Gunsmithing Help! AR-15 Issue - Bolt cycle issue

[GBMaryland]

Ok,

I've having an odd issue with my AR-15:

The bolt is not always completely cycling.

That is to say that it appear to fire a round, then the bolt carrier blows 9/10s of the way back, and another round does NOT camber.

The action will throw the brass from 1 foot to 4/5 feet. (That's the other symptom)

I've replaced the gas rings, and the upper is an Armalite 20 inch upper that has only a few hundred rounds through it.

It's completely intermittant... and it happens with Remington factory ammo, or my own reloads (the reloads are not more that +/- .1gr from each other and they are using 22.9gr of N130). Both are 50 Gr bullets...

What the heck is causing this?

GB

 

[LoneWolfUSMC]

Re: Help! AR-15 Issue - Bolt cycle issue

CHeck the gas key, make sure it's tight and staked.

Check the gas tube for obstructions.

Check the gas block and make sure it hasn't moved.

If it's happened since you assembled/bought it, then the gas port in the barrel may be undersized.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

I swapped out of the front sight (gas block) with an Armalite picatinny gas block.

I will dissemble the rifle (yet again), but completely this time (as tube and all)... then get back to you!

Thanks.

GB

 

[USMCj]

Re: Help! AR-15 Issue - Bolt cycle issue

+1 to what LoneWolf said, plus check and make sure the gas port on your barrel and gas block are aligned properly. Thats pretty much the #1 cause of what you are describing. Ask me how I know.....

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

Ok,

I put the military style front sight on the barrel.

What I noticed with the picatinny gas block...

1) Under the hand guard, where the gas tube leaves the gas block, there are clear signs of leakage.

2) Original config: Heavy barrel -> step down -> forearm retainer -> military front sight / gas block -> step down -> compensator

The replacement picatinny gas block is only fastened by 2 screws and presure. I'm wondering if it's not supposed to use the forearm retainer?

(Which could cause a slight alignment issue, BUT the hole in the barrel is smaller than the one in the gas block... which leads me to believe that it was not the issue.)

I wonder if there is a slightly larger gas tube for that block, as when I re-installed the original sight/gas block, the tube was a much tighter fit.

Gerhard

 

[Dans40X]

Re: Help! AR-15 Issue - Bolt cycle issue

Replace Gas Rings for the 1pc McFarland gas ring.
Gas Block blow-by-LocTite the block to the Bbl to prevent blow by.
Add weight to buffer to prevent short stroking.

 

[Insayn]

Re: Help! AR-15 Issue - Bolt cycle issue

Aside from possible gas issues, you aren't running a rifle buffer on a carbine buffer tube or a carbine buffer in a rifle buffer tube are you? Then there is the "H" weighted carbine buffers.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

DAN:

Using the McFarland gas ring.

Thought about the loctite, so I'm thinking I'll try that.

General:

Buffer spring. I've got two springs, and the one that's on there feels exactly like it should during the entire length of the stroke.

The stock was/is and A2 stock... and the only thing I did there was replace A2 with a MagPul PRS.

I suppose I could make a tester if someone can tell me what the spring tension should be. (I also have a spare buffer and sping that came with the lower... no idea if it's different than the one in there now...)

I'm gonna try a function test today and see if the original gas block fixes the issue. If that's the case, I know where the problem is.

 

[LoneWolfUSMC]

Re: Help! AR-15 Issue - Bolt cycle issue

I would put money on the gas block alignment.

 

[skurtz]

Re: Help! AR-15 Issue - Bolt cycle issue

A little gas leakage around the front gas block is normal. You can seal it up with some Loc Tite. I usually us some where the block is clamped on the barrel, not on the gas tube. Check the gas key as mentioned earlier.
Make sure the gas rings you have are aligned properly.
My bet is the same as the others gas port/block alignment.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LoneWolfUSMC</div><div class="ubbcode-body">I would put money on the gas block alignment. </div></div>

We'll know in two hours when I get to the firing line.

(There is no guessing when you use pins to attach the block. I'm even thinking of using a milling machine to put pin holes in the picatinny gas block to ensure perfect alignment.)

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

Well,

Here is what I found:

- 50 Rnds of Black Hills 77gr Match Ammo: no issues

- 30 Rnds of LC stamped XM193A: no issues

- 10 Rnds of 52gr A-Max w/ H4198 (1 gr under Max): short stroke every few rnds.

- 10 Rnds of 52gr V-Max w/ N130 (Max Load): Short strokes consistantly.

- 20 Rnds of 55gr SMK w/ H4198 (1 gr under Max): 2 rnds jammed on injection (headspaced slightly long)

- 30 Rnds of Remington 50gr factory ammo: Short strokes randomly, and 3 rounds that did not fire, but were dimpled by the firing pin. (...and subsequently fired when retired.)

It appears to like ammo at max load / on the hotter side.

I'm also not sure that I'm not over lubing the darned thing, if possible. I did note that many of the rounds had breakfree in the firing pin dimple of the primer.

