I've got a 7" AR-15 that just wont cycle.

Here is the current setup:

DPMS 7" Kitty Kat Barrel
Magpul UBR Stock
Denny's Guns custom Bolt and Carrier Group
H3 Buffer
Carbine Buffer Spring (i think its a carbine spring... its shorter than the other spring I have)


I've also tried the above with different combinations, I've have carbine and H2 buffers, and a rifle length spring.  I've tried a couple different bolts and carriers from friends with ARs, and from the other AR in my closet.

Here's the two problems I've noticed:
5.56 Ammunition (Winchester 5.56 and Federal XM193)
The majority of rounds used will fire, extract, and chamber the next round no problem.  2 or 3 out of 30 have problems extracting.  The bolt will pull the spent casing part way out of the chamber but lets go of it before it can be ejected.  The bolt will then pick up the next round from the mag and attempt to chamber it, usually smashing it up against the existing spent casing.  With the last round in the mag it will not lock the bolt back with the H3 buffer, but i recall it would lock back using other buffers.

Commercial Ammunition (PMC, Remington .223) Sorry I haven't tried as much commercial ammunition so I don't have as much info.
One round will fire and extract.  No problems ejecting the round here, but the bolt will not travel far enough back to pick up the next round from the mag and chamber it.


So based on my observations, 5.56 ammunition seems to have too much bolt speed, and enough bolt travel to grab the next round, but not enough travel to lock the bolt back when the mag runs empty.  Commercial ammunition doesn't have enough bolt travel.

Any thoughts on how to fix this?  Would going back to a lighter buffer and/or spring be the answer?  Thanks in advance :)

first try an o-ring on your extractor and a carbine buffer.  If that doesnt help, you probably need to open up your gas port a bit.

Thanks.  Does it matter what kind or size o-ring?  I'd probably just pick whichever one fits from the hardware store unless there is a recommended type.

I was talking to a USAMU instructor this week and he has a 7" SBR.  He had to enlarge his gas port in the barrel and put an AR10 buffer in his gun.  I shot it at the school this week, and it ran like a dream.

This for heat and chemical resistance.

but, first I'd pick up a similarly sized one at the local hardware store for a few pennies and see if it solves your problem.

do you know what size gas port the barrel currently has? I forgot what the ideal diameter is suppose to be but it can be a factor.

It's whatever the stock DPMS 7" Kitty Kat port size is.  I'll give the extractor o-ring upgrade and different buffer setup a try. I don't want to have to open up the gas port, since thats a permanent modification as far as I know.

I had the same problem first I started with a light buffer and carbine spring
second I use a ar-15 bolt carrier and not M-16
I did not have to open my gas port and spoke to a couple of armors
they all said not to
mine runs great shot it in a couple up matches no problems

I remember when you first posted your SBR, it was the main reason I got this...thanks for the inspiration. I also have another Noveske 7.5" spare barrel.

Special run Noveske 8.5" upper

thanks...... just got my tan ubr ready to go on Monday I will post some more

Mine was set up using a bigger gas port- .078, a 9mm buffer, BCM extractor upgrade(with O-ring) and an M16 carrier. It would even run Wolf reliably.
My DPMS Kitty Cat barrel had an undersized gas port, but then thats what I get for using DPMS crap.

My 7.5" SBR runs well with a Wolff extra power extractor spring (use with existing rubber insert, but not a rubber o-ring), an M16 BC, and a 9mm buffer (heavier than an H-buffer) with a standard carbine buffer spring. I would not recommend a rifle buffer spring. The gun should not be so sensitive as to run only with one type of ammo. As you may have two different problems, try to separate them:

1) Verify, for all your ammo types, that the bolt locks open when fired with an empty mag inserted - test this multiple times to make sure it does so consistently. If it locks open every time, then it has enough gas (don't touch the gas port). Even if it does not do so, check everything else - gas tube, gas block alignment, carrier key etc. - before messing with the gas port.

2) If the gun passes the above test, do you still see the ejection problem ? Make sure the ejector and extractor move freely. Check the fired cases to see if the extractor is slipping off the rim (not enough tension) or ripping through the rim (case gripping the chamber too tight). If slipping off, try the stronger spring or the o-ring. If ripping through, then change the ammo type... if it still happens, check your chamber, as it may be rough.

Thanks for the reply.
Just got back from the range this morning.  Of all the ammunition I had with me (PMC, XM193, Winchester 5.56 and Remington .223) none of them locked the bolt back when the mag ran empty.  I did pick up a couple rubber o-rings from the hardware store to try in the extractor, and after putting one in I had no problems ejecting cases with any type of ammunition.  Now it seems the only problem is that the bolt wont pick up the next round from the mag.  Here's what I tried:

I had three buffer springs with me, two shorter ones and one longer one.  (And I cant for the life of me remember which is a rifle spring and which is a carbine spring.) and four buffers, carbine, 9mm, h2 and h3.  the only combination that seemed to work was the long spring + carbine buffer which cycled PMC about 30% of the time and XM193/Winchester 5.56 80% of the time.  The only combination that worked was the h2 or h3 buffer and the shorter spring, which cycled XM193/Winchester 5.56 about 80% of the time but did not cycle PMC or Remington .223.

But, I never had the bolt lock back with an empty magazine.

So, do all signs point to a gas port that needs to be opened up?  :(  Has anyone ever tried trimming a buffer spring to a slightly shorter length, or is that not recommended?

you need to try the lightest buffer you have.  The bolt isnt going back all the way due to a combination of a couple things, too little gas, underpowered ammo, or too much buffer (resistance).  Use a carbine buffer and spring with M193 and see if the bolt locks back.  If it doesnt, you need more gas.

H3 buffer is way too heavy for that set-up.  I'd go with a std buffer and spring first, then add an semi carrier if you're using a full auto carrier, and finally get a flash suppressor that increases back pressure.  A krink type like the Noveske KX3 or one by Schuster Manufacturing.  If that fails, send it to a gunsmith that can enlarge your gas port.

If its not cycling, you could also have a gas leak or a misaligned gas port/FSB. I would investigate everything else before drilling the gas port.

I have the same problem with my 10.5 upper.  I don't mean to hijack this thread, but anyone know if there's a place that I can send my upper to and they'll open up the gas port?  No gunsmiths near me can do it.

Thanks, that was one of the first things I checked.  Everything is aligned correctly.



No problem, in fact here's what I did to get things working.  Picked up a tin snips from the garage and trimmed the buffer spring.  I loaded one round in a mag at a time and fired until the bolt locked back.  For every time the bolt didn't lock back, I trimmed the spring by one revolution.

Here's the final result, and it works consistantly with all the different types of ammunition I tried.  (thats a carbine buffer pictured)

http://www.adamkonieska.com/akblog/Images/ARPARTS/lower.jpg

The spring kinda scares me being that short. It doesn't look like it from the picture with the UBR, but the spring does reach the back of the buffer tube, so the buffer and spring are still compressed a bit and not floating loose.... But if it works it that works for me I guess.  Thanks to everyone for their help!

Hmmmm - I would not run the gun that way. If you had to cut back the buffer spring there is something very wrong. If you have eliminated gas loss (gas port misalignment, blocked/loose gas tube, carrier key problems or worn gas rings), you should indeed pull the FSB and measure the gas port. Post the gas port size here and determine if drilling it out makes sense (or send it back to DPMS for them to fix).

check the gas rings on your bolt as well

Obviously, all the H buffers are too heavy.   I'd recommend checking the extractor for chips/breakage, replace gas rings and ensure the gas key is not leaking.

You may also want to consider using a KX3 flash hider.

The only  things I did for no reason was add a #60 o-ring over  extractor spring, the claw not slipping on mine.  I do have a spare M16 bolt-carrier group that is neutered that I use also with no ill effects.

Gas port size from DPMS is .070"

Thanks again everyone for the good information, especially the DPMS factory gas port size.

If anything is worth doing, its worth doing right.  I took it to a gun smith who opened the gas port.  He measured it originally at .05 and opened it up to .063 and it runs like a top now.  Also, it cost me 14% for the tin snip i used to cut down the buffer spring, and it cost me 10$ to have the gun smith do it right.  

Moral of the story:  Do it right.  Do it safe.

Thanks again arfcommers.