Looking for AR builders, a little help please

[Cannonman]

I assume that you're not riding the CH, but from you're brief description, I'm not sure I would let the BCG fly home as it may lock up. As posted above, any meaningful help is going to require more detail of your build list and function test. For my quick recommendation, unless you bought your bolt and barrel/extension as a matched set, I would start by checking headspace with the stripped bolt and the appropriate gauges. You may have a Armalite AR-10 cut chamber while using a DPMS spec bolt. You may also have the wrong gas tube, it should end at the center of the cam pin cut-out.

Ok, good thoughts. I think I'll try to get a few pics up with some measurements. The upper and lower were DPMS clones from a now defunct mnfg. I traded out the BCG with one from a .308 that runs. And your right about it locking up. I do not have the appropriate gauges to check tolerances. You do seem to be spot on with my issues. Thanks. I hope to get more info to you guys soon.

 

[Cannonman]

1) So you swapped in the B.R.O. BOLT only or the complete BCG (bolt carrier group) ?

Instead of waiting for your answer, I'll jump ahead.
One of the "things" that could be causing this, and it's not uncommon, is the gas tube/gas tube key alignment.
The portion of the gas tube that projects into the "carrier tunnel" has to align perfectly with the gas tube key on top of the CARRIER.
If it doesn't, this could very well be your issue.

The way to check this is to remove the BOLT from the CARRIER.
Then slide in the empty carrier in and verify alignment.
IF you have good alignment, then it could be an issue with the bolt/BE (barrel extension) interface.

As always, make sure that your bolt and bolt carrier group is assembled correctly.

From what you report, it sounds like gas tube alignment.
It's an easy fix.

Hey thanks DD! I've put together a few AR15's and never had a problem. I guess I got a bit cocky and thought a .308 should be no difference. Really appreciate everyone trying to help.

 

[Cannonman]

1) So you swapped in the B.R.O. BOLT only or the complete BCG (bolt carrier group) ?

Instead of waiting for your answer, I'll jump ahead.
One of the "things" that could be causing this, and it's not uncommon, is the gas tube/gas tube key alignment.
The portion of the gas tube that projects into the "carrier tunnel" has to align perfectly with the gas tube key on top of the CARRIER.
If it doesn't, this could very well be your issue.

The way to check this is to remove the BOLT from the CARRIER.
Then slide in the empty carrier in and verify alignment.
IF you have good alignment, then it could be an issue with the bolt/BE (barrel extension) interface.

As always, make sure that your bolt and bolt carrier group is assembled correctly.

From what you report, it sounds like gas tube alignment.
It's an easy fix.

Thanks, The bolt that works jams as msgriff notes, Will looking into the gas system cause the bolts to stick?

 

[DirectDrive]

Thanks, The bolt that works jams as msgriff notes, Will looking into the gas system cause the bolts to stick?

Most AR's are "direct impingement" where gas is directed to the carrier key which activates the BCG.
Some are piston driven which use a rod to push the BCG.
I'm assuming you have the more common direct impingement system.

With that out of the way...
The bolt doesn't directly interface with the portion of the gas tube that projects into the receiver.
In the AR system, there is a bolt and a carrier. They are very different but they work together as group.
It's called the Bolt Carrier Group or BCG.
The carrier is what you see when you open the dust door on your receiver.
The actual bolt is inside of the carrier.

The little top-mounted device on the carrier is called the Gas Key.
It needs to line up perfectly with the portion of the gas tube that projects into the upper receiver.
Sometimes the gas tube is crooked or bent and in turn drags on the gas key.
This can produce the type of malfunction you describe.
It's an easy check and an easy fix.

 

[Ops]

carbine, rifle, mid length. the size of the gas tube

 

[P7M13]

Great site for info and resources when building AR10 and the many derivatives.

The Ultimate AR-10 AR308 Resource Guide (Visual Tutorials) | 308 AR

We are the Ultimate Online Community and Technical Authority for Information related to AR-10 and 308 AR, 308AR, AR 308 Semi Auto Rifles.Learn and Understand the differences AR15 vs AR-10, AR-10 vs AR15, 308 ar15. Many VISUALS and TUTORIALS to help you assemble your rifle

308ar.com

 

[Cannonman]

You guys are great! I'm reading and re-reading your thoughts. I'd buy dinner and drinks to spend a day with anyone of you. I've never had a mentor and tend to learn by doing. This time it's not my gun and I feel sick about not getting it right. I'll get some pics and better questions shortly. Appreciate you guys.

 

[Tony617]

Not the about the internals of an AR10 since I have only built a couple of AR15 rifles. Did you build the upper receiver and if so you did make sure you did not misalign the gas tube.

 

[msgriff]

carbine, rifle, mid length. the size of the gas tube

Here is a list I compiled some time ago but should still be accurate. This is not 100% complete. Using Armalite AR10 length gas tubes in DPMS pattern barrels can prevent the BCG from going into battery.

DPMS LR-308 Gas Tube (also AR15):

Pistol 6-5/8 - 6-3/4
Carbine 9-3/4 - 9-7/8
Mid-Length 11-3/4 - 11-7/8
Intermediate 13-3/4
Rifle 15-1/8 - 15-1/4
Rifle+2 17-1/8


Armalite Gas Tube:

Super SASS Carbine (308) 11"
Mid length Carbine (308) 12 1/16"
Rifle (308) 15-1/2 "
Carbine (223) 9-3/4 "
Mid (223) 11-3/4"
Rifle (223) 15-3/16"


Oddballs:

Wilson Combat Match Grade Intermediate 13 1/4"
Knight's Armament SR-15 Mod 1. Mid-Length 13"


There are a couple of Mfg's who make DPMS patterned barrels/extensions and drill the gas ports at Armalite lengths (Fulton Armory Criterion).

 

[osprey]

So does it go into battery with no ammo? If it does, the first thing I would do is buy or borrow a set of go, no go gauges and see if headspace is correct.

 

[Cannonman]

Here is a list I compiled some time ago but should still be accurate. This is not 100% complete. Using Armalite AR10 length gas tubes in DPMS pattern barrels can prevent the BCG from going into battery.

DPMS LR-308 Gas Tube (also AR15):

Pistol 6-5/8 - 6-3/4
Carbine 9-3/4 - 9-7/8
Mid-Length 11-3/4 - 11-7/8
Intermediate 13-3/4
Rifle 15-1/8 - 15-1/4
Rifle+2 17-1/8


Armalite Gas Tube:

Super SASS Carbine (308) 11"
Mid length Carbine (308) 12 1/16"
Rifle (308) 15-1/2 "
Carbine (223) 9-3/4 "
Mid (223) 11-3/4"
Rifle (223) 15-3/16"


Oddballs:

Wilson Combat Match Grade Intermediate 13 1/4"


There are a couple of Mfg's who make DPMS patterned barrels/extensions and drill the gas ports at Armalite lengths (Fulton Armory Criterion).

I'm more than excited to have your skills and guidance. Yesterday was full of cooking for my Wife of almost 30 years. Today a head cold. I also live in a very small town of about 25K. The gun shop here does not have these go/no go's. But I'll have them soon. GGeeezzzee I hope you guys can help figure this out for me. Knoxville and Chattanooga are about 45 miles in opposite directions. One of these places should have them. If not, Larry Porterfield? [as if I knew him]

 

[Cannonman]

So does it go into battery with no ammo? If it does, the first thing I would do is buy or borrow a set of go, no go gauges and see if headspace is correct.

Didn't even check. Stupid me. The first round out of the magazine feeds so I "Assumed" it would. I'll know soon. More thanks to you sir.

 

[308]

Are you using reloads? If the brass came from something with an oversized chamber, ie machine gun, and you don't use small base dies, the bottom of the brass may be bulged, not allowing it to fully chamber.

My .308 reloads would never chamber well if at all in my M1A even using the correct dies. I suspect the OP may be having the same issue if he is using reloads in his semi-auto.

 

[BAJ475]

I have an Anderson AM-10 that was purchased new as a complete rifle. My local FFL ordered it directly from Anderson. This is the only AR style rifle that I did not assemble myself. The bolt would go into battery with no ammo but not with a live round (factory, not a reload). I was puzzled because I assumed that the rifle had been test fired before it left Anderson. My first thought was that it may have been a head space problem so had the gunsmith at the local range check it out. The problem turned out to be two O-rings around the extractor spring. He removed the extra O-right and it functioned correctly. Not suggesting that this is your problem but just to let you know that things like this happen.

 

[Mr Smith]

Not sure if anyone asked... Did you build the upper yourself, or did you get a pre-assembled upper?

If pre-assembled: Did it come with the BCG or did you buy that separate?

Can you freely and easily move the BCG into and out of battery with your hand/charging handle? I wouldn't even worry about the lower until we make sure the upper is sorted. Keep it simple and ensure proper function before we complicate it any more than necessary.

 

[Cannonman]

Thanks to you folks I've discovered these differences. The gas tube is considerably longer in the one I'm having problems with. And there's a "nick" caused by the bolt. It's the green one that's giving me the grief. Please why and what do I do next?

 

[spookshack]

I'm no expert, nor did I stay at a Holiday Inn Express, but I'll take "wrong length gas tube" for $200, Alex.

 

[BAJ475]

I'm no expert, nor did I stay at a Holiday Inn Express, but I'll take "wrong length gas tube" for $200, Alex.

Looks that way, especially looking at my AM-10, which is DPMS. But this leaves the question of whether there are other parts that are incompatible between AR-10 and DPMS style rifles. He mentioned nicks on the upper receiver, IIRC.

 

[osprey]

You never answered whether it would go into battery with no ammo. If that gas tube is the wrong application and too long like it appears, it probably will not go into battery as it is bottoming out in gas key. You need to get the right length gas tube for the application. To be totally blunt it is apparent you got in over your head and do not have the proper understanding of the components involved and how they interact. My suggestion at this point is to involve someone directly who has experience with this type of build and get it sorted out correctly. I also suggest not building for someone else until you have many successful builds of your own under your belt.

 

[msgriff]

The gas tube in the green receiver is the correct length. It ends at about the center of the cam key cut-out ("D" shaped, left side from shooters perspective). The light colored receiver actually has the wrong gas tube (too short) and would be prone to short stroking if the gas port was properly sized, but if it ain't broke... . FYI, the circular hole next to the barrel extension is where the T-slot cutter is inserted to mill the charging handle slots. The green receiver gas tube alignment along the x-axis (left-right in pic) doesn't look that far off. I can't tell about the z-axis (up-down). If the tube has a burr, clean it up and try without a cartridge as osprey recommended.
It really needs to be headspaced no matter what. Headspace gauges aren't cheap, and from what I hear, they are hard to find these days. If you know how to strip a bolt, many gunsmiths will check headspace for free or cheap. If they have to strip the bolt, it's usually shop minimum. Stripping a 308 bolt is much more difficult than a AR15, especially double ejector HP bolts, and a bolt jig is usually required.

Edit to add: The gas tube length in the light colored receiver is very typical of using DPMS gas tubes with an Armalite barrel.