 

[LoneWolfUSMC]

Re: Help! AR-15 Issue - Bolt cycle issue

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: GBMaryland</div><div class="ubbcode-body">
I'm also not sure that I'm not over lubing the darned thing, if possible. I did note that many of the rounds had breakfree in the firing pin dimple of the primer. </div></div>

That's not really possible with an AR if you are using CLP. We used to lube with a spray bottle on the M16A2 when we were doing extended firing.

I would bet you have an undersized gas port. If you have a good set of drill bits you can use them to check the diameter of the port. I don't recall the spec for the size on a 20" right now.

 

[sobrbiker883]

Re: Help! AR-15 Issue - Bolt cycle issue

Is this a new problem on a rifle that functioned fine with the ammo now causing problems previously?

What length barrel/gas system?

Never heard of checking a buffer spring by seeing if it "feels right" through its stroke.......they're cheap, get a new one from a quality source.

Clean your gas tube and check its alignment. If you got the new block off center a little (or if your tube is bent slightly) the tube can be dragging your gas key.

Good luck with replicating the angle and fit of the two taper pins on your fixed FSB to your railed gas block.

If you are going to dick with the gas port, know what you are doing or send it to someone who does. Lots of AR's get jacked up by opening their gas ports unecessarily (the timing of the system is critical to long term health of the weapon). Too much gas can make the system try to extract before the case has started to shrink back from the chamber walls. AR15barrels.com may be able to help you on the proper size (but would need to know the length of barrel, length of gas system).

I'd clean gas system, clean the bolt carrier and everywhere the gas travels to oprate the bolt in the carrier, verify the gas tube's staight and the gasblock's straight and centered over the port, and put in a new buffer spring.
Be sure your chamber is clean too. With the weak ejection, I'd almost bet your tube and somewhere else in your gas's travel is being impeded or leaked.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

sobrbiker38:

Barrel: 20 inch (armalite 20 inch upper, flat top)

Let's just say the spring didn't feel different than any other AR I've shot, and is consistant in presure during the length of a pull on the charging handle.

What spring / buffer would you recommend for a full length A2 stocked AR?

I'll check the alignment, but it appears to have a good seal and be in line.

Never under estimate a milling macihne with very good digital indicator. ...and a little math. ...but that's a project for a day when I care. -grin-

The weapon was firing without issue initally. However, I did switch over to generic UMC/Remington Ammo, and that's not working well at all.

My loads are all under maximum by about 1gr. ...and that seems to be making a difference.

The buffer / spring should be good... but I'd love to know what to check for.

 

[rankroddin250]

Re: Help! AR-15 Issue - Bolt cycle issue

are you sure it is short stroking and you dont just have crappy magazines failing to feed rounds?

try running a pipe cleaner with some solevent down your gas tube.

how clean is your action? is your buffer lubed well?

 

[sobrbiker883]

Re: Help! AR-15 Issue - Bolt cycle issue

This is the rifle buffer you should have:

If you have a shorter buffer you have a carbine buffer.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

It appears that I have a full length buffer...

Two in fact...

I did a little checking on my reloads and the N130 based 50gr loads max at 24.5grs. I reloaded them for testing.

I then went ahead and weighed the Remington plinking ammo: The weights range from 173.2gr to 176.9gr.

I suspect the ammo is THAT crappy.

I've sorted it in 3 sets with the heaviest in one mag, the mid range in another, and the lighted in another.

When you look at reloading tables it shows that powders like N130 change CUP pressures from 36000PSI to 44000PSI with just 1gr difference.

That would make a huge difference in a gas operated firearms.

Mags are all new: Brownells 20 rounders, Magpul 30 rounders...

The buffer spring should have a total of about 500 rounds on it...

I'll order a replacement silicon carbide for giggles. At the moment I'm thinking ammo, unless you guys really thing an AR should shoot anything.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

Update:

Remington UMC 50gr MC rounds: Weighed them all...

There is a 3.5gr difference between bullets in the same lots.

If I sort them out at 175.0gr+, they function without issue in the AR.

If I use anything that weights less than 174.8gr total, it will short-stroke.

Match ammo and XM193A work without issue.

Loading 50gr A-MAX BTs with N130 at 24.5Grs it just enough to literally throw the bolt back and stick the spent case in the ejection port.

So the 20inch upper doesn't like N130, even at max load.

 

[rankroddin250]

Re: Help! AR-15 Issue - Bolt cycle issue

try using heavier bullets and se what that does for your reloads.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

Wilco

 

[bigwheeler]

Re: Help! AR-15 Issue - Bolt cycle issue

I saw a thread on a barrel being ported to .076 in .556 for the heavier military
rounds. He redrilled to .093 and it functions fine now. The barrel maker told him to
drill to .10 . Seems the 62 grain and up .556 stuff is quite a bit hotter than .223.

 

[scotharr]

Re: Help! AR-15 Issue - Bolt cycle issue

In your second post, you said "I swapped out of the front sight (gas block) with an Armalite picatinny gas block."

Was the rifle functioning properly before you did this? Or was this a troubleshooting effort?

In short, was your rifle ever functioning properly? If so, what changes have been made since it was functioning well? A good AR should be able to handle any type of decent factory ammo without any problems.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

The rifle always shot fine with heavy loads and military ammo.

I think I've just begun to relize that the barrel is made for NATO / MILSPEC ammo, not for .223 loads.

...and especially for 50/52gr .223 loads.

 

[scotharr]

Re: Help! AR-15 Issue - Bolt cycle issue

A NATO barrel can shoot 223 just fine. Your rifle should be able to shoot 50 to 77gr ammo without issue.

Did the rifle have any problems before you changed the gas block???? If not, then that is probably your issue. It is simple trouble-shooting. If your gun was shooting fine and then you changed something and then its not fine......well the change is the problem.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

When I first picked up the upper, and took it to the range... it seemed to be shooting everything fine.

Of course, I was only putting factory ammo through it at the time.

When I changed the gas block, I started to notice that it was not cycling properly. However, I was then using different ammo.

M193 and 77gr Match ammo seem to work without issue.

Replaceing the new gas block with the original one didn't make any difference... and the gas system appears to be fine.

If there is an alignment issue with the gas key... I can't see it.

At this point NOTHING has actually changed. It bone OEM...

 

[scotharr]

Re: Help! AR-15 Issue - Bolt cycle issue

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: GBMaryland</div><div class="ubbcode-body">When I first picked up the upper, and took it to the range... it seemed to be shooting everything fine.
...
When I changed the gas block, I started to notice that it was not cycling properly. ... </div></div>

Was working fine, you changed the gas block, then its not working fine: odds are that is your problem. Just because you replaced the gas system does not mean it is back to the original state. you might have: it misaligned, clogged the gas tube, bent the gas tube, created a leak around the gas block. Also, shooting ammo of different weights should not be important.

What is the original gas system: is it a pinned gas block or screw on? What type of chamber: NATO? How many trouble-free round went through it before you changed the gas block? Normally, 20" rifle-length systems are very reliable. So, I would not be satisfied until your gun feeds any type of ammo with perfect reliability.

The problem you describe can come from several sources:
1. a gas system not providing enough juice because of a problem/leakage somewhere. Possibilities:
-a leak somewhere in the end to end system
-gas port of improper size for the barrel length and thickness.
-blockage somewhere in the gas system: cotton swab or the like.
-gas block not centered over the gas port.
-bent gas tube not mating with carrier key.
2. A buffer/buffer spring/bolt carrier system that provides too much resistance. Possibilities:
-"upgraded" bolt carriers(bigger, heavier)
-"upgraded" buffer springs
-"upgraded heavy buffers
-mechanical problem preventing BCG from cycling smoothly
3. Very tight match chamber or rough chamber that makes extraction difficult.
-A NATO chamber is very loose by design
-A Wylde chamber is tighter but not tight as other match chambers. It has proven to provide the best accuracy and reliability and can shoot 556 and 223 ammo.
-Tight match chambers are known to cause issues with some brass.

Based on the info you provided. I think "1." & "2." are the areas you should focus on.

Regarding gas port size: A 20" barrel with a ".75 gas block should have a .093" to .096" gas block. Can you measure that and report back?

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

The barrel has between a .093 and .096 gas port. The port on the gas block is much larger...

I'm wonder about the buffer...

How does one check the key to tube alignment, etc.?

 

[scotharr]

Re: Help! AR-15 Issue - Bolt cycle issue

Sounds like the gas port and block are fine. They make the block with a much bigger hole so it is easy to align with the gas port. If your bolt carrier/bolt lock up smoothly and you can see the end of the gas tube in the receiver, then that is probably good to go also.

NOW, you mentioned on another post that you may have identified the problem.....so spill the beans!

 

[MJC]

Re: Help! AR-15 Issue - Bolt cycle issue


How does one check the key to tube alignment, etc.?

Run a piece of soft wire or a pipe cleaner into the gas key and see if it will show inside the bolt carrier. A borescope will let you look into the bolt carrier.

 

[GBMaryland]

Re: Help! AR-15 Issue - Bolt cycle issue

Well, if that's how you check the gas key, then it's fine.

New barrel, adjusted the trigger, and it seems to be working well.

The issue for me what that the Armalite 20 inch heavy barrel was chambered in 5.56x45 NATO, not 223 Remington. So, my hot 223 Rem loads using 50 and 52gr rounds were just not allowing the action to function.

In the end, I guess it was a matter of fast burning power and light rounds.

I have found that the rifle is less sensitive to those loads with a barrel chambered in 223 Wylde. (I picked up a Lilja AR740 barrel.)

Since I had multiple buffer springs I opted to remove a few coils from one of the spring and see how the rifle functioned.

So the buffer worked fine with fast burning light rounds...

The factory ammo works fine (standard buffer), as do hot loads with slower burning powders (W748 as opposed to N130).