I am shooting a 24" DPMS Super Bull Barrel Rifle, incase that helps any.

Well, whenever I go to the range I run into a feeding problem with my AR.  When I begin to fire, it will always fire the first round but sometimes it will half feed the next round so it sticks in the bolt in  a diagonal angle or sometime it wont even send it into the chamber and just dry-fires.  It will only do this for me if i dont press the rifle relatively firm into my shoulder.  But since my rifle weighs around 12 pounds since it is a super bull barrel it shouldnt have to be be firmly pressed into my shoulder.  Is it the mag?  Is it the Rifle or bolt?  Is it me?  What can I do about it?

What kind of magazines?

2 DPMS 10 rounders that come with the rifle and one 30 rounder without a stamped floorplate or label.

Sounds like short-stroking to me.  What ammo are you using ?  Has any work been recently done to the rifle ?  Did it function ok the last time you used the rifle ?

Try USGI mags and ammo then get back to us.

I had a bipod fixed and handguards changed, I am using winchester 5.56 FMG, and I used some ultramx .223 that they sell at the range.

If the rifle ran before, then started having problems after the float tube change, then my guess would be that the gas block is out of alignment with the gas port on the barrel.

Is there anyway that I can fix this or check this on my own, or should I take it into a smithy?

It’s easy to check for alignment since some smiths index the top of block with the upper instead of the barrel port.

Remove the two bottom allen screws on the bottom of the block, then pull the gas block straight off the barrel.  Do not twist the block or you will bend the gas tube.

Once the gas block is off the barrel, mic the center of the barrel gas port from the barrel step, then the distance from the back face of the block to the center of the gas  block port hole.  You do this to figure out if the block need be off the barrel shoulder to center up the ports or if it needs to be flushed.

The next item is pencil to line mark the barrel port down the top of the barrel forward past the port so you can index the gas block in this plane as well when you reinstall the block. You shouldn’t need to mark the block, just use the gas tube as a reference of the block port.

Once the block is in both correct index positions on the barrel, apply a thin coat of blue loctite on the screws and tighten them down to re-secure the block to the barrel.

Note: Since you have the screws out of the block, may as well check to make sure that the gas tube hole is aligned with the gas block port.  If needed, chuck the block in a press with the gas tube still attached, and drill out any portion of the gas tube that is blocking the port. The treads are larger than the port passage, so if you chuck up true, you will be able to remove the overlap without touching the set screw threads (remember to set the press stop to keep from over drilling and punching threw the other side of the gas tube).

I don't really understand how to do that, if you could somewhere get instructions with pictures that would be nice.

So i take off the gas block with the gas rod, then how do i realign it?

Quoted: So i take off the gas block with the gas rod, then how do i realign it?

Since you think that the rifle has a gas rod, we need to step back and have you spend some time learning the rifle/system.

Please read the repair manual (cover to cover), and then get back to us once your up to speed on the system.


www.ar15.com/content/manuals/TM9-1005-319-23.pdf

Well, i ment gas tube, all i really need to know is how to align the gas tube.

And that manual doesnt mention anything about free-float tubes.

If you mean that gas tube end to enter cleanly into the carrier key on bolt closer, then there's two ways to attack the problem.  

The first is to remove the end tube of the hand guard (leaving the barrel nut intact) and slightly bending the gas tube above the barrel to get the end in the correct position.

The second way to adjust the tube is to just pull the tube and gas block and slightly bend the tube, then re-install the tube/block to see if you have tweaked it correctly.

Even on the float tube, the gas tube enters the top hole of the float tube barrel nut, and then threw the upper gas tube channel.  The float tube barrel nut, like the standard barrel nut must be indexed with the upper receiver gas channel to allow the tube to glide threw both openings.

BTW, the tube to the key shouldn’t be the problem; it’s the gas block alignment to the barrel port that may be out of whack.  Since the gas block uses two allen screws that press against the bottom of the barrel, verses taper pins on a standard barrel that dictate alignment, the gas block can be installed wrongly, with the exit of the barrel port not aligning with the entry of the gas block.

Have you performed a short stroke check on this rifle?

M11293,

I think your next queston will be "what is a short-stroke test?"  This involves placing just one round in the magazine, then firing it.  With a healthy AR, the bolt should lock-open on the empty mag.  This is important because the bolt will lock open only if the gas system is healthy.  If your AR was short-stroking, it would Not be able to lock open the bolt.  

It helps if you do Not hold the gun too firmly against your shoulder.   Holding the gun loosely will aggrevaite the short stroking problem - and after all, we do what to know if it exist or not.  When I test my ARs for short-stroking, I allow it to free float (no support against the shoulder).  I just hold the gun out in front of me.  I even jerk the gun backward as I fire it.  If it pass this test by locking the bolt open, then you know that you do Not have a short stroke problem.

I do recommend that you fire approx 20 shoots.  If one of the 20 shoots fail to lock open the bolt, you probably have short stroking.  If approx 5 of the 20 shoots fail to lock open, you definitlely have short stroke problems.

well, the feeding on my rifle is really random, if i load a 30 round mag to 28 it wont feed but if it get's down to 20 I can zip through it.  Like I said it will feed with me if I give it a snug fit but if you hold it loosely, it will half feed.

We need to eliminate some variables here.  It will really help us in our troubleshooting if you could perform the short-stroking test and get back to us with the results.

Also, do you have the same problem regardless of which magazine you are using ?

I'll go to the range tomorrow, and ill do the test and bring results back.

Quoted: I think your next question will be "what is a short-stroke test?"

Nah, couldn't be, if he's posting then he must have read the FAQs and The Rules right? No one would start posting without first doing that, that would be disrespectful.

He's using two 10 rounds mags of questionable utility and one no name 30 rounder. He might have the problem regardless of which mag he uses.

[sp][bk]

Short-Stroke

So either too little gas is getting into the system or too much gas is leaking out.

Shortstroking is covered here and in the FAQ.

First make sure the bolt locks fully open by hand (I'm assuming you followed G35 instructions closely) then check that the carrier keys are tight and that the ID of the carrier key and the OD of the rear end of the gas tube are very close to the other. Gas ring check is outlined in the TM. Check the little stuff first, then move onto the big stuff  like gas ports and buffers and springs.

You may want to retest with known good ammo first tho.

Just to elaborate on Tweak's advice.

1) Remove magazine.  With gun pointing in safe direction, pull back the charging handle and verify that chamber is empty.  Allow bolt/carrier to return closed by riding the charging handle back into gun.

2) Place EMPTY magazine in gun.

3) Pull charging handle all the way back as far as it will travel.  The bolt should remain open {WITHOUT you touching the bolt catch) as you ride the charging handle to the closed position.

Please post back with the results of this test.  Did the bolt remain open after you finished step 3 ?

Quoted: I had a bipod fixed and handguards changed, I am using winchester 5.56 FMG, and I used some ultramx .223 that they sell at the range.

Guys, I still say that if the rifle ran before being worked on, and then started to have problems after the hand guard change, the gas block is out of alignment with the barrel port.

I completely agree Dano but I haven't heard him say that it worked fine until the hanguards were changed. He said initially that "every time" he went to the range the rifle didn't work. He then said that the handguards had been changed anda  bipod installed. THEN you asked if the rifle had ran before the change and I haven't seen an answer to that question.

Indeed, if a rifle works fine then stops after a change it's probably the fault of the change.

M11293,

Can you clarify?

I do not recall any problems with the rifle before the change, it just isnt feeding now most likely short stroking since the bolt doesn't lock open when you reach the end of your mag.  And the bolt does lock open with an empty mag if you pull it back.

Tell us about the new HandGuard.  What kinda HG was installed ?  Do you know if the front sight body had to be removed in order to install the new HG ?  Who installed the new HG ?  

I think we are getting close to reaching a verdict in this trial.

Well, I had a normal DPMS float hand guard, and I decided to get one that was vented.  Well, I took it to a gun smith had him order it and install it.  He has done other work for me and has done a great job but he may have flubbed up this time.

Quoted: Quoted: I had a bipod fixed and handguards changed, I am using winchester 5.56 FMG, and I used some ultramx .223 that they sell at the range. Guys, I still say that if the rifle ran before being worked on, and then started to have problems after the hand guard change, the gas block is out of alignment with the barrel port.

Seems like Dano had it right.  I advise to take it back to the gunsmith that did the HG swap.  Politely tell him all the test that you perform so that he will be convienced that it probably was something that he caused with the gas system (probably the gas block as Dano said).

Thanks alot guys, I'll be going to the shop tomorrow and I'll talk to him about it.

Once again thanks.

Edited Update:  She is now in the hospital receiving testing and immediate attention.