AR 308 Short Stroke...

I recently built a DPMS patterned 308. It’s my first build. Every time I fire it, the bolt carrier only goes back far enough to extract the spent casing but not far enough to load a new round. The round that is left in the magazine has a pretty decent dent on it.

I built the gun with a heavy buffer. I changed to a different buffer and spring to see if that was the problem but it did no good. I also checked the position of the gas block and it is lined up correctly. The bolt carrier group is NIB and I even oiled it till it was dripping wet. Still, every shot is a short stroke.

I called the company that made the barrel for advice and they told me to drill the gas port a little bigger. I have read online that there are many causes including too small of a gas port and too big of a gas port.

Even though I am no gunsmith, I suspect that the problem is with the bolt carrier group itself. It is an SI Defense Nickel Boron 308 bolt carrier group. When I first received it, I had to send it back because they forgot to stake the gas key. When I told them about the short stroking, they told me to check to make sure the gas rings were installed correctly. WTF?! So I checked and they were staggered like they are supposed to be. They then told me to send the gun to them and they would make it work. Nope. They are in Montana and it takes a week to ship something one way and I really don't trust them at this point. So I taped a can of computer duster onto the gas key with the bolt in the rear position and when I sprayed air through it, the bolt did not move (most likely not enough pressure for the can of air) and air came out of the bolt carrier where the firing pin is. It also felt like air was coming from the bottom of the gas key and one of the gas ports on the side of the carrier.

Are there any gunsmiths in Saint Mary’s County that could give me a hand figuring this thing out? Maybe someone with a DPMS patterned 308 that could swap the bcg for one that actually works so i can see if that is the problem before I go drilling out the gas port on the barrel?

You did the right thing by swapping buffers and buffer springs. I assume you are using .308 Buffers and Springs right? They are different in regards to Bolt travel.

Have you tried other ammo? Some ammo is just problematic on some guns. I assume you tried a seperate magazine as well?

Gas Ring staggering is a myth, even the military GM mentions it being needed, but they rotate on their own and will align with use.

I believe you have it tracked down if its not ammo or mag related. Its likely something in the bolt group or the gas block/port. Oversizing the port is the LAST thing to do.

To troubleshoot the Carrier or Bolt you'll need help from another dpms owner to troubleshoot it.

If no one local can help you could always drive to GB on a Saturday and I'll troubleshoot it while you wait.

Are you using an AR15 heavy buffer or a 308AR buffer like this?
http://heavybuffers.com/ar10carbine.html

Does it cycle by hand?

What buffer spring are you using? Mine did the EXACT same thing originally. I think I was sent a AR15 buffer spring originally (can't remember where I got it.) I bought the correct Armalite AR10 spring (from heavybuffers) and works like a champ now.

Edit: Upon re-reading a couple times, not exact same thing. Mine would 1/2 way try to strip a round and would dent it up. I can't see how the bcg could tear up a round still in the magazine...

Both buffers are for the 308. The heavy buffer is from heavybuffers.com and the standard buffer is from DPMS. The spring from heavybuffers.com is about an inch longer than the spring from DPMS. I have tried a combination of both springs and buffers. It does cycle by hand with both.

Right now all I have used is Remington Core Lokt 150 grain ammo. I am going to try a few different types tomorrow. Any recommendation on a type of ammo? I have also tried 2 different pmags. I have 8 so I will try them all.

Thank you all for the info.

Also, my wife took a video of me firing the first shot. While watching the video I noticed that the bcg on travels about half way back.

Make the trip uptown to let Clandestine figure it out. It's what the man does.

*And I can say from personal experience, it's worth the trip.

BradMacc82 said:

Make the trip uptown to let Clandestine figure it out. It's what the man does.

*And I can say from personal experience, it's worth the trip.

Click to expand...

What he said.

6x45 build gave me fits like you're having. Never apply drill bit to barrel unless you're sure that's the problem. Incidentally, my problem was the BCG, in part.

Essentially the gas is not making its was to the carrier. Causes:
1) Bad Ammo/Underpowered Ammo
2) Clogged Gas Port or more likely, Gas Port blocked by Gas Block.

That's all I got for you, it seems like you have already concluded the the bolts range of motion is fine with hand cycling it. So it moves the correct amount back with hand cycling, so its the gas that has to be the issue.

I would look at the gas block alignment before I do anything else.

I agree, I will take a trip this weekend if clandestine has the time to look at it.

I am 99% sure that the gas block is lined up correctly. I checked it three times. But, I could be wrong.

Thank you guys for the info.

Note, your gas block normally can't line up against the 'step' in the barrel. There is typ a handguard retaining plate sandwiched in there. So if you have a free floating rail, you have to space the gas block forward slightly. Have to measure the gas block to determine the edge to hole distance and then the barrel to determine where the edge of the gas block should line up. The rotational alignment is the obvious one, but many miss the fore/aft alignment step. Don't know if its your problem, just throwing out a common AR build problem.

The round being "dented" should be a concern. What company manufactured the lower? This sounds identical to an issue I had with a very expensive factory gun that I had....I'll stay away from naming the manufacturer. The problem was the magazine catch was machined slightly low causing the mag to sit lower than it should, and that caused bolt override. Magazines could also be the problem. What kind are they? The only readily available ones I totally trust are Knight's Armament and Magpul.

The lower is from SI Defense as is the upper, the handguard and the bolt carrier group. I am using magpul mags.

I really hope that the lower is not the problem. I should be taking the gun to clandestine next Saturday. Hopefully it is just something that I messed up and it can be fixed.

what type of buffer tube did you use? The only308 rifle parts that can be used with the ar15 buffer tubes are the DPMS buffer and springs everyone else has to use a special ar10 buffer tube

assuming you bcg is dpms compatible.

I have a spare stock DPMS buffer and spring you are welcome to try if you like as I upgraded to a heavy buffer in mine.

I also have a spare DPMS gas block and rifle length gas tube you can swap in if you want to be sure they are not the issue on your rifle. Send me a PM if you are interested.

I am using the buffer tube that came with the Magpul PRS AR10 stock. The rifle is DPMS patterened. I am using rifle length buffers and springs from both heavybuffers.com (AR10 heavy buffer and spring) and DPMS (308 buffer and spring). The spring that I recieved from heavybuffers.com is about an inch longer than the spring from DPMS. I tried all combinations of buffers and springs and it did not help. I have ruled out the buffer system at this point because I have used both types of buffers and springs and there is no problems when I cycle the rifle by hand. I have a 20" barrel with a rifle length gas system, using a DPMS rifle length gas tube. When I lined up the gas block, I put the gas tube gas port directly over the barrel gas port and gently slid the gas block over it and tightned it. I tried three different positions until I saw the carbon build up on the barrel was symetrical with the gas port on the barrel (an even circle over the gas port). The gas block neck on the barrel is .875 in diameter. My orginial though was that it was not getting enough gas. I called the company who made the barrel and spoke with their gunsmith. He said to drill out the gas port about .2 thousandths. I later learned that too much gas can make the rifle do the same thing. I figured that if I was going to make the port larger, then I should just buy an adjustable gas block in case the port was too big. But, drilling out the gas port is going to be the very last thing I try. This wekend, I am going to try all 8 of my magpul mags and a whole bunch of different ammo. I have only used Remington Core Lokt 150 grain ammo so far. I am also going to try to measure the gas port on the barrel and the gas block and quadruple check to make sure it is lined up. If that doesn't work, I am going to bring the gun to clandestine next Saturday. He is out of town this weekend.

Nice!

venomousranger said:

buffer tube that came with the Magpul PRS AR10 stock. ... I am using rifle length buffers and springs from both heavybuffers.com (AR10 heavy buffer and spring)

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Im using same on my DPMS pattern Maten.

I don't see how overgassing would cause short stroking like you said you saw on video. Overgassing can cause the buffer to bounce off the bumper and come back too fast before a round feeds up. But if its not coming back far enough at all it can't be overgassed IMO (though Im no expert.) If you have slo-mo video, fire it w/o a mag installed. Curious if the bolt will cycle fully back w/o a round dragging on the bottom of the BCG.

I had the exact same issue with an upper I had a friend build and ship me. Properly spacing/locating the gas block and drilling out the hole just a tad bit more was all it took to clear that up.

When you shoot it you should be able to tell whether its getting too little or too much gas. When I shot mine for the first time I could feel the AR's cycling felt weaker than it should be... can't quite explain it...

Well, I tried new ammo today. Actually, I tried to try new ammo. I went to chamber a round and the bolt wouldn't seat and it became jammed. I couldn't even pull the charging handle back. I had to pry it back with a screw driver. That happened with all five different types of bullets, including the same kind I had been able to shoot. Yes, I did clean the gun including the bolt carrier group before putting it away last time I shot it.

venomousranger said:

Well, I tried new ammo today. Actually, I tried to try new ammo. I went to chamber a round and the bolt wouldn't seat and it became jammed. I couldn't even pull the charging handle back. I had to pry it back with a screw driver. That happened with all five different types of bullets, including the same kind I had been able to shoot. Yes, I did clean the gun including the bolt carrier group before putting it away last time I shot it.

Click to expand...

Sounds like you have a Bad Chamber or the Bolt Head is for the wrong type of Barrel Extension. There are essentially 2 Types of .308 Bolt Heads/Barrel Extensions and they are not cross compatible. A Headspace test will tell you about the chamber (Undersized Chamber, or the throat is to shallow)

I have some 308 headspace gauges if the op would like to borrow.