly since you should be good on the spring and buffer.

[Adamc55]

9 hours ago, 98Z5V said:

I just went through the exact dwell time (length) numbers, and on an 18" rifle-gas system, you should be at exactly 4.875", from gas port location centerline to the muzzle end. 

Looks like this number is dead on.

 

I'll have to measure the buffer tube in a little bit