Looking for an AR-15 expert, intermittent feed issue

[Winkfish]

If I drop the bolt on a full mag it does go into battery and the first couple rounds will fire and cycle as expected.

I will attempt to open the edges of the magazine to get the round out of a level position in the magazine and raise the tip. This may reduce the drag of hitting the feed ramps.

If I can get a chance to get out after work today I may try and fire a few rounds to see what happens.

I have attempted to open the gas up and it did not impact the feeding mid magazine. As I adjusted the gas block you could feel a difference in the recoil impulse.

At this time I do not have a lighter buffer to try. If the magazine does not change the behavior that will be next.

I did also change the buffer tube as well as the buffer springs. Currently I have a carbine buffer tube in place with a +15% power spring.

 

[cantfixstupid]

I had this same trouble on a 6.5 Grendel, its the lips on the magazine and leave the magazine one short of capacity, thats what worked for me, but out of the several different calibers I have the Grendel was the only one that gave me grief.

 

[cohunt]

I ask about ejection patten to help diagnose --- with the gas block all the way open ( no bleed off if it's a bleed off design gb) --- where does your brass eject to? 1-3 o'clock, 3-4 o'clock, 5-6 o'clock?

This will tell us if in the full open position if the gas system has proper pressure/flow.

You can tune an ar15 gas system in multiple ways--- buffer springs, buffer weights, gas hole size, etc

We know you have a rifle+2 length gas system/tube), so we are trying to get more info to help diagnose .

You said you used a rifle spring but you have a carbine spring now-- std buffer right now, I'm assuming you have a carbine length buffer tube? ( edit just saw carbine tube and +15% spring)

Remember to only change 1 thing at a time to diagnose

 

[cohunt]

As far as magazines go--- one thing to look at is the "protrusion" of the mag/rounds above the top of the lower receiver.

If you take you upper off of the lower and then insert a loaded mag into just the lower-- you can measure the protrusion height of the mag and rounds---

This is more along the lines of failure to pick up a round, but it also jas an effect on how much pressure the rounds put on the bottom of the bolt carrier.

There are tolerances allowed for mag catch height, mag catch dimentions' mag catch hole in side of mag, etc--' you may have some "tolerance stacking" going on due to variances


You be surprised at the variances in things like bolt catch height, mag height in lower, feed lip angles etc that can affect how the platform runs.

 

[Winkfish]

I was changing only one item at a time and gathering info. Agreed if you change more than 1 variable at a time one will not be able to determine which change is impacting the results.

Step 1 was change gas. Regardless of gas setting I was getting a failure mid mag.
To answer your question about brass position I will have to go and verify, which I will do.

Step 2: put gas back at 4.5 turns open. change buffer spring. fail

Step 3: with new buffer spring change gas. Fail

Step 4 replace buffer tube, go back to original spring. fail
with new buffer tube play with gas settings. fail
with +15% spring, and 4.5 turns open gas fail
with +15% spring and playing with gas fail.

Step 5 replace bolt carrier group then go through all the above again. fail

I will go take some pictures of where it sits with the magazine in it as you mentioned and report back.

 

[Winkfish]

Here are a couple pictures with the upper off.
Magazine inserted. The measurement from the top of magazine feed lips near the bolt catch to the lower is approximately .4" There is little to no play in the magazine vertically

 

[cohunt]

The SA gb is a bleed off style--4.5 total turns ( 18 clicks) is full open-- if you go less then you limit the gas, if you go more you limit the Gas by using the bleed off design function.

When at 4.5 turns out you should be ejecting brass in the 3 to 330 positions....if it's further back you either need lighter spring/buffer or more gas.

I saw a few other threads saying they ate running .100-.106" gas hole size with the +2" gas length system.

As a last resort you can open up the gas orifice 1 or 2 drill sizes BUT try everything else first.

Make sure your gas block hole is lined up with the hole in your barrel too--- if you pull the gb off you should be able to see a " soot ring"-- make sure it's centered around the hole in your barrel-- also check the barrel around the block for soot and the has tube where it goes into the block for soot--- if you have a bunch of soot in one of those 2 places you have a gas leak

At this point I'm thinking 1 of 2 issues, either a simple mag lip tweak, or you are right on the edge of enough gas and when you add that extra pressure on the bottom of the bolt it's slowing down the carrier enough to cause issues

I know originally, the Grendel mags had some cirvature/geometry issues-- but that was years ago with high cap mags and was under assumption that was long past

 

[xsn10s]

It sounds like you tried all the options I would. To me my best guess is the buffer spring cannot overcome the mag when fully loaded. And with all the options you tried my last best guess would be a polymer mag. Maybe being polymer it'll feed with less resistance.

Amend2 Polymer 6.5mm Grendel AR-15 Magazines — First Look - Firearms News

They said it couldn't be done! Amend2 Magazines introduces the first polymer 6.5mm Grendel/6mm ARC AR-15 magazine to hit the market!

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[tbird86]

You and I are thinking the same. He tried a different spring. I asked about increasing gas and a superbolt (less Drag). All that is left is the buffer. Lighter will increase bolt velocity. I still think the gas is too low for the load.

I still haven't been able to determine which has a bigger effect in this situation: lighter buffer for speed, or heavier buffer for momentum. Gas or spring has taken care of every situation I've encountered.

Sounds like mag feed lips need a look first, then gas block alignment, before coming back to this.

 

[Winkfish]

I did verify gas block alignment with a bore scope and it appears aligned. At this time there isn't any soot around where the gas tube goes into the gas block.


I also tried excessive amounts of profanity when working with this rifle with little to no success either.

 

[cohunt]

For now-- try the lip tweak and take the std carbine spring to the range with you.

Shoot as is and note brass ejection pattern

If it's more than 3-330, make sure gas block is open (4.5 turns) -- if it is, try lighter spring.

For now-- that's all my ideas-- maybe someone else has more suggestions

 

[xsn10s]

I also tried excessive amounts of profanity when working with this rifle with little to no success either.

I've found positive re-enforcement to be a more productive option. Maybe try lubing the bolt heavy. And when I say heavy until it's dripping and then shake off the excess a little bit.

 

[Orange Dust]

Just throwing this out since i don't have a 6.5. All my stuff is on the 6.8 case or the .556 case. It almost works. It works with 5 rounds or less. It works for a couple of rounds with a full mag. Issue is round feeds but bolt won't fully close with 3/4 of a magazine. One would logically conclude that due to the way the rounds are sitting in the mag when 3/4 full it takes all the energy available plus a little for the bolt to fully go into battery. So we either need more energy in the bolt or the rounds need to release easier from the mag when 3/4 full. I no longer think we need more bolt speed, we need more energy. Heavier buffer? Or adjust the mag so the rounds release easier when 3/4 full. Either of these make sense to me. Intermittent issues are always the most difficult to solve with anything mechanical, usually intermittent is an electrical problem!!

 

[del2les]

Now we are going to try and run a normal 10 shot stage. start shooting the gun. around shots 5-7 I will get a failure for the bolt to lockup up / go into battery. In some cases I am able to hit the forward assist and close it, then fire the gun. Other times the round is not far enough out of the magazine to allow that to happen. In some cases it is close to going into battery and when you pull the trigger it just goes click. If I drop the magazine, I am able to pull the charging handle, let it fly and it locks up.

I have seen this with minimum dimension target chambers with minimum head space that once dirty via a few shots, began to have very similar issues. Also, we have seen a few experience this with handloaded ammo that was not small base resized. I have a very tight minimum AR 223 Target chamber/barrel that must have all reloaded ammo small base resized and shoulder bumped to feed properly, and it has experienced similar issues with certain brands of factory target ammo. When the bolt is dropped on a single, it has sufficient forward force to basically bump-size the shoulder via the chamber, but when using a fully loaded mag, the increased drag reduces the forward bolt thrust until a slightly dirty chamber prevents lockup.

The round not feeding completely from the mag is a separate issue, but it can slow down the forward thrust.

EDIT: One thing I forgot was we used a dry silicone spray on a clean cloth to rub the rounds prior to loading, and this gave improved feeding and clambering for a while. But this is not a longer term solution for a minimum chamber and tight headspace. Polishing the inside of the mag lips can also help